Summary:
The following are risks associated with pointer-to-pointer reinterpret_cast:
* Can produce the "wrong result" (crash or memory corruption). IIRC, in theory this can happen for any up-cast or down-cast for a non-standard-layout type, though in practice would only happen for multiple inheritance cases (where the base class pointer might be "inside" the derived object). We don't use multiple inheritance a lot, but we do.
* Can mask useful compiler errors upon code change, including converting between unrelated pointer types that you are expecting to be related, and converting between pointer and scalar types unintentionally.
I can only think of some obscure cases where static_cast could be troublesome when it compiles as a replacement:
* Going through `void*` could plausibly cause unnecessary or broken pointer arithmetic. Suppose we have
`struct Derived: public Base1, public Base2`. If we have `Derived*` -> `void*` -> `Base2*` -> `Derived*` through reinterpret casts, this could plausibly work (though technical UB) assuming the `Base2*` is not dereferenced. Changing to static cast could introduce breaking pointer arithmetic.
* Unnecessary (but safe) pointer arithmetic could arise in a case like `Derived*` -> `Base2*` -> `Derived*` where before the Base2 pointer might not have been dereferenced. This could potentially affect performance.
With some light scripting, I tried replacing pointer-to-pointer reinterpret_casts with static_cast and kept the cases that still compile. Most occurrences of reinterpret_cast have successfully been changed (except for java/ and third-party/). 294 changed, 257 remain.
A couple of related interventions included here:
* Previously Cache::Handle was not actually derived from in the implementations and just used as a `void*` stand-in with reinterpret_cast. Now there is a relationship to allow static_cast. In theory, this could introduce pointer arithmetic (as described above) but is unlikely without multiple inheritance AND non-empty Cache::Handle.
* Remove some unnecessary casts to void* as this is allowed to be implicit (for better or worse).
Most of the remaining reinterpret_casts are for converting to/from raw bytes of objects. We could consider better idioms for these patterns in follow-up work.
I wish there were a way to implement a template variant of static_cast that would only compile if no pointer arithmetic is generated, but best I can tell, this is not possible. AFAIK the best you could do is a dynamic check that the void* conversion after the static cast is unchanged.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12308
Test Plan: existing tests, CI
Reviewed By: ltamasi
Differential Revision: D53204947
Pulled By: pdillinger
fbshipit-source-id: 9de23e618263b0d5b9820f4e15966876888a16e2
Summary:
After refactoring of FilePrefetchBuffer, PREFETCH_BYTES_USEFUL was miscalculated. Instead of calculating how many requested bytes are already in the buffer, it took into account alignment as well because aligned_useful_len takes into consideration alignment too.
Also refactored the naming of chunk_offset_in_buffer to make it similar to aligned_useful_len
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12251
Test Plan:
1. Validated internally through release validation benchmarks.
2. Updated unit test that fails without the fix.
Reviewed By: ajkr
Differential Revision: D52891112
Pulled By: akankshamahajan15
fbshipit-source-id: 2526a0b0572d473beaf8b841f2f9c2f6275d9779
Summary:
Summary - Refactor FilePrefetchBuffer code
- Implementation:
FilePrefetchBuffer maintains a deque of free buffers (free_bufs_) of size num_buffers_ and buffers (bufs_) which contains the prefetched data. Whenever a buffer is consumed or is outdated (w.r.t. to requested offset), that buffer is cleared and returned to free_bufs_.
If a buffer is available in free_bufs_, it's moved to bufs_ and is sent for prefetching. num_buffers_ defines how many buffers are maintained that contains prefetched data.
If num_buffers_ == 1, it's a sequential read flow. Read API will be called on that one buffer whenever the data is requested and is not in the buffer.
If num_buffers_ > 1, then the data is prefetched asynchronosuly in the buffers whenever the data is consumed from the buffers and that buffer is freed.
If num_buffers > 1, then requested data can be overlapping between 2 buffers. To return the continuous buffer overlap_bufs_ is used. The requested data is copied from 2 buffers to the overlap_bufs_ and overlap_bufs_ is returned to
the caller.
- Merged Sync and Async code flow into one in FilePrefetchBuffer.
Test Plan -
- Crash test passed
- Unit tests
- Pending - Benchmarks
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12097
Reviewed By: ajkr
Differential Revision: D51759552
Pulled By: akankshamahajan15
fbshipit-source-id: 69a352945affac2ed22be96048d55863e0168ad5
Summary:
Add support for tuning of readahead_size by block cache lookup for async_io.
**Design/ Implementation** -
**BlockBasedTableIterator.cc** -
`BlockCacheLookupForReadAheadSize` callback API lookups in the block cache and tries to reduce the start
and end offset passed. This function looks into the block cache for the blocks between `start_offset`
and `end_offset` and add all the handles in the queue.
It then iterates from the end in the handles to find first miss block and update the end offset to that block.
It also iterates from the start and find first miss block and update the start offset to that block.
```
_read_curr_block_ argument : True if this call was due to miss in the cache and caller wants to read that block
synchronously.
False if current call is to prefetch additional data in extra buffers
(due to ReadAsync call in FilePrefetchBuffer)
```
In case there is no data to be read in that callback (because of upper_bound or all blocks are in cache),
it updates start and end offset to be equal and that `FilePrefetchBuffer` interprets that as 0 length to be read.
**FilePrefetchBuffer.cc** -
FilePrefetchBuffer calls the callback - `ReadAheadSizeTuning` and pass the start and end offset to that
callback to get updated start and end offset to read based on cache hits/misses.
1. In case of Read calls (when offset passed to FilePrefetchBuffer is on cache miss and that data needs to be read), _read_curr_block_ is passed true.
2. In case of ReadAsync calls, when buffer is all consumed and can go for additional prefetching, the start offset passed is the initial end offset of prev buffer (without any updated offset based on cache hit/miss).
Foreg. if following are the data blocks with cache hit/miss and start offset
and Read API found miss on DB1 and based on readahead_size (50) it passes end offset to be 50.
[DB1 - miss- 0 ] [DB2 - hit -10] [DB3 - miss -20] [DB4 - miss-30] [DB5 - hit-40]
[DB6 - hit-50] [DB7 - miss-60] [DB8 - miss - 70] [DB9 - hit - 80] [DB6 - hit 90]
- For Read call - updated start offset remains 0 but end offset updates to DB4, as DB5 is in cache.
- Read calls saves initial end offset 50 as that was meant to be prefetched.
- Now for next ReadAsync call - the start offset will be 50 (previous buffer initial end offset) and based on readahead_size, end offset will be 100
- On callback, because of cache hits - callback will update the start offset to 60 and end offset to 80 to read only 2 data blocks (DB7 and DB8).
- And for that ReadAsync call - initial end offset will be set to 100 which will again used by next ReadAsync call as start offset.
- `initial_end_offset_` in `BufferInfo` is used to save the initial end offset of that buffer.
- If let's say DB5 and DB6 overlaps in 2 buffers (because of alignment), `prev_buf_end_offset` is passed to make sure already prefetched data is not prefetched again in second buffer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11936
Test Plan:
- Ran crash_test several times.
- New unit tests added.
Reviewed By: anand1976
Differential Revision: D50906217
Pulled By: akankshamahajan15
fbshipit-source-id: 0d75d3c98274e98aa34901b201b8fb05232139cf
Summary:
Add stats for better observability of scan prefetching. Its only implemented for sync scan right now. These stats can help inform future improvements in scan prefetching.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11981
Test Plan: Add a new unit test
Reviewed By: akankshamahajan15
Differential Revision: D50516505
Pulled By: anand1976
fbshipit-source-id: cb1cc6cf02df8295930a49c62b11870020df3f97
Summary:
... when compiled with ASSERT_STATUS_CHECKED = 1.
The main change is in iterator_wrapper.h. The remaining changes are just fixing existing unit tests. Adding this check to IteratorWrapper gives a good coverage as the class is used in many places, including child iterators under merging iterator, merging iterator under DB iter, file_iter under level iterator, etc. This change can catch the bug fixed in https://github.com/facebook/rocksdb/issues/11782.
Future follow up: enable `ASSERT_STATUS_CHECKED=1` for stress test and for DEBUG_LEVEL=0.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11975
Test Plan:
* `ASSERT_STATUS_CHECKED=1 DEBUG_LEVEL=2 make -j32 J=32 check`
* I tried to run stress test with `ASSERT_STATUS_CHECKED=1`, but there are a lot of existing stress code that ignore status checking, and fail without the change in this PR. So defer that to a follow up task.
Reviewed By: ajkr
Differential Revision: D50383790
Pulled By: cbi42
fbshipit-source-id: 1a28ce0f5fdf1890f93400b26b3b1b3a287624ce
Summary:
1. **Error** in TestIterateAgainstExpected API - `Assertion index < pre_read_expected_values.size() && index < post_read_expected_values.size() failed.`
**Fix** - `Prev` op is not supported with `auto_readahead_size`. So added support to Reseek in db_iter, if Prev is called. In BlockBasedTableIterator, index_iter_ already moves forward. So there is no way to do Prev from BlockBasedTableIterator.
2. **Error** - `void rocksdb::BlockBasedTableIterator::BlockCacheLookupForReadAheadSize(uint64_t, size_t, size_t&): Assertion index_iter_->value().handle.offset() == offset`
**Fix** - Remove prefetch_buffer to be used when uncompressed dict is read.
3. ** Error in TestPrefixScan API - `db_stress: db/db_iter.cc:369: bool rocksdb::DBIter::FindNextUserEntryInternal(bool, const rocksdb::Slice*): Assertion !skipping_saved_key || CompareKeyForSkip(ikey_.user_key, saved_key_.GetUserKey()) > 0 failed.
Received signal 6 (Aborted)
Invoking GDB for stack trace...
db_stress: table/merging_iterator.cc:1036: bool rocksdb::MergingIterator::SkipNextDeleted(): Assertion comparator_->Compare(range_tombstone_iters_[i]->start_key(), pik) <= 0 failed`
**Fix** - SeekPrev also calls 1) SeekPrev , 2)Seek and then 3)Prev in some cases in db_iter.cc leading to failure of Prev operation. These backward operations also call Seek. Added direction to disable lookup once direction is backwards in BlockBasedTableIterator.cc
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11884
Test Plan: Ran various flavors of crash tests locally for the whole duration
Reviewed By: anand1976
Differential Revision: D49834201
Pulled By: akankshamahajan15
fbshipit-source-id: 9a007b4d46a48002c43dc4623a400ecf47d997fe
Summary:
Implement block cache lookup to determine readahead_size during scans. It's enabled if auto_readahead_size, block_cache and iterate_upper_bound - all three are set.
Design -
1. Whenever there is a cache miss and FilePrefetchBuffer is called, a callback is made to determine readahead_size for that prefetching.
2. The callback iterates over index and do block cache lookup for each data block handle until existing readahead_size is reached. Then It removes the cache hit data blocks from end to calculate optimized readahead_size.
3. Since index_iter_ is moved, it stores block handles in a queue, and use that queue to get block handle instead of doing index_iter_->Next().
4. This is for Sync scans. Async scans support is in progress.
NOTE:
The issue right now is after Seek and Next, if Prev is called, there is no way to do Prev operation. index_iter_ is already pointing to a different block. So it returns "Not supported" in that case with error message - "auto tuning of readahead size is not supported with Prev op"
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11860
Test Plan:
- Added new unit test
- crash_tests
- Running scans locally to check for any regression
Reviewed By: anand1976
Differential Revision: D49548118
Pulled By: akankshamahajan15
fbshipit-source-id: f1aee409a71b4ad9e5bf3610f43edf30c6630c78
Summary:
When auto_readahead_size is enabled in async_io, during seek, first buffer will prefetch the data - (current block + readahead till upper_bound). There can be cases where
1. first buffer prefetched all the data till upper bound, or
2. first buffer already has the data from prev seek call
and second buffer prefetch further leading to alignment issues.
This PR fixes that assertion and second buffer won't go for prefetching if first buffer has already prefetched till upper_bound.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11852
Test Plan:
- Added new unit test that failed without this fix.
- crash tests passed locally
Reviewed By: pdillinger
Differential Revision: D49384138
Pulled By: akankshamahajan15
fbshipit-source-id: 54417e909e4d986f1e5a17dbaea059cd4962fd4d
Summary:
With the async_io option, the Seek happens in 2 phases. Phase 1 starts an asynchronous read on a block cache miss, and phase 2 waits for it to complete and finishes the seek. In both phases, BlockBasedTable::NewDataBlockIterator is called, which tries to lookup the block cache for the data block first before looking in the prefetch buffer. It's optimized by doing the block cache lookup only in the first phase and save some CPU.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11616
Test Plan: Added unit test
Reviewed By: jaykorean
Differential Revision: D47477887
Pulled By: akankshamahajan15
fbshipit-source-id: 0355e0a68fc0ea2eb92340ae42735afcdbcbfd79
Summary:
Update the logic in FilePrefetchBuffer to update `upper_bound_offset_` during reseek. During Reseek, `iterate_upper_bound` can be changed dynamically. So added an API to update that in FilePrefetchBuffer.
Added unit test to confirm the behavior.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11775
Test Plan:
- Check stress tests in case there is any failure after this diff.
- make crash_test -j32 with auto_readahead_size=1 passed locally
Reviewed By: anand1976
Differential Revision: D48815177
Pulled By: akankshamahajan15
fbshipit-source-id: 5f44fbb3af06c86a1c38f139c5fa4543891837f4
Summary:
During Seek, the iterator seeks every file on L0. In async_io, it submit the requests to seek on every file on L0 asynchronously using RocksDB FilePrefetchBuffer.
However, FilePrefetchBuffer does alignment and reads extra bytes then needed that can increase the throughput.
In case of non direct io, the alignment can be avoided.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11793
Test Plan:
- Added a unit test that fails without this PR.
- make crash_test -j32 completed successfully
Reviewed By: anand1976
Differential Revision: D48985051
Pulled By: akankshamahajan15
fbshipit-source-id: 2d130a9e7c3df9c4fcd0408406e6277ab75a4389
Summary:
Fix seg fault in auto_readahead_size with async_io when readahead_size = 0. If readahead_size is trimmed and is 0, it's not eligible for further prefetching and should return.
Error occured when the first buffer already contains data and it goes for prefetching in second buffer leading to assertion failure -
`assert(roundup_len1 >= alignment);
`
because roundup_len1 = length + readahead_size.
length is 0 and readahead_size is also 0.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11769
Test Plan: Reproducible with db_stress with async_io enabled.
Reviewed By: anand1976
Differential Revision: D48743031
Pulled By: akankshamahajan15
fbshipit-source-id: 0e08c41f862f6287ca223fbfaf6cd42fc97b3c87
Summary:
**Context/Summary:** as title, should be harmless. And it's a guessed fix to https://github.com/facebook/rocksdb/issues/11717 while no repro has obtained on my end yet.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11720
Test Plan: existing tests
Reviewed By: cbi42
Differential Revision: D48475661
Pulled By: hx235
fbshipit-source-id: 7c7390319f094c540e703fe2e78a8d601b7a894b
Summary:
Implement trimming of readahead_size under a new option ReadOptions.auto_readahead_size. It'll trim the readahead_size during prefetching upto iterate_upper_bound offset only when ReadOptions.iterate_upper_bound is set, therefore reducing the prefetching of data beyond upper_bound.
It's enabled for both implicit auto readahead size and when ReadOptions.readahead_size is specified and for sync and async_io.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11684
Test Plan: Added new unit test
Reviewed By: anand1976
Differential Revision: D48479723
Pulled By: akankshamahajan15
fbshipit-source-id: 2b1703579caf779105e836b580866ffd7db076fc
Summary:
It seems the flag `-fno-elide-constructors` is incorrectly overwritten in Makefile by 9c2ebcc2c3/Makefile (L243)
Applying the change in PR https://github.com/facebook/rocksdb/issues/11675 shows a lot of missing status checks. This PR adds the missing status checks.
Most of changes are just adding asserts in unit tests. I'll add pr comment around more interesting changes that need review.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11686
Test Plan: change Makefile as in https://github.com/facebook/rocksdb/issues/11675, and run `ASSERT_STATUS_CHECKED=1 TEST_UINT128_COMPAT=1 ROCKSDB_MODIFY_NPHASH=1 LIB_MODE=static OPT="-DROCKSDB_NAMESPACE=alternative_rocksdb_ns" make V=1 -j24 J=24 check`
Reviewed By: hx235
Differential Revision: D48176132
Pulled By: cbi42
fbshipit-source-id: 6758946cfb1c6ff84c4c1e0ca540d05e6fc390bd
Summary:
**Context/Summary:**
- Similar to https://github.com/facebook/rocksdb/pull/11288 but for user read such as `Get(), MultiGet(), DBIterator::XXX(), Verify(File)Checksum()`.
- For this, I refactored some user-facing `MultiGet` calls in `TransactionBase` and various types of `DB` so that it does not call a user-facing `Get()` but `GetImpl()` for passing the `ReadOptions::io_activity` check (see PR conversation)
- New user read stats breakdown are guarded by `kExceptDetailedTimers` since measurement shows they have 4-5% regression to the upstream/main.
- Misc
- More refactoring: with https://github.com/facebook/rocksdb/pull/11288, we complete passing `ReadOptions/IOOptions` to FS level. So we can now replace the previously [added](https://github.com/facebook/rocksdb/pull/9424) `rate_limiter_priority` parameter in `RandomAccessFileReader`'s `Read/MultiRead/Prefetch()` with `IOOptions::rate_limiter_priority`
- Also, `ReadAsync()` call time is measured in `SST_READ_MICRO` now
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11444
Test Plan:
- CI fake db crash/stress test
- Microbenchmarking
**Build** `make clean && ROCKSDB_NO_FBCODE=1 DEBUG_LEVEL=0 make -jN db_basic_bench`
- google benchmark version: 604f6fd3f4
- db_basic_bench_base: upstream
- db_basic_bench_pr: db_basic_bench_base + this PR
- asyncread_db_basic_bench_base: upstream + [db basic bench patch for IteratorNext](https://github.com/facebook/rocksdb/compare/main...hx235:rocksdb:micro_bench_async_read)
- asyncread_db_basic_bench_pr: asyncread_db_basic_bench_base + this PR
**Test**
Get
```
TEST_TMPDIR=/dev/shm ./db_basic_bench_{null_stat|base|pr} --benchmark_filter=DBGet/comp_style:0/max_data:134217728/per_key_size:256/enable_statistics:1/negative_query:0/enable_filter:0/mmap:1/threads:1 --benchmark_repetitions=1000
```
Result
```
Coming soon
```
AsyncRead
```
TEST_TMPDIR=/dev/shm ./asyncread_db_basic_bench_{base|pr} --benchmark_filter=IteratorNext/comp_style:0/max_data:134217728/per_key_size:256/enable_statistics:1/async_io:1/include_detailed_timers:0 --benchmark_repetitions=1000 > syncread_db_basic_bench_{base|pr}.out
```
Result
```
Base:
1956,1956,1968,1977,1979,1986,1988,1988,1988,1990,1991,1991,1993,1993,1993,1993,1994,1996,1997,1997,1997,1998,1999,2001,2001,2002,2004,2007,2007,2008,
PR (2.3% regression, due to measuring `SST_READ_MICRO` that wasn't measured before):
1993,2014,2016,2022,2024,2027,2027,2028,2028,2030,2031,2031,2032,2032,2038,2039,2042,2044,2044,2047,2047,2047,2048,2049,2050,2052,2052,2052,2053,2053,
```
Reviewed By: ajkr
Differential Revision: D45918925
Pulled By: hx235
fbshipit-source-id: 58a54560d9ebeb3a59b6d807639692614dad058a
Summary:
**Context/Summary:**
After https://github.com/facebook/rocksdb/pull/11631, file hint is not longer needed for compaction read. Therefore we can deprecate `Options::access_hint_on_compaction_start`. As this is a public API change, we should first mark the relevant APIs (including the Java's) deprecated and remove it in next major release 9.0.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11658
Test Plan: No code change
Reviewed By: ajkr
Differential Revision: D47997856
Pulled By: hx235
fbshipit-source-id: 16e015ae7728c224b1caef73143aa9915668f4ac
Summary:
**Context/Summary**
As titled. The benefit of doing so is to explicitly call readahead() instead of relying page cache behavior for compaction read when we know that we most likely need readahead as compaction read is sequential read .
**Test**
Extended the existing UT to cover compaction read case
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11631
Reviewed By: ajkr
Differential Revision: D47681437
Pulled By: hx235
fbshipit-source-id: 78792f64985c4dc44aa8f2a9c41ab3e8bbc0bc90
Summary:
When num_file_reads_for_auto_readahead = 1, during seek, it would go for prefetchingextra data in second buffer along with seek data, that would lead to increase in read data and
discarded bytes.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11560
Test Plan: Added unit test
Reviewed By: anand1976
Differential Revision: D47008102
Pulled By: akankshamahajan15
fbshipit-source-id: 566c6131cb5f968d5efb81fd0ab233ff7e534ab0
Summary:
1. Public API change: Replace `use_async_io` API in file_system with `SupportedOps` API which is used by underlying FileSystem to indicate to upper layers whether the FileSystem supports different operations introduced in `enum FSSupportedOps `. Right now operations are `async_io` and whether FS will provide its own buffer during reads or not. The api is changed to extend it to various FileSystem operations in one API rather than creating a separate API for each operation.
2. Provide support for underlying FS to pass their own buffer during Reads (async and sync read) instead of using RocksDB provided `scratch` (buffer) in `FSReadRequest`. Currently only MultiRead supports it and later will be extended to other reads as well (point lookup, scan etc). More details in how to enable in file_system.h
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11324
Test Plan: Tested locally
Reviewed By: anand1976
Differential Revision: D44465322
Pulled By: akankshamahajan15
fbshipit-source-id: 9ec9e08f839b5cc815e75d5dade6cd549998d0ec
Summary:
**Context/Summary:**
When db is upgrading to adopt [pr11406](https://github.com/facebook/rocksdb/pull/11406/), it's possible for RocksDB to infer a small tail size to prefetch for pre-upgrade files. Such small tail size would have caused 1 file read per index or filter partition if partitioned index or filer is used. This PR provides a UT to show this would not happen.
Misc: refactor the related UTs a bit to make this new UT more readable.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11522
Test Plan:
- New UT
If logic of upgrade is wrong e.g,
```
--- a/table/block_based/partitioned_index_reader.cc
+++ b/table/block_based/partitioned_index_reader.cc
@@ -166,7 +166,8 @@ Status PartitionIndexReader::CacheDependencies(
uint64_t prefetch_len = last_off - prefetch_off;
std::unique_ptr<FilePrefetchBuffer> prefetch_buffer;
if (tail_prefetch_buffer == nullptr || !tail_prefetch_buffer->Enabled() ||
- tail_prefetch_buffer->GetPrefetchOffset() > prefetch_off) {
+ (false && tail_prefetch_buffer->GetPrefetchOffset() > prefetch_off)) {
```
, then the UT will fail like below
```
[ RUN ] PrefetchTailTest/PrefetchTailTest.UpgradeToTailSizeInManifest/0
file/prefetch_test.cc:461: Failure
Expected: (db_open_file_read.count) < (num_index_partition), actual: 38 vs 33
Received signal 11 (Segmentation fault)
```
Reviewed By: pdillinger
Differential Revision: D46546707
Pulled By: hx235
fbshipit-source-id: 9897b0a975e9055963edac5451fd1cd9d6c45d0e
Summary:
**Context:**
We prefetch the tail part of a SST file (i.e, the blocks after data blocks till the end of the file) during each SST file open in hope to prefetch all the stuff at once ahead of time for later read e.g, footer, meta index, filter/index etc. The existing approach to estimate the tail size to prefetch is through `TailPrefetchStats` heuristics introduced in https://github.com/facebook/rocksdb/pull/4156, which has caused small reads in unlucky case (e.g, small read into the tail buffer during table open in thread 1 under the same BlockBasedTableFactory object can make thread 2's tail prefetching use a small size that it shouldn't) and is hard to debug. Therefore we decide to record the exact tail size and use it directly to prefetch tail of the SST instead of relying heuristics.
**Summary:**
- Obtain and record in manifest the tail size in `BlockBasedTableBuilder::Finish()`
- For backward compatibility, we fall back to TailPrefetchStats and last to simple heuristics that the tail size is a linear portion of the file size - see PR conversation for more.
- Make`tail_start_offset` part of the table properties and deduct tail size to record in manifest for external files (e.g, file ingestion, import CF) and db repair (with no access to manifest).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11406
Test Plan:
1. New UT
2. db bench
Note: db bench on /tmp/ where direct read is supported is too slow to finish and the default pinning setting in db bench is not helpful to profile # sst read of Get. Therefore I hacked the following to obtain the following comparison.
```
diff --git a/table/block_based/block_based_table_reader.cc b/table/block_based/block_based_table_reader.cc
index bd5669f0f..791484c1f 100644
--- a/table/block_based/block_based_table_reader.cc
+++ b/table/block_based/block_based_table_reader.cc
@@ -838,7 +838,7 @@ Status BlockBasedTable::PrefetchTail(
&tail_prefetch_size);
// Try file system prefetch
- if (!file->use_direct_io() && !force_direct_prefetch) {
+ if (false && !file->use_direct_io() && !force_direct_prefetch) {
if (!file->Prefetch(prefetch_off, prefetch_len, ro.rate_limiter_priority)
.IsNotSupported()) {
prefetch_buffer->reset(new FilePrefetchBuffer(
diff --git a/tools/db_bench_tool.cc b/tools/db_bench_tool.cc
index ea40f5fa0..39a0ac385 100644
--- a/tools/db_bench_tool.cc
+++ b/tools/db_bench_tool.cc
@@ -4191,6 +4191,8 @@ class Benchmark {
std::shared_ptr<TableFactory>(NewCuckooTableFactory(table_options));
} else {
BlockBasedTableOptions block_based_options;
+ block_based_options.metadata_cache_options.partition_pinning =
+ PinningTier::kAll;
block_based_options.checksum =
static_cast<ChecksumType>(FLAGS_checksum_type);
if (FLAGS_use_hash_search) {
```
Create DB
```
./db_bench --bloom_bits=3 --use_existing_db=1 --seed=1682546046158958 --partition_index_and_filters=1 --statistics=1 -db=/dev/shm/testdb/ -benchmarks=readrandom -key_size=3200 -value_size=512 -num=1000000 -write_buffer_size=6550000 -disable_auto_compactions=false -target_file_size_base=6550000 -compression_type=none
```
ReadRandom
```
./db_bench --bloom_bits=3 --use_existing_db=1 --seed=1682546046158958 --partition_index_and_filters=1 --statistics=1 -db=/dev/shm/testdb/ -benchmarks=readrandom -key_size=3200 -value_size=512 -num=1000000 -write_buffer_size=6550000 -disable_auto_compactions=false -target_file_size_base=6550000 -compression_type=none
```
(a) Existing (Use TailPrefetchStats for tail size + use seperate prefetch buffer in PartitionedFilter/IndexReader::CacheDependencies())
```
rocksdb.table.open.prefetch.tail.hit COUNT : 3395
rocksdb.sst.read.micros P50 : 5.655570 P95 : 9.931396 P99 : 14.845454 P100 : 585.000000 COUNT : 999905 SUM : 6590614
```
(b) This PR (Record tail size + use the same tail buffer in PartitionedFilter/IndexReader::CacheDependencies())
```
rocksdb.table.open.prefetch.tail.hit COUNT : 14257
rocksdb.sst.read.micros P50 : 5.173347 P95 : 9.015017 P99 : 12.912610 P100 : 228.000000 COUNT : 998547 SUM : 5976540
```
As we can see, we increase the prefetch tail hit count and decrease SST read count with this PR
3. Test backward compatibility by stepping through reading with post-PR code on a db generated pre-PR.
Reviewed By: pdillinger
Differential Revision: D45413346
Pulled By: hx235
fbshipit-source-id: 7d5e36a60a72477218f79905168d688452a4c064
Summary:
when I use g++-13 to exec the `make all` command, the output throws the warnings.
```
db/compaction/compaction_job_test.cc: In member function ‘void rocksdb::CompactionJobTestBase::AddMockFile(const rocksdb::mock::KVVector&, int)’:
db/compaction/compaction_job_test.cc:376:57: error: redundant move in initialization [-Werror=redundant-move]
376 | env_, GenerateFileName(file_number), std::move(contents)));
| ~~~~~~~~~^~~~~~~~~~
db/compaction/compaction_job_test.cc:375:7: note: in expansion of macro ‘EXPECT_OK’
375 | EXPECT_OK(mock_table_factory_->CreateMockTable(
| ^~~~~~~~~
db/compaction/compaction_job_test.cc:376:57: note: remove ‘std::move’ call
376 | env_, GenerateFileName(file_number), std::move(contents)));
| ~~~~~~~~~^~~~~~~~~~
db/compaction/compaction_job_test.cc:375:7: note: in expansion of macro ‘EXPECT_OK’
375 | EXPECT_OK(mock_table_factory_->CreateMockTable(
| ^~~~~~~~~
cc1plus: all warnings being treated as errors
make: *** [Makefile:2507: db/compaction/compaction_job_test.o] Error 1
```
and I also add some `(void)unused_variable` statements because of the cmake argument `-Wunused-but-set-variable -Wunused-but-set-variable`
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11418
Reviewed By: akankshamahajan15
Differential Revision: D45528223
Pulled By: ajkr
fbshipit-source-id: fee1a77c30039a56b481de953f0a834cc788abbc
Summary:
In PosixFileSystem, IO uring support is opt-in. If the support is not enabled by the user, then ignore the async_io ReadOption in MultiGet and iteration at the top, rather than follow the async_io codepath and transparently switch to sync IO at the FileSystem layer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11296
Test Plan: Add new unit tests
Reviewed By: akankshamahajan15
Differential Revision: D44045776
Pulled By: anand1976
fbshipit-source-id: a0881bf763ca2fde50b84063d0068bb521edd8b9
Summary:
- Return NotSupported in scan if IOUring not supported if async_io is enabled
- Enable IOUring in db_stress for async_io testing
- Disable async_io in circleci crash testing as circleci doesn't support IOUring
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11197
Test Plan: CircleCI jobs
Reviewed By: anand1976
Differential Revision: D43096313
Pulled By: akankshamahajan15
fbshipit-source-id: c2c53a87636950c0243038b9f5bd0d91608e4fda
Summary:
We haven't been actively mantaining RocksDB LITE recently and the size must have been gone up significantly. We are removing the support.
Most of changes were done through following comments:
unifdef -m -UROCKSDB_LITE `git grep -l ROCKSDB_LITE | egrep '[.](cc|h)'`
by Peter Dillinger. Others changes were manually applied to build scripts, CircleCI manifests, ROCKSDB_LITE is used in an expression and file db_stress_test_base.cc.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11147
Test Plan: See CI
Reviewed By: pdillinger
Differential Revision: D42796341
fbshipit-source-id: 4920e15fc2060c2cd2221330a6d0e5e65d4b7fe2
Summary:
Add more coverage in unit tests for async scan. The added unit test fails without PR https://github.com/facebook/rocksdb/pull/10939.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11087
Test Plan: CircleCI jobs status for new unit tests.
Reviewed By: anand1976
Differential Revision: D42487931
Pulled By: akankshamahajan15
fbshipit-source-id: d59ed7666599bd0d2733ac5d76bd70984b54c5a9
Summary:
Instead of existing calls to ps from gnu_parallel, call a new wrapper that does ps, looks for unit test like processes, and uses pstack or gdb to print thread stack traces. Also, using `ps -wwf` instead of `ps -wf` ensures output is not cut off.
For security, CircleCI runs with security restrictions on ptrace (/proc/sys/kernel/yama/ptrace_scope = 1), and this change adds a work-around to `InstallStackTraceHandler()` (only used by testing tools) to allow any process from the same user to debug it. (I've also touched >100 files to ensure all the unit tests call this function.)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10828
Test Plan: local manual + temporary infinite loop in a unit test to observe in CircleCI
Reviewed By: hx235
Differential Revision: D40447634
Pulled By: pdillinger
fbshipit-source-id: 718a4c4a5b54fa0f9af2d01a446162b45e5e84e1
Summary:
RocksDB does auto-readahead for iterators on noticing more
than two reads for a table file if user doesn't provide readahead_size and reads are sequential.
A new option num_file_reads_for_auto_readahead is added which can be
configured and indicates after how many sequential reads prefetching should
be start.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10556
Test Plan: Existing and new unit test
Reviewed By: anand1976
Differential Revision: D38947147
Pulled By: akankshamahajan15
fbshipit-source-id: c9eeab495f84a8df7f701c42f04894e46440ad97
Summary:
This PR adds few optimizations for async_io for shorter scans.
1. If async_io is enabled, seek would create FilePrefetchBuffer object to fetch the data asynchronously. However `FilePrefetchbuffer::num_file_reads_` wasn't taken into consideration if it calls Next after Seek and would go for Prefetching. This PR fixes that and Next will go for prefetching only if `FilePrefetchbuffer::num_file_reads_` is greater than 2 along with if blocks are sequential. This scenario is only for implicit auto readahead.
2. For seek, when it calls TryReadFromCacheAsync to poll it makes async call as well because TryReadFromCacheAsync flow wasn't changed. So I updated to return after poll instead of further prefetching any data.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10140
Test Plan:
1. Added a unit test
2. Ran crash_test with async_io = 1 to make sure nothing crashes.
Reviewed By: anand1976
Differential Revision: D37042242
Pulled By: akankshamahajan15
fbshipit-source-id: b8e6b7cb2ee0886f37a8f53951948b9084e8ffda
Summary:
In case of non sequential reads with `async_io`, `FilePRefetchBuffer::TryReadFromCacheAsync` can be called for previous blocks with `offset < bufs_[curr_].offset_` which wasn't handled correctly resulting wrong data being returned from buffer.
Since `FilePRefetchBuffer::PrefetchAsync` can be called for any data block, it sets `prev_len_` to 0 indicating `FilePRefetchBuffer::TryReadFromCacheAsync` to go for the prefetching even though offset < bufs_[curr_].offset_ This is because async prefetching is always done in second buffer (to avoid mutex) even though curr_ is empty leading to offset < bufs_[curr_].offset_ in some cases.
If prev_len_ is non zero then `TryReadFromCacheAsync` returns false if `offset < bufs_[curr_].offset_ && prev_len != 0` indicating reads are not sequential and previous call wasn't PrefetchAsync.
- This PR also simplifies `FilePRefetchBuffer::TryReadFromCacheAsync` as it was getting complicated covering different scenarios based on `async_io` enabled/disabled. If `for_compaction` is set true, it now calls `FilePRefetchBufferTryReadFromCache` following synchronous flow as before. Its decided in BlockFetcher.cc
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10032
Test Plan:
1. export CRASH_TEST_EXT_ARGS=" --async_io=1"
make crash_test -j completed successfully locally
2. make crash_test -j completed successfully locally
3. Reran CircleCi mini crashtest job 4 - 5 times.
4. Updated prefetch_test for more coverage.
Reviewed By: anand1976
Differential Revision: D36579858
Pulled By: akankshamahajan15
fbshipit-source-id: 0c428d62b45e12e082a83acf533a5e37a584bedf
Summary:
The RocksDB iterator is a hierarchy of iterators. MergingIterator maintains a heap of LevelIterators, one for each L0 file and for each non-zero level. The Seek() operation naturally lends itself to parallelization, as it involves positioning every LevelIterator on the correct data block in the correct SST file. It lookups a level for a target key, to find the first key that's >= the target key. This typically involves reading one data block that is likely to contain the target key, and scan forward to find the first valid key. The forward scan may read more data blocks. In order to find the right data block, the iterator may read some metadata blocks (required for opening a file and searching the index).
This flow can be parallelized.
Design: Seek will be called two times under async_io option. First seek will send asynchronous request to prefetch the data blocks at each level and second seek will follow the normal flow and in FilePrefetchBuffer::TryReadFromCacheAsync it will wait for the Poll() to get the results and add the iterator to min_heap.
- Status::TryAgain is passed down from FilePrefetchBuffer::PrefetchAsync to block_iter_.Status indicating asynchronous request has been submitted.
- If for some reason asynchronous request returns error in submitting the request, it will fallback to sequential reading of blocks in one pass.
- If the data already exists in prefetch_buffer, it will return the data without prefetching further and it will be treated as single pass of seek.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9994
Test Plan:
- **Run Regressions.**
```
./db_bench -db=/tmp/prefix_scan_prefetch_main -benchmarks="fillseq" -key_size=32 -value_size=512 -num=5000000 -use_direct_io_for_flush_and_compaction=true -target_file_size_base=16777216
```
i) Previous release 7.0 run for normal prefetching with async_io disabled:
```
./db_bench -use_existing_db=true -db=/tmp/prefix_scan_prefetch_main -benchmarks="seekrandom" -key_size=32 -value_size=512 -num=5000000 -use_direct_reads=true -seek_nexts=327680 -duration=120 -ops_between_duration_checks=1
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
RocksDB: version 7.0
Date: Thu Mar 17 13:11:34 2022
CPU: 24 * Intel Core Processor (Broadwell)
CPUCache: 16384 KB
Keys: 32 bytes each (+ 0 bytes user-defined timestamp)
Values: 512 bytes each (256 bytes after compression)
Entries: 5000000
Prefix: 0 bytes
Keys per prefix: 0
RawSize: 2594.0 MB (estimated)
FileSize: 1373.3 MB (estimated)
Write rate: 0 bytes/second
Read rate: 0 ops/second
Compression: Snappy
Compression sampling rate: 0
Memtablerep: SkipListFactory
Perf Level: 1
------------------------------------------------
DB path: [/tmp/prefix_scan_prefetch_main]
seekrandom : 483618.390 micros/op 2 ops/sec; 338.9 MB/s (249 of 249 found)
```
ii) normal prefetching after changes with async_io disable:
```
./db_bench -use_existing_db=true -db=/tmp/prefix_scan_prefetch_main -benchmarks="seekrandom" -key_size=32 -value_size=512 -num=5000000 -use_direct_reads=true -seek_nexts=327680 -duration=120 -ops_between_duration_checks=1
Set seed to 1652922591315307 because --seed was 0
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
RocksDB: version 7.3
Date: Wed May 18 18:09:51 2022
CPU: 32 * Intel Xeon Processor (Skylake)
CPUCache: 16384 KB
Keys: 32 bytes each (+ 0 bytes user-defined timestamp)
Values: 512 bytes each (256 bytes after compression)
Entries: 5000000
Prefix: 0 bytes
Keys per prefix: 0
RawSize: 2594.0 MB (estimated)
FileSize: 1373.3 MB (estimated)
Write rate: 0 bytes/second
Read rate: 0 ops/second
Compression: Snappy
Compression sampling rate: 0
Memtablerep: SkipListFactory
Perf Level: 1
------------------------------------------------
DB path: [/tmp/prefix_scan_prefetch_main]
seekrandom : 483080.466 micros/op 2 ops/sec 120.287 seconds 249 operations; 340.8 MB/s (249 of 249 found)
```
iii) db_bench with async_io enabled completed succesfully
```
./db_bench -use_existing_db=true -db=/tmp/prefix_scan_prefetch_main -benchmarks="seekrandom" -key_size=32 -value_size=512 -num=5000000 -use_direct_reads=true -seek_nexts=327680 -duration=120 -ops_between_duration_checks=1 -async_io=1 -adaptive_readahead=1
Set seed to 1652924062021732 because --seed was 0
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
RocksDB: version 7.3
Date: Wed May 18 18:34:22 2022
CPU: 32 * Intel Xeon Processor (Skylake)
CPUCache: 16384 KB
Keys: 32 bytes each (+ 0 bytes user-defined timestamp)
Values: 512 bytes each (256 bytes after compression)
Entries: 5000000
Prefix: 0 bytes
Keys per prefix: 0
RawSize: 2594.0 MB (estimated)
FileSize: 1373.3 MB (estimated)
Write rate: 0 bytes/second
Read rate: 0 ops/second
Compression: Snappy
Compression sampling rate: 0
Memtablerep: SkipListFactory
Perf Level: 1
------------------------------------------------
DB path: [/tmp/prefix_scan_prefetch_main]
seekrandom : 553913.576 micros/op 1 ops/sec 120.199 seconds 217 operations; 293.6 MB/s (217 of 217 found)
```
- db_stress with async_io disabled completed succesfully
```
export CRASH_TEST_EXT_ARGS=" --async_io=0"
make crash_test -j
```
I**n Progress**: db_stress with async_io is failing and working on debugging/fixing it.
Reviewed By: anand1976
Differential Revision: D36459323
Pulled By: akankshamahajan15
fbshipit-source-id: abb1cd944abe712bae3986ae5b16704b3338917c
Summary:
Add stats PREFETCHED_BYTES_DISCARDED and POLL_WAIT_MICROS.
PREFETCHED_BYTES_DISCARDED records number of prefetched bytes discarded by
FilePrefetchBuffer. POLL_WAIT_MICROS records the time taken by underling
file_system Poll API.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9845
Test Plan: Update existing tests
Reviewed By: anand1976
Differential Revision: D35909694
Pulled By: akankshamahajan15
fbshipit-source-id: e009ef940bb9ed72c9446f5529095caabb8a1e36
Summary:
Currently async prefetching is enabled for implicit internal auto readahead in FilePrefetchBuffer if `ReadOptions.async_io` is set. This PR enables async prefetching for `ReadOptions.readahead_size` when `ReadOptions.async_io` is set true.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9827
Test Plan: Update unit test
Reviewed By: anand1976
Differential Revision: D35552129
Pulled By: akankshamahajan15
fbshipit-source-id: d9f9a96672852a591375a21eef15355cf3289f5c
Summary:
Since all plaftorms don't support io_uring. So updated the unit
test to take that into consideration when testing async reads in unit tests.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9819
Test Plan:
valgrind --error-exitcode=2 --leak-check=full ./prefetch_test
--gtest_filter=PrefetchTest2.ReadAsyncWithPosixFS
CircleCI jobs
Reviewed By: pdillinger
Differential Revision: D35469959
Pulled By: akankshamahajan15
fbshipit-source-id: b170459ec816487fc0a13b1d55dbbe4f754b2eba
Summary:
Update stats in random_access_file_reader for Read and
ReadAsync API to take into account the read latency for async
prefetching.
It also fixes ERROR_HANDLER_AUTORESUME_RETRY_COUNT stat whose value was
incorrect in portal.h
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9810
Test Plan: Update unit test
Reviewed By: anand1976
Differential Revision: D35433081
Pulled By: akankshamahajan15
fbshipit-source-id: aeec3901270e58a003ce6b5214bd25ddcb3a12a9
Summary:
If FilePrefetchBuffer object is destroyed and then later Poll() calls callback on object which has been destroyed, it gives segfault on accessing destroyed object. It was caught after adding unit tests that tests Posix implementation of ReadAsync and Poll APIs.
This PR also updates and fixes existing IOURing tests which were not running locally because RocksDbIOUringEnable function wasn't defined and IOUring was disabled for those tests
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9777
Test Plan: Added new unit test
Reviewed By: anand1976
Differential Revision: D35254002
Pulled By: akankshamahajan15
fbshipit-source-id: 68e80054ffb14ae25c255920ebc6548ca5f130a1
Summary:
In FilePrefetchBuffer if reads are sequential, after prefetching call ReadAsync API to prefetch data asynchronously so that in next prefetching data will be available. Data prefetched asynchronously will be readahead_size/2. It uses two buffers, one for synchronous prefetching and one for asynchronous. In case, the data is overlapping, the data is copied from both buffers to third buffer to make it continuous.
This feature is under ReadOptions::async_io and is under experimental.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9674
Test Plan:
1. Add new unit tests
2. Run **db_stress** to make sure nothing crashes.
- Normal prefetch without `async_io` ran successfully:
```
export CRASH_TEST_EXT_ARGS=" --async_io=0"
make crash_test -j
```
3. **Run Regressions**.
i) Main branch without any change for normal prefetching with async_io disabled:
```
./db_bench -db=/tmp/prefix_scan_prefetch_main -benchmarks="fillseq" -key_size=32 -value_size=512 -num=5000000 -
use_direct_io_for_flush_and_compaction=true -target_file_size_base=16777216
```
```
./db_bench -use_existing_db=true -db=/tmp/prefix_scan_prefetch_main -benchmarks="seekrandom" -key_size=32 -value_size=512 -num=5000000 -use_direct_reads=true -seek_nexts=327680 -duration=120 -ops_between_duration_checks=1
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
RocksDB: version 7.0
Date: Thu Mar 17 13:11:34 2022
CPU: 24 * Intel Core Processor (Broadwell)
CPUCache: 16384 KB
Keys: 32 bytes each (+ 0 bytes user-defined timestamp)
Values: 512 bytes each (256 bytes after compression)
Entries: 5000000
Prefix: 0 bytes
Keys per prefix: 0
RawSize: 2594.0 MB (estimated)
FileSize: 1373.3 MB (estimated)
Write rate: 0 bytes/second
Read rate: 0 ops/second
Compression: Snappy
Compression sampling rate: 0
Memtablerep: SkipListFactory
Perf Level: 1
------------------------------------------------
DB path: [/tmp/prefix_scan_prefetch_main]
seekrandom : 483618.390 micros/op 2 ops/sec; 338.9 MB/s (249 of 249 found)
```
ii) normal prefetching after changes with async_io disable:
```
./db_bench -use_existing_db=true -db=/tmp/prefix_scan_prefetch_withchange -benchmarks="seekrandom" -key_size=32 -value_size=512 -num=5000000 -use_direct_reads=true -seek_nexts=327680 -duration=120 -ops_between_duration_checks=1
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
RocksDB: version 7.0
Date: Thu Mar 17 14:11:31 2022
CPU: 24 * Intel Core Processor (Broadwell)
CPUCache: 16384 KB
Keys: 32 bytes each (+ 0 bytes user-defined timestamp)
Values: 512 bytes each (256 bytes after compression)
Entries: 5000000
Prefix: 0 bytes
Keys per prefix: 0
RawSize: 2594.0 MB (estimated)
FileSize: 1373.3 MB (estimated)
Write rate: 0 bytes/second
Read rate: 0 ops/second
Compression: Snappy
Compression sampling rate: 0
Memtablerep: SkipListFactory
Perf Level: 1
------------------------------------------------
DB path: [/tmp/prefix_scan_prefetch_withchange]
seekrandom : 471347.227 micros/op 2 ops/sec; 348.1 MB/s (255 of 255 found)
```
Reviewed By: anand1976
Differential Revision: D34731543
Pulled By: akankshamahajan15
fbshipit-source-id: 8e23aa93453d5fe3c672b9231ad582f60207937f
Summary:
RocksDB does auto-readahead for iterators on noticing more than two sequential reads for a table file if user doesn't provide readahead_size. The readahead starts at 8KB and doubles on every additional read up to max_auto_readahead_size. However at each level, if iterator moves over next file, readahead_size starts again from 8KB.
This PR introduces a new ReadOption "adaptive_readahead" which when set true will maintain readahead_size at each level. So when iterator moves from one file to another, new file's readahead_size will continue from previous file's readahead_size instead of scratch. However if reads are not sequential it will fall back to 8KB (default) with no prefetching for that block.
1. If block is found in cache but it was eligible for prefetch (block wasn't in Rocksdb's prefetch buffer), readahead_size will decrease by 8KB.
2. It maintains readahead_size for L1 - Ln levels.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9056
Test Plan:
Added new unit tests
Ran db_bench for "readseq, seekrandom, seekrandomwhilewriting, readrandom" with --adaptive_readahead=true and there was no regression if new feature is enabled.
Reviewed By: anand1976
Differential Revision: D31773640
Pulled By: akankshamahajan15
fbshipit-source-id: 7332d16258b846ae5cea773009195a5af58f8f98
