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rocksdb/cache/cache_bench_tool.cc
Peter Dillinger 86a1e3e0e7 Derive cache keys from SST unique IDs (#10394) 
Summary:
... so that cache keys can be derived from DB manifest data
before reading the file from storage--so that every part of the file
can potentially go in a persistent cache.

See updated comments in cache_key.cc for technical details. Importantly,
the new cache key encoding uses some fancy but efficient math to pack
data into the cache key without depending on the sizes of the various
pieces. This simplifies some existing code creating cache keys, like
cache warming before the file size is known.

This should provide us an essentially permanent mapping between SST
unique IDs and base cache keys, with the ability to "upgrade" SST
unique IDs (and thus cache keys) with new SST format_versions.

These cache keys are of similar, perhaps indistinguishable quality to
the previous generation. Before this change (see "corrected" days
between collision):

```
./cache_bench -stress_cache_key -sck_keep_bits=43
18 collisions after 2 x 90 days, est 10 days between (1.15292e+19 corrected)
```

After this change (keep 43 bits, up through 50, to validate "trajectory"
is ok on "corrected" days between collision):
```
19 collisions after 3 x 90 days, est 14.2105 days between (1.63836e+19 corrected)
16 collisions after 5 x 90 days, est 28.125 days between (1.6213e+19 corrected)
15 collisions after 7 x 90 days, est 42 days between (1.21057e+19 corrected)
15 collisions after 17 x 90 days, est 102 days between (1.46997e+19 corrected)
15 collisions after 49 x 90 days, est 294 days between (2.11849e+19 corrected)
15 collisions after 62 x 90 days, est 372 days between (1.34027e+19 corrected)
15 collisions after 53 x 90 days, est 318 days between (5.72858e+18 corrected)
15 collisions after 309 x 90 days, est 1854 days between (1.66994e+19 corrected)
```

However, the change does modify (probably weaken) the "guaranteed unique" promise from this

> SST files generated in a single process are guaranteed to have unique cache keys, unless/until number session ids * max file number = 2**86

to this (see https://github.com/facebook/rocksdb/issues/10388)

> With the DB id limitation, we only have nice guaranteed unique cache keys for files generated in a single process until biggest session_id_counter and offset_in_file reach combined 64 bits

I don't think this is a practical concern, though.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10394

Test Plan: unit tests updated, see simulation results above

Reviewed By: jay-zhuang

Differential Revision: D38667529

Pulled By: pdillinger

fbshipit-source-id: 49af3fe7f47e5b61162809a78b76c769fd519fba
2022-08-12 13:49:49 -07:00

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#include "cache_key.h"
#ifdef GFLAGS
#include <cinttypes>
#include <cstddef>
#include <cstdio>
#include <limits>
#include <memory>
#include <set>
#include <sstream>
#include "cache/clock_cache.h"
#include "cache/fast_lru_cache.h"
#include "db/db_impl/db_impl.h"
#include "monitoring/histogram.h"
#include "port/port.h"
#include "rocksdb/cache.h"
#include "rocksdb/convenience.h"
#include "rocksdb/db.h"
#include "rocksdb/env.h"
#include "rocksdb/secondary_cache.h"
#include "rocksdb/system_clock.h"
#include "rocksdb/table_properties.h"
#include "table/block_based/block_based_table_reader.h"
#include "table/block_based/cachable_entry.h"
#include "util/coding.h"
#include "util/distributed_mutex.h"
#include "util/gflags_compat.h"
#include "util/hash.h"
#include "util/mutexlock.h"
#include "util/random.h"
#include "util/stop_watch.h"
#include "util/string_util.h"
using GFLAGS_NAMESPACE::ParseCommandLineFlags;
static constexpr uint32_t KiB = uint32_t{1} << 10;
static constexpr uint32_t MiB = KiB << 10;
static constexpr uint64_t GiB = MiB << 10;
DEFINE_uint32(threads, 16, "Number of concurrent threads to run.");
DEFINE_uint64(cache_size, 1 * GiB,
"Number of bytes to use as a cache of uncompressed data.");
DEFINE_uint32(num_shard_bits, 6, "shard_bits.");
DEFINE_double(resident_ratio, 0.25,
"Ratio of keys fitting in cache to keyspace.");
DEFINE_uint64(ops_per_thread, 2000000U, "Number of operations per thread.");
DEFINE_uint32(value_bytes, 8 * KiB, "Size of each value added.");
DEFINE_uint32(skew, 5, "Degree of skew in key selection");
DEFINE_bool(populate_cache, true, "Populate cache before operations");
DEFINE_uint32(lookup_insert_percent, 87,
"Ratio of lookup (+ insert on not found) to total workload "
"(expressed as a percentage)");
DEFINE_uint32(insert_percent, 2,
"Ratio of insert to total workload (expressed as a percentage)");
DEFINE_uint32(lookup_percent, 10,
"Ratio of lookup to total workload (expressed as a percentage)");
DEFINE_uint32(erase_percent, 1,
"Ratio of erase to total workload (expressed as a percentage)");
DEFINE_bool(gather_stats, false,
"Whether to periodically simulate gathering block cache stats, "
"using one more thread.");
DEFINE_uint32(
gather_stats_sleep_ms, 1000,
"How many milliseconds to sleep between each gathering of stats.");
DEFINE_uint32(gather_stats_entries_per_lock, 256,
"For Cache::ApplyToAllEntries");
DEFINE_bool(skewed, false, "If true, skew the key access distribution");
DEFINE_bool(lean, false,
"If true, no additional computation is performed besides cache "
"operations.");
#ifndef ROCKSDB_LITE
DEFINE_string(secondary_cache_uri, "",
"Full URI for creating a custom secondary cache object");
static class std::shared_ptr<ROCKSDB_NAMESPACE::SecondaryCache> secondary_cache;
#endif // ROCKSDB_LITE
DEFINE_string(cache_type, "lru_cache", "Type of block cache.");
// ## BEGIN stress_cache_key sub-tool options ##
// See class StressCacheKey below.
DEFINE_bool(stress_cache_key, false,
"If true, run cache key stress test instead");
DEFINE_uint32(
sck_files_per_day, 2500000,
"(-stress_cache_key) Simulated files generated per simulated day");
// NOTE: Giving each run a specified lifetime, rather than e.g. "until
// first collision" ensures equal skew from start-up, when collisions are
// less likely.
DEFINE_uint32(sck_days_per_run, 90,
"(-stress_cache_key) Number of days to simulate in each run");
// NOTE: The number of observed collisions directly affects the relative
// accuracy of the predicted probabilities. 15 observations should be well
// within factor-of-2 accuracy.
DEFINE_uint32(
sck_min_collision, 15,
"(-stress_cache_key) Keep running until this many collisions seen");
// sck_file_size_mb can be thought of as average file size. The simulation is
// not precise enough to care about the distribution of file sizes; other
// simulations (https://github.com/pdillinger/unique_id/tree/main/monte_carlo)
// indicate the distribution only makes a small difference (e.g. < 2x factor)
DEFINE_uint32(
sck_file_size_mb, 32,
"(-stress_cache_key) Simulated file size in MiB, for accounting purposes");
DEFINE_uint32(sck_reopen_nfiles, 100,
"(-stress_cache_key) Simulate DB re-open average every n files");
DEFINE_uint32(sck_newdb_nreopen, 1000,
"(-stress_cache_key) Simulate new DB average every n re-opens");
DEFINE_uint32(sck_restarts_per_day, 24,
"(-stress_cache_key) Average simulated process restarts per day "
"(across DBs)");
DEFINE_uint32(
sck_db_count, 100,
"(-stress_cache_key) Parallel DBs in simulation sharing a block cache");
DEFINE_uint32(
sck_table_bits, 20,
"(-stress_cache_key) Log2 number of tracked (live) files (across DBs)");
// sck_keep_bits being well below full 128 bits amplifies the collision
// probability so that the true probability can be estimated through observed
// collisions. (More explanation below.)
DEFINE_uint32(
sck_keep_bits, 50,
"(-stress_cache_key) Number of bits to keep from each cache key (<= 64)");
// sck_randomize is used to validate whether cache key is performing "better
// than random." Even with this setting, file offsets are not randomized.
DEFINE_bool(sck_randomize, false,
"(-stress_cache_key) Randomize (hash) cache key");
// See https://github.com/facebook/rocksdb/pull/9058
DEFINE_bool(sck_footer_unique_id, false,
"(-stress_cache_key) Simulate using proposed footer unique id");
// ## END stress_cache_key sub-tool options ##
namespace ROCKSDB_NAMESPACE {
class CacheBench;
namespace {
// State shared by all concurrent executions of the same benchmark.
class SharedState {
public:
explicit SharedState(CacheBench* cache_bench)
: cv_(&mu_),
num_initialized_(0),
start_(false),
num_done_(0),
cache_bench_(cache_bench) {}
~SharedState() {}
port::Mutex* GetMutex() { return &mu_; }
port::CondVar* GetCondVar() { return &cv_; }
CacheBench* GetCacheBench() const { return cache_bench_; }
void IncInitialized() { num_initialized_++; }
void IncDone() { num_done_++; }
bool AllInitialized() const { return num_initialized_ >= FLAGS_threads; }
bool AllDone() const { return num_done_ >= FLAGS_threads; }
void SetStart() { start_ = true; }
bool Started() const { return start_; }
private:
port::Mutex mu_;
port::CondVar cv_;
uint64_t num_initialized_;
bool start_;
uint64_t num_done_;
CacheBench* cache_bench_;
};
// Per-thread state for concurrent executions of the same benchmark.
struct ThreadState {
uint32_t tid;
Random64 rnd;
SharedState* shared;
HistogramImpl latency_ns_hist;
uint64_t duration_us = 0;
ThreadState(uint32_t index, SharedState* _shared)
: tid(index), rnd(1000 + index), shared(_shared) {}
};
struct KeyGen {
char key_data[27];
Slice GetRand(Random64& rnd, uint64_t max_key, int max_log) {
uint64_t key = 0;
if (!FLAGS_skewed) {
uint64_t raw = rnd.Next();
// Skew according to setting
for (uint32_t i = 0; i < FLAGS_skew; ++i) {
raw = std::min(raw, rnd.Next());
}
key = FastRange64(raw, max_key);
} else {
key = rnd.Skewed(max_log);
if (key > max_key) {
key -= max_key;
}
}
// Variable size and alignment
size_t off = key % 8;
key_data[0] = char{42};
EncodeFixed64(key_data + 1, key);
key_data[9] = char{11};
EncodeFixed64(key_data + 10, key);
key_data[18] = char{4};
EncodeFixed64(key_data + 19, key);
assert(27 >= kCacheKeySize);
return Slice(&key_data[off], kCacheKeySize);
}
};
char* createValue(Random64& rnd) {
char* rv = new char[FLAGS_value_bytes];
// Fill with some filler data, and take some CPU time
for (uint32_t i = 0; i < FLAGS_value_bytes; i += 8) {
EncodeFixed64(rv + i, rnd.Next());
}
return rv;
}
// Callbacks for secondary cache
size_t SizeFn(void* /*obj*/) { return FLAGS_value_bytes; }
Status SaveToFn(void* obj, size_t /*offset*/, size_t size, void* out) {
memcpy(out, obj, size);
return Status::OK();
}
// Different deleters to simulate using deleter to gather
// stats on the code origin and kind of cache entries.
void deleter1(const Slice& /*key*/, void* value) {
delete[] static_cast<char*>(value);
}
void deleter2(const Slice& /*key*/, void* value) {
delete[] static_cast<char*>(value);
}
void deleter3(const Slice& /*key*/, void* value) {
delete[] static_cast<char*>(value);
}
Cache::CacheItemHelper helper1(SizeFn, SaveToFn, deleter1);
Cache::CacheItemHelper helper2(SizeFn, SaveToFn, deleter2);
Cache::CacheItemHelper helper3(SizeFn, SaveToFn, deleter3);
} // namespace
class CacheBench {
static constexpr uint64_t kHundredthUint64 =
std::numeric_limits<uint64_t>::max() / 100U;
public:
CacheBench()
: max_key_(static_cast<uint64_t>(FLAGS_cache_size / FLAGS_resident_ratio /
FLAGS_value_bytes)),
lookup_insert_threshold_(kHundredthUint64 *
FLAGS_lookup_insert_percent),
insert_threshold_(lookup_insert_threshold_ +
kHundredthUint64 * FLAGS_insert_percent),
lookup_threshold_(insert_threshold_ +
kHundredthUint64 * FLAGS_lookup_percent),
erase_threshold_(lookup_threshold_ +
kHundredthUint64 * FLAGS_erase_percent),
skewed_(FLAGS_skewed) {
if (erase_threshold_ != 100U * kHundredthUint64) {
fprintf(stderr, "Percentages must add to 100.\n");
exit(1);
}
max_log_ = 0;
if (skewed_) {
uint64_t max_key = max_key_;
while (max_key >>= 1) max_log_++;
if (max_key > (static_cast<uint64_t>(1) << max_log_)) max_log_++;
}
if (FLAGS_cache_type == "clock_cache") {
cache_ = ExperimentalNewClockCache(
FLAGS_cache_size, FLAGS_value_bytes, FLAGS_num_shard_bits,
false /*strict_capacity_limit*/, kDefaultCacheMetadataChargePolicy);
if (!cache_) {
fprintf(stderr, "Clock cache not supported.\n");
exit(1);
}
} else if (FLAGS_cache_type == "fast_lru_cache") {
cache_ = NewFastLRUCache(
FLAGS_cache_size, FLAGS_value_bytes, FLAGS_num_shard_bits,
false /*strict_capacity_limit*/, kDefaultCacheMetadataChargePolicy);
} else if (FLAGS_cache_type == "lru_cache") {
LRUCacheOptions opts(FLAGS_cache_size, FLAGS_num_shard_bits, false, 0.5);
#ifndef ROCKSDB_LITE
if (!FLAGS_secondary_cache_uri.empty()) {
Status s = SecondaryCache::CreateFromString(
ConfigOptions(), FLAGS_secondary_cache_uri, &secondary_cache);
if (secondary_cache == nullptr) {
fprintf(
stderr,
"No secondary cache registered matching string: %s status=%s\n",
FLAGS_secondary_cache_uri.c_str(), s.ToString().c_str());
exit(1);
}
opts.secondary_cache = secondary_cache;
}
#endif // ROCKSDB_LITE
cache_ = NewLRUCache(opts);
} else {
fprintf(stderr, "Cache type not supported.");
exit(1);
}
}
~CacheBench() {}
void PopulateCache() {
Random64 rnd(1);
KeyGen keygen;
for (uint64_t i = 0; i < 2 * FLAGS_cache_size; i += FLAGS_value_bytes) {
Status s = cache_->Insert(keygen.GetRand(rnd, max_key_, max_log_),
createValue(rnd), &helper1, FLAGS_value_bytes);
assert(s.ok());
}
}
bool Run() {
const auto clock = SystemClock::Default().get();
PrintEnv();
SharedState shared(this);
std::vector<std::unique_ptr<ThreadState> > threads(FLAGS_threads);
for (uint32_t i = 0; i < FLAGS_threads; i++) {
threads[i].reset(new ThreadState(i, &shared));
std::thread(ThreadBody, threads[i].get()).detach();
}
HistogramImpl stats_hist;
std::string stats_report;
std::thread stats_thread(StatsBody, &shared, &stats_hist, &stats_report);
uint64_t start_time;
{
MutexLock l(shared.GetMutex());
while (!shared.AllInitialized()) {
shared.GetCondVar()->Wait();
}
// Record start time
start_time = clock->NowMicros();
// Start all threads
shared.SetStart();
shared.GetCondVar()->SignalAll();
// Wait threads to complete
while (!shared.AllDone()) {
shared.GetCondVar()->Wait();
}
}
// Stats gathering is considered background work. This time measurement
// is for foreground work, and not really ideal for that. See below.
uint64_t end_time = clock->NowMicros();
stats_thread.join();
// Wall clock time - includes idle time if threads
// finish at different times (not ideal).
double elapsed_secs = static_cast<double>(end_time - start_time) * 1e-6;
uint32_t ops_per_sec = static_cast<uint32_t>(
1.0 * FLAGS_threads * FLAGS_ops_per_thread / elapsed_secs);
printf("Complete in %.3f s; Rough parallel ops/sec = %u\n", elapsed_secs,
ops_per_sec);
// Total time in each thread (more accurate throughput measure)
elapsed_secs = 0;
for (uint32_t i = 0; i < FLAGS_threads; i++) {
elapsed_secs += threads[i]->duration_us * 1e-6;
}
ops_per_sec = static_cast<uint32_t>(1.0 * FLAGS_threads *
FLAGS_ops_per_thread / elapsed_secs);
printf("Thread ops/sec = %u\n", ops_per_sec);
printf("\nOperation latency (ns):\n");
HistogramImpl combined;
for (uint32_t i = 0; i < FLAGS_threads; i++) {
combined.Merge(threads[i]->latency_ns_hist);
}
printf("%s", combined.ToString().c_str());
if (FLAGS_gather_stats) {
printf("\nGather stats latency (us):\n");
printf("%s", stats_hist.ToString().c_str());
}
printf("\n%s", stats_report.c_str());
return true;
}
private:
std::shared_ptr<Cache> cache_;
const uint64_t max_key_;
// Cumulative thresholds in the space of a random uint64_t
const uint64_t lookup_insert_threshold_;
const uint64_t insert_threshold_;
const uint64_t lookup_threshold_;
const uint64_t erase_threshold_;
const bool skewed_;
int max_log_;
// A benchmark version of gathering stats on an active block cache by
// iterating over it. The primary purpose is to measure the impact of
// gathering stats with ApplyToAllEntries on throughput- and
// latency-sensitive Cache users. Performance of stats gathering is
// also reported. The last set of gathered stats is also reported, for
// manual sanity checking for logical errors or other unexpected
// behavior of cache_bench or the underlying Cache.
static void StatsBody(SharedState* shared, HistogramImpl* stats_hist,
std::string* stats_report) {
if (!FLAGS_gather_stats) {
return;
}
const auto clock = SystemClock::Default().get();
uint64_t total_key_size = 0;
uint64_t total_charge = 0;
uint64_t total_entry_count = 0;
std::set<Cache::DeleterFn> deleters;
StopWatchNano timer(clock);
for (;;) {
uint64_t time;
time = clock->NowMicros();
uint64_t deadline = time + uint64_t{FLAGS_gather_stats_sleep_ms} * 1000;
{
MutexLock l(shared->GetMutex());
for (;;) {
if (shared->AllDone()) {
std::ostringstream ostr;
ostr << "Most recent cache entry stats:\n"
<< "Number of entries: " << total_entry_count << "\n"
<< "Total charge: " << BytesToHumanString(total_charge) << "\n"
<< "Average key size: "
<< (1.0 * total_key_size / total_entry_count) << "\n"
<< "Average charge: "
<< BytesToHumanString(static_cast<uint64_t>(
1.0 * total_charge / total_entry_count))
<< "\n"
<< "Unique deleters: " << deleters.size() << "\n";
*stats_report = ostr.str();
return;
}
if (clock->NowMicros() >= deadline) {
break;
}
uint64_t diff = deadline - std::min(clock->NowMicros(), deadline);
shared->GetCondVar()->TimedWait(diff + 1);
}
}
// Now gather stats, outside of mutex
total_key_size = 0;
total_charge = 0;
total_entry_count = 0;
deleters.clear();
auto fn = [&](const Slice& key, void* /*value*/, size_t charge,
Cache::DeleterFn deleter) {
total_key_size += key.size();
total_charge += charge;
++total_entry_count;
// Something slightly more expensive as in (future) stats by category
deleters.insert(deleter);
};
timer.Start();
Cache::ApplyToAllEntriesOptions opts;
opts.average_entries_per_lock = FLAGS_gather_stats_entries_per_lock;
shared->GetCacheBench()->cache_->ApplyToAllEntries(fn, opts);
stats_hist->Add(timer.ElapsedNanos() / 1000);
}
}
static void ThreadBody(ThreadState* thread) {
SharedState* shared = thread->shared;
{
MutexLock l(shared->GetMutex());
shared->IncInitialized();
if (shared->AllInitialized()) {
shared->GetCondVar()->SignalAll();
}
while (!shared->Started()) {
shared->GetCondVar()->Wait();
}
}
thread->shared->GetCacheBench()->OperateCache(thread);
{
MutexLock l(shared->GetMutex());
shared->IncDone();
if (shared->AllDone()) {
shared->GetCondVar()->SignalAll();
}
}
}
void OperateCache(ThreadState* thread) {
// To use looked-up values
uint64_t result = 0;
// To hold handles for a non-trivial amount of time
Cache::Handle* handle = nullptr;
KeyGen gen;
const auto clock = SystemClock::Default().get();
uint64_t start_time = clock->NowMicros();
StopWatchNano timer(clock);
for (uint64_t i = 0; i < FLAGS_ops_per_thread; i++) {
Slice key = gen.GetRand(thread->rnd, max_key_, max_log_);
uint64_t random_op = thread->rnd.Next();
Cache::CreateCallback create_cb = [](const void* buf, size_t size,
void** out_obj,
size_t* charge) -> Status {
*out_obj = reinterpret_cast<void*>(new char[size]);
memcpy(*out_obj, buf, size);
*charge = size;
return Status::OK();
};
timer.Start();
if (random_op < lookup_insert_threshold_) {
if (handle) {
cache_->Release(handle);
handle = nullptr;
}
// do lookup
handle = cache_->Lookup(key, &helper2, create_cb, Cache::Priority::LOW,
true);
if (handle) {
if (!FLAGS_lean) {
// do something with the data
result += NPHash64(static_cast<char*>(cache_->Value(handle)),
FLAGS_value_bytes);
}
} else {
// do insert
Status s = cache_->Insert(key, createValue(thread->rnd), &helper2,
FLAGS_value_bytes, &handle);
assert(s.ok());
}
} else if (random_op < insert_threshold_) {
if (handle) {
cache_->Release(handle);
handle = nullptr;
}
// do insert
Status s = cache_->Insert(key, createValue(thread->rnd), &helper3,
FLAGS_value_bytes, &handle);
assert(s.ok());
} else if (random_op < lookup_threshold_) {
if (handle) {
cache_->Release(handle);
handle = nullptr;
}
// do lookup
handle = cache_->Lookup(key, &helper2, create_cb, Cache::Priority::LOW,
true);
if (handle) {
if (!FLAGS_lean) {
// do something with the data
result += NPHash64(static_cast<char*>(cache_->Value(handle)),
FLAGS_value_bytes);
}
}
} else if (random_op < erase_threshold_) {
// do erase
cache_->Erase(key);
} else {
// Should be extremely unlikely (noop)
assert(random_op >= kHundredthUint64 * 100U);
}
thread->latency_ns_hist.Add(timer.ElapsedNanos());
}
if (handle) {
cache_->Release(handle);
handle = nullptr;
}
// Ensure computations on `result` are not optimized away.
if (result == 1) {
printf("You are extremely unlucky(2). Try again.\n");
exit(1);
}
thread->duration_us = clock->NowMicros() - start_time;
}
void PrintEnv() const {
#if defined(__GNUC__) && !defined(__OPTIMIZE__)
printf(
"WARNING: Optimization is disabled: benchmarks unnecessarily slow\n");
#endif
#ifndef NDEBUG
printf("WARNING: Assertions are enabled; benchmarks unnecessarily slow\n");
#endif
printf("RocksDB version : %d.%d\n", kMajorVersion, kMinorVersion);
printf("DMutex impl name : %s\n", DMutex::kName());
printf("Number of threads : %u\n", FLAGS_threads);
printf("Ops per thread : %" PRIu64 "\n", FLAGS_ops_per_thread);
printf("Cache size : %s\n",
BytesToHumanString(FLAGS_cache_size).c_str());
printf("Num shard bits : %u\n", FLAGS_num_shard_bits);
printf("Max key : %" PRIu64 "\n", max_key_);
printf("Resident ratio : %g\n", FLAGS_resident_ratio);
printf("Skew degree : %u\n", FLAGS_skew);
printf("Populate cache : %d\n", int{FLAGS_populate_cache});
printf("Lookup+Insert pct : %u%%\n", FLAGS_lookup_insert_percent);
printf("Insert percentage : %u%%\n", FLAGS_insert_percent);
printf("Lookup percentage : %u%%\n", FLAGS_lookup_percent);
printf("Erase percentage : %u%%\n", FLAGS_erase_percent);
std::ostringstream stats;
if (FLAGS_gather_stats) {
stats << "enabled (" << FLAGS_gather_stats_sleep_ms << "ms, "
<< FLAGS_gather_stats_entries_per_lock << "/lock)";
} else {
stats << "disabled";
}
printf("Gather stats : %s\n", stats.str().c_str());
printf("----------------------------\n");
}
};
// cache_bench -stress_cache_key is an independent embedded tool for
// estimating the probability of CacheKey collisions through simulation.
// At a high level, it simulates generating SST files over many months,
// keeping them in the DB and/or cache for some lifetime while staying
// under resource caps, and checking for any cache key collisions that
// arise among the set of live files. For efficient simulation, we make
// some simplifying "pessimistic" assumptions (that only increase the
// chance of the simulation reporting a collision relative to the chance
// of collision in practice):
// * Every generated file has a cache entry for every byte offset in the
// file (contiguous range of cache keys)
// * All of every file is cached for its entire lifetime. (Here "lifetime"
// is technically the union of DB and Cache lifetime, though we only
// model a generous DB lifetime, where space usage is always maximized.
// In a effective Cache, lifetime in cache can only substantially exceed
// lifetime in DB if there is little cache activity; cache activity is
// required to hit cache key collisions.)
//
// It would be possible to track an exact set of cache key ranges for the
// set of live files, but we would have no hope of observing collisions
// (overlap in live files) in our simulation. We need to employ some way
// of amplifying collision probability that allows us to predict the real
// collision probability by extrapolation from observed collisions. Our
// basic approach is to reduce each cache key range down to some smaller
// number of bits, and limiting to bits that are shared over the whole
// range. Now we can observe collisions using a set of smaller stripped-down
// (reduced) cache keys. Let's do some case analysis to understand why this
// works:
// * No collision in reduced key - because the reduction is a pure function
// this implies no collision in the full keys
// * Collision detected between two reduced keys - either
// * The reduction has dropped some structured uniqueness info (from one of
// session counter or file number; file offsets are never materialized here).
// This can only artificially inflate the observed and extrapolated collision
// probabilities. We only have to worry about this in designing the reduction.
// * The reduction has preserved all the structured uniqueness in the cache
// key, which means either
// * REJECTED: We have a uniqueness bug in generating cache keys, where
// structured uniqueness info should have been different but isn't. In such a
// case, increasing by 1 the number of bits kept after reduction would not
// reduce observed probabilities by half. (In our observations, the
// probabilities are reduced approximately by half.)
// * ACCEPTED: The lost unstructured uniqueness in the key determines the
// probability that an observed collision would imply an overlap in ranges.
// In short, dropping n bits from key would increase collision probability by
// 2**n, assuming those n bits have full entropy in unstructured uniqueness.
//
// But we also have to account for the key ranges based on file size. If file
// sizes are roughly 2**b offsets, using XOR in 128-bit cache keys for
// "ranges", we know from other simulations (see
// https://github.com/pdillinger/unique_id/) that that's roughly equivalent to
// (less than 2x higher collision probability) using a cache key of size
// 128 - b bits for the whole file. (This is the only place we make an
// "optimistic" assumption, which is more than offset by the real
// implementation stripping off 2 lower bits from block byte offsets for cache
// keys. The simulation assumes byte offsets, which is net pessimistic.)
//
// So to accept the extrapolation as valid, we need to be confident that all
// "lost" bits, excluding those covered by file offset, are full entropy.
// Recall that we have assumed (verifiably, safely) that other structured data
// (file number and session counter) are kept, not lost. Based on the
// implementation comments for OffsetableCacheKey, the only potential hole here
// is that we only have ~103 bits of entropy in "all new" session IDs, and in
// extreme cases, there might be only 1 DB ID. However, because the upper ~39
// bits of session ID are hashed, the combination of file number and file
// offset only has to add to 25 bits (or more) to ensure full entropy in
// unstructured uniqueness lost in the reduction. Typical file size of 32MB
// suffices (at least for simulation purposes where we assume each file offset
// occupies a cache key).
//
// Example results in comments on OffsetableCacheKey.
class StressCacheKey {
public:
void Run() {
if (FLAGS_sck_footer_unique_id) {
// Proposed footer unique IDs are DB-independent and session-independent
// (but process-dependent) which is most easily simulated here by
// assuming 1 DB and (later below) no session resets without process
// reset.
FLAGS_sck_db_count = 1;
}
// Describe the simulated workload
uint64_t mb_per_day =
uint64_t{FLAGS_sck_files_per_day} * FLAGS_sck_file_size_mb;
printf("Total cache or DBs size: %gTiB Writing %g MiB/s or %gTiB/day\n",
FLAGS_sck_file_size_mb / 1024.0 / 1024.0 *
std::pow(2.0, FLAGS_sck_table_bits),
mb_per_day / 86400.0, mb_per_day / 1024.0 / 1024.0);
// For extrapolating probability of any collisions from a number of
// observed collisions
multiplier_ = std::pow(2.0, 128 - FLAGS_sck_keep_bits) /
(FLAGS_sck_file_size_mb * 1024.0 * 1024.0);
printf(
"Multiply by %g to correct for simulation losses (but still assume "
"whole file cached)\n",
multiplier_);
restart_nfiles_ = FLAGS_sck_files_per_day / FLAGS_sck_restarts_per_day;
double without_ejection =
std::pow(1.414214, FLAGS_sck_keep_bits) / FLAGS_sck_files_per_day;
// This should be a lower bound for -sck_randomize, usually a terribly
// rough lower bound.
// If observation is worse than this, then something has gone wrong.
printf(
"Without ejection, expect random collision after %g days (%g "
"corrected)\n",
without_ejection, without_ejection * multiplier_);
double with_full_table =
std::pow(2.0, FLAGS_sck_keep_bits - FLAGS_sck_table_bits) /
FLAGS_sck_files_per_day;
// This is an alternate lower bound for -sck_randomize, usually pretty
// accurate. Our cache keys should usually perform "better than random"
// but always no worse. (If observation is substantially worse than this,
// then something has gone wrong.)
printf(
"With ejection and full table, expect random collision after %g "
"days (%g corrected)\n",
with_full_table, with_full_table * multiplier_);
collisions_ = 0;
// Run until sufficient number of observed collisions.
for (int i = 1; collisions_ < FLAGS_sck_min_collision; i++) {
RunOnce();
if (collisions_ == 0) {
printf(
"No collisions after %d x %u days "
" \n",
i, FLAGS_sck_days_per_run);
} else {
double est = 1.0 * i * FLAGS_sck_days_per_run / collisions_;
printf("%" PRIu64
" collisions after %d x %u days, est %g days between (%g "
"corrected) \n",
collisions_, i, FLAGS_sck_days_per_run, est, est * multiplier_);
}
}
}
void RunOnce() {
// Re-initialized simulated state
const size_t db_count = std::max(size_t{FLAGS_sck_db_count}, size_t{1});
dbs_.reset(new TableProperties[db_count]{});
const size_t table_mask = (size_t{1} << FLAGS_sck_table_bits) - 1;
table_.reset(new uint64_t[table_mask + 1]{});
if (FLAGS_sck_keep_bits > 64) {
FLAGS_sck_keep_bits = 64;
}
// Details of which bits are dropped in reduction
uint32_t shift_away = 64 - FLAGS_sck_keep_bits;
// Shift away fewer potential file number bits (b) than potential
// session counter bits (a).
uint32_t shift_away_b = shift_away / 3;
uint32_t shift_away_a = shift_away - shift_away_b;
process_count_ = 0;
session_count_ = 0;
newdb_count_ = 0;
ResetProcess(/*newdbs*/ true);
Random64 r{std::random_device{}()};
uint64_t max_file_count =
uint64_t{FLAGS_sck_files_per_day} * FLAGS_sck_days_per_run;
uint32_t report_count = 0;
uint32_t collisions_this_run = 0;
size_t db_i = 0;
for (uint64_t file_count = 1; file_count <= max_file_count;
++file_count, ++db_i) {
// Round-robin through DBs (this faster than %)
if (db_i >= db_count) {
db_i = 0;
}
// Any other periodic actions before simulating next file
if (!FLAGS_sck_footer_unique_id && r.OneIn(FLAGS_sck_reopen_nfiles)) {
ResetSession(db_i, /*newdb*/ r.OneIn(FLAGS_sck_newdb_nreopen));
} else if (r.OneIn(restart_nfiles_)) {
ResetProcess(/*newdbs*/ false);
}
// Simulate next file
OffsetableCacheKey ock;
dbs_[db_i].orig_file_number += 1;
// skip some file numbers for other file kinds, except in footer unique
// ID, orig_file_number here tracks process-wide generated SST file
// count.
if (!FLAGS_sck_footer_unique_id) {
dbs_[db_i].orig_file_number += (r.Next() & 3);
}
bool is_stable;
BlockBasedTable::SetupBaseCacheKey(&dbs_[db_i], /* ignored */ "",
/* ignored */ 42, &ock, &is_stable);
assert(is_stable);
// Get a representative cache key, which later we analytically generalize
// to a range.
CacheKey ck = ock.WithOffset(0);
uint64_t reduced_key;
if (FLAGS_sck_randomize) {
reduced_key = GetSliceHash64(ck.AsSlice()) >> shift_away;
} else if (FLAGS_sck_footer_unique_id) {
// Special case: keep only file number, not session counter
reduced_key = DecodeFixed64(ck.AsSlice().data()) >> shift_away;
} else {
// Try to keep file number and session counter (shift away other bits)
uint32_t a = DecodeFixed32(ck.AsSlice().data()) << shift_away_a;
uint32_t b = DecodeFixed32(ck.AsSlice().data() + 4) >> shift_away_b;
reduced_key = (uint64_t{a} << 32) + b;
}
if (reduced_key == 0) {
// Unlikely, but we need to exclude tracking this value because we
// use it to mean "empty" in table. This case is OK as long as we
// don't hit it often.
printf("Hit Zero! \n");
file_count--;
continue;
}
uint64_t h =
NPHash64(reinterpret_cast<char*>(&reduced_key), sizeof(reduced_key));
// Skew expected lifetimes, for high variance (super-Poisson) variance
// in actual lifetimes.
size_t pos =
std::min(Lower32of64(h) & table_mask, Upper32of64(h) & table_mask);
if (table_[pos] == reduced_key) {
collisions_this_run++;
// Our goal is to predict probability of no collisions, not expected
// number of collisions. To make the distinction, we have to get rid
// of observing correlated collisions, which this takes care of:
ResetProcess(/*newdbs*/ false);
} else {
// Replace (end of lifetime for file that was in this slot)
table_[pos] = reduced_key;
}
if (++report_count == FLAGS_sck_files_per_day) {
report_count = 0;
// Estimate fill %
size_t incr = table_mask / 1000;
size_t sampled_count = 0;
for (size_t i = 0; i <= table_mask; i += incr) {
if (table_[i] != 0) {
sampled_count++;
}
}
// Report
printf(
"%" PRIu64 " days, %" PRIu64 " proc, %" PRIu64 " sess, %" PRIu64
" newdb, %u coll, occ %g%%, ejected %g%% \r",
file_count / FLAGS_sck_files_per_day, process_count_,
session_count_, newdb_count_ - FLAGS_sck_db_count,
collisions_this_run, 100.0 * sampled_count / 1000.0,
100.0 * (1.0 - sampled_count / 1000.0 * table_mask / file_count));
fflush(stdout);
}
}
collisions_ += collisions_this_run;
}
void ResetSession(size_t i, bool newdb) {
dbs_[i].db_session_id = DBImpl::GenerateDbSessionId(nullptr);
if (newdb) {
++newdb_count_;
if (FLAGS_sck_footer_unique_id) {
// Simulate how footer id would behave
dbs_[i].db_id = "none";
} else {
// db_id might be ignored, depending on the implementation details
dbs_[i].db_id = std::to_string(newdb_count_);
dbs_[i].orig_file_number = 0;
}
}
session_count_++;
}
void ResetProcess(bool newdbs) {
process_count_++;
DBImpl::TEST_ResetDbSessionIdGen();
for (size_t i = 0; i < FLAGS_sck_db_count; ++i) {
ResetSession(i, newdbs);
}
if (FLAGS_sck_footer_unique_id) {
// For footer unique ID, this tracks process-wide generated SST file
// count.
dbs_[0].orig_file_number = 0;
}
}
private:
// Use db_session_id and orig_file_number from TableProperties
std::unique_ptr<TableProperties[]> dbs_;
std::unique_ptr<uint64_t[]> table_;
uint64_t process_count_ = 0;
uint64_t session_count_ = 0;
uint64_t newdb_count_ = 0;
uint64_t collisions_ = 0;
uint32_t restart_nfiles_ = 0;
double multiplier_ = 0.0;
};
int cache_bench_tool(int argc, char** argv) {
ParseCommandLineFlags(&argc, &argv, true);
if (FLAGS_stress_cache_key) {
// Alternate tool
StressCacheKey().Run();
return 0;
}
if (FLAGS_threads <= 0) {
fprintf(stderr, "threads number <= 0\n");
exit(1);
}
ROCKSDB_NAMESPACE::CacheBench bench;
if (FLAGS_populate_cache) {
bench.PopulateCache();
printf("Population complete\n");
printf("----------------------------\n");
}
if (bench.Run()) {
return 0;
} else {
return 1;
}
} // namespace ROCKSDB_NAMESPACE
} // namespace ROCKSDB_NAMESPACE
#endif // GFLAGS
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