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Igor Canadi 42ea795209 Fix concurrency issue in CompactionPicker 
Summary:
I am currently working on a project that uses RocksDB. While debugging some perf issues, I came up across interesting compaction concurrency issue. Namely, I had 15 idle threads and a good comapction to do, but CompactionPicker returned "Compaction nothing to do". Here's how Internal stats looked:

    2014/08/22-08:08:04.551982 7fc7fc3f5700 ------- DUMPING STATS -------
    2014/08/22-08:08:04.552000 7fc7fc3f5700
    ** Compaction Stats [default] **
    Level   Files   Size(MB) Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) RW-Amp W-Amp Rd(MB/s) Wr(MB/s)  Rn(cnt) Rnp1(cnt) Wnp1(cnt) Wnew(cnt)  Comp(sec) Comp(cnt) Avg(sec) Stall(sec) Stall(cnt) Avg(ms)
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
      L0     7/5        353   1.0      0.0     0.0      0.0       2.3      2.3    0.0   0.0      0.0      9.4        0         0         0         0        247        46    5.359       8.53          1 8526.25
      L1     2/2         86   1.3      2.6     1.9      0.7       2.6      1.9    2.7   1.3     24.3     24.0       39        19        71        52        109        11    9.938       0.00          0    0.00
      L2    26/0        833   1.3      5.7     1.7      4.0       5.2      1.2    6.3   3.0     15.6     14.2       47       112       147        35        373        44    8.468       0.00          0    0.00
      L3    12/0        505   0.1      0.0     0.0      0.0       0.0      0.0    0.0   0.0      0.0      0.0        0         0         0         0          0         0    0.000       0.00          0    0.00
     Sum    47/7       1778   0.0      8.3     3.6      4.6      10.0      5.4    8.1   4.4     11.6     14.1       86       131       218        87        728       101    7.212       8.53          1 8526.25
     Int     0/0          0   0.0      2.4     0.8      1.6       2.7      1.2   11.5   6.1     12.0     13.6       20        43        63        20        203        23    8.845       0.00          0    0.00
    Flush(GB): accumulative 2.266, interval 0.444
    Stalls(secs): 0.000 level0_slowdown, 0.000 level0_numfiles, 8.526 memtable_compaction, 0.000 leveln_slowdown_soft, 0.000 leveln_slowdown_hard
    Stalls(count): 0 level0_slowdown, 0 level0_numfiles, 1 memtable_compaction, 0 leveln_slowdown_soft, 0 leveln_slowdown_hard

    ** DB Stats **
    Uptime(secs): 336.8 total, 60.4 interval
    Cumulative writes: 61584000 writes, 6480589 batches, 9.5 writes per batch, 1.39 GB user ingest
    Cumulative WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 GB written
    Interval writes: 11235257 writes, 1175050 batches, 9.6 writes per batch, 259.9 MB user ingest
    Interval WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 MB written

To see what happened, go here: 47b452cfcf/db/compaction_picker.cc (L430)
* The for loop started with level 1, because it has the worst score.
* PickCompactionBySize on L429 returned nullptr because all files were being compacted
* ExpandWhileOverlapping(c) returned true (because that's what it does when it gets nullptr!?)
* for loop break-ed, never trying compactions for level 2 :( :(

This bug was present at least since January. I have no idea how we didn't find this sooner.

Test Plan:
Unit testing compaction picker is hard. I tested this by running my service and observing L0->L1 and L2->L3 compactions in parallel. However, for long-term, I opened the task #4968469. @yhchiang is currently refactoring CompactionPicker, hopefully the new version will be unit-testable ;)

Here's how my compactions look like after the patch:

    2014/08/22-08:50:02.166699 7f3400ffb700 ------- DUMPING STATS -------
    2014/08/22-08:50:02.166722 7f3400ffb700
    ** Compaction Stats [default] **
    Level   Files   Size(MB) Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) RW-Amp W-Amp Rd(MB/s) Wr(MB/s)  Rn(cnt) Rnp1(cnt) Wnp1(cnt) Wnew(cnt)  Comp(sec) Comp(cnt) Avg(sec) Stall(sec) Stall(cnt) Avg(ms)
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
      L0     8/5        404   1.5      0.0     0.0      0.0       4.3      4.3    0.0   0.0      0.0      9.6        0         0         0         0        463        88    5.260       0.00          0    0.00
      L1     2/2         60   0.9      4.8     3.9      0.8       4.7      3.9    2.4   1.2     23.9     23.6       80        23       131       108        204        19   10.747       0.00          0    0.00
      L2    23/3        697   1.0     11.6     3.5      8.1      10.9      2.8    6.4   3.1     17.7     16.6       95       242       317        75        669        92    7.268       0.00          0    0.00
      L3    58/14      2207   0.3      6.2     1.6      4.6       5.9      1.3    7.4   3.6     14.6     13.9       43       121       159        38        436        36   12.106       0.00          0    0.00
     Sum    91/24      3368   0.0     22.5     9.1     13.5      25.8     12.4   11.2   6.0     13.0     14.9      218       386       607       221       1772       235    7.538       0.00          0    0.00
     Int     0/0          0   0.0      3.2     0.9      2.3       3.6      1.3   15.3   8.0     12.4     13.7       24        66        89        23        266        27    9.838       0.00          0    0.00
    Flush(GB): accumulative 4.336, interval 0.444
    Stalls(secs): 0.000 level0_slowdown, 0.000 level0_numfiles, 0.000 memtable_compaction, 0.000 leveln_slowdown_soft, 0.000 leveln_slowdown_hard
    Stalls(count): 0 level0_slowdown, 0 level0_numfiles, 0 memtable_compaction, 0 leveln_slowdown_soft, 0 leveln_slowdown_hard

    ** DB Stats **
    Uptime(secs): 577.7 total, 60.1 interval
    Cumulative writes: 116960736 writes, 11966220 batches, 9.8 writes per batch, 2.64 GB user ingest
    Cumulative WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 GB written
    Interval writes: 11643735 writes, 1206136 batches, 9.7 writes per batch, 269.2 MB user ingest
    Interval WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 MB written

Yay for concurrent L0->L1 and L2->L3 compactions!

Reviewers: sdong, yhchiang, ljin

Reviewed By: yhchiang

Subscribers: yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D22305
2014-08-22 11:32:40 -07:00
build_tools Changes to support unity build: 2014-08-11 13:22:47 -04:00
coverage Disable the html-based coverage report by default 2014-02-06 12:58:13 -08:00
db Fix concurrency issue in CompactionPicker 2014-08-22 11:32:40 -07:00
doc Remove seek compaction 2014-06-20 10:23:02 +02:00
examples Make it easier to start using RocksDB 2014-05-10 10:49:33 -07:00
hdfs hdfs cleanup and compile test against CDH 4.4. 2014-05-20 17:22:12 -04:00
helpers/memenv Expose in memory Env to the world 2014-04-14 12:28:15 -07:00
include Improve Options sanitization and add MmapReadRequired() to TableFactory 2014-08-20 15:53:39 -07:00
java [Java] Add purgeOldBackups API 2014-08-14 10:58:09 -07:00
linters allow lambda function syntax in cpplint 2014-02-20 12:47:05 -08:00
port generic rate limiter 2014-07-08 11:41:57 -07:00
table Fix compilation issue on OSX 2014-08-21 18:11:33 -07:00
third-party/rapidjson Fix a rapidjson compile error in mac. 2014-06-23 17:09:24 -06:00
tools fix_sst_dump_for_old_sst_format 2014-08-14 09:59:41 -07:00
util Eliminate VersionSet memory leak 2014-08-20 13:52:03 -07:00
utilities Optimize storage parameters for spatialDB 2014-08-20 11:14:01 -07:00
.arcconfig Improve/fix bugs for the cpp linter 2014-02-13 17:48:11 -08:00
.clang-format A script that automatically reformat affected lines 2014-01-14 12:21:24 -08:00
.gitignore Changes to support unity build: 2014-08-11 13:22:47 -04:00
.travis.yml Turn off travis notifications 2014-05-14 16:08:02 -07:00
CONTRIBUTING.md facebook accounts are not required for CLA signers 2014-07-08 05:57:54 -04:00
HISTORY.md WriteBatchWithIndex: a wrapper of WriteBatch, with a searchable index 2014-08-18 16:37:38 -07:00
INSTALL.md specify the command to install build_tools/mac-install-gflags.sh file in doc 2014-06-17 17:03:21 -05:00
LICENSE Fix copyright year 2014-03-12 12:06:58 -07:00
Makefile WriteBatchWithIndex: a wrapper of WriteBatch, with a searchable index 2014-08-18 16:37:38 -07:00
PATENTS Fix the patent format 2013-10-16 15:37:32 -07:00
README.md Update README.md 2014-06-23 15:58:54 -07:00
ROCKSDB_LITE.md RocksDBLite 2014-04-15 13:39:26 -07:00

README.md

RocksDB: A Persistent Key-Value Store for Flash and RAM Storage

Build Status

RocksDB is developed and maintained by Facebook Database Engineering Team. It is built on on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)

This code is a library that forms the core building block for a fast key value server, especially suited for storing data on flash drives. It has a Log-Structured-Merge-Database (LSM) design with flexible tradeoffs between Write-Amplification-Factor (WAF), Read-Amplification-Factor (RAF) and Space-Amplification-Factor (SAF). It has multi-threaded compactions, making it specially suitable for storing multiple terabytes of data in a single database.

Start with example usage here: https://github.com/facebook/rocksdb/tree/master/examples

See the github wiki for more explanation.

The public interface is in include/. Callers should not include or rely on the details of any other header files in this package. Those internal APIs may be changed without warning.

Design discussions are conducted in https://www.facebook.com/groups/rocksdb.dev/