rocksdb/table/persistent_cache_helper.cc
Peter Dillinger 0050a73a4f New stable, fixed-length cache keys (#9126)
Summary:
This change standardizes on a new 16-byte cache key format for
block cache (incl compressed and secondary) and persistent cache (but
not table cache and row cache).

The goal is a really fast cache key with practically ideal stability and
uniqueness properties without external dependencies (e.g. from FileSystem).
A fixed key size of 16 bytes should enable future optimizations to the
concurrent hash table for block cache, which is a heavy CPU user /
bottleneck, but there appears to be measurable performance improvement
even with no changes to LRUCache.

This change replaces a lot of disjointed and ugly code handling cache
keys with calls to a simple, clean new internal API (cache_key.h).
(Preserving the old cache key logic under an option would be very ugly
and likely negate the performance gain of the new approach. Complete
replacement carries some inherent risk, but I think that's acceptable
with sufficient analysis and testing.)

The scheme for encoding new cache keys is complicated but explained
in cache_key.cc.

Also: EndianSwapValue is moved to math.h to be next to other bit
operations. (Explains some new include "math.h".) ReverseBits operation
added and unit tests added to hash_test for both.

Fixes https://github.com/facebook/rocksdb/issues/7405 (presuming a root cause)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9126

Test Plan:
### Basic correctness
Several tests needed updates to work with the new functionality, mostly
because we are no longer relying on filesystem for stable cache keys
so table builders & readers need more context info to agree on cache
keys. This functionality is so core, a huge number of existing tests
exercise the cache key functionality.

### Performance
Create db with
`TEST_TMPDIR=/dev/shm ./db_bench -bloom_bits=10 -benchmarks=fillrandom -num=3000000 -partition_index_and_filters`
And test performance with
`TEST_TMPDIR=/dev/shm ./db_bench -readonly -use_existing_db -bloom_bits=10 -benchmarks=readrandom -num=3000000 -duration=30 -cache_index_and_filter_blocks -cache_size=250000 -threads=4`
using DEBUG_LEVEL=0 and simultaneous before & after runs.
Before ops/sec, avg over 100 runs: 121924
After ops/sec, avg over 100 runs: 125385 (+2.8%)

### Collision probability
I have built a tool, ./cache_bench -stress_cache_key to broadly simulate host-wide cache activity
over many months, by making some pessimistic simplifying assumptions:
* 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

We use a simple table with skewed address assignment and replacement on address collision
to simulate files coming & going, with quite a variance (super-Poisson) in ages. Some output
with `./cache_bench -stress_cache_key -sck_keep_bits=40`:

```
Total cache or DBs size: 32TiB  Writing 925.926 MiB/s or 76.2939TiB/day
Multiply by 9.22337e+18 to correct for simulation losses (but still assume whole file cached)
```

These come from default settings of 2.5M files per day of 32 MB each, and
`-sck_keep_bits=40` means that to represent a single file, we are only keeping 40 bits of
the 128-bit cache key.  With file size of 2\*\*25 contiguous keys (pessimistic), our simulation
is about 2\*\*(128-40-25) or about 9 billion billion times more prone to collision than reality.

More default assumptions, relatively pessimistic:
* 100 DBs in same process (doesn't matter much)
* Re-open DB in same process (new session ID related to old session ID) on average
every 100 files generated
* Restart process (all new session IDs unrelated to old) 24 times per day

After enough data, we get a result at the end:

```
(keep 40 bits)  17 collisions after 2 x 90 days, est 10.5882 days between (9.76592e+19 corrected)
```

If we believe the (pessimistic) simulation and the mathematical generalization, we would need to run a billion machines all for 97 billion days to expect a cache key collision. To help verify that our generalization ("corrected") is robust, we can make our simulation more precise with `-sck_keep_bits=41` and `42`, which takes more running time to get enough data:

```
(keep 41 bits)  16 collisions after 4 x 90 days, est 22.5 days between (1.03763e+20 corrected)
(keep 42 bits)  19 collisions after 10 x 90 days, est 47.3684 days between (1.09224e+20 corrected)
```

The generalized prediction still holds. With the `-sck_randomize` option, we can see that we are beating "random" cache keys (except offsets still non-randomized) by a modest amount (roughly 20x less collision prone than random), which should make us reasonably comfortable even in "degenerate" cases:

```
197 collisions after 1 x 90 days, est 0.456853 days between (4.21372e+18 corrected)
```

I've run other tests to validate other conditions behave as expected, never behaving "worse than random" unless we start chopping off structured data.

Reviewed By: zhichao-cao

Differential Revision: D33171746

Pulled By: pdillinger

fbshipit-source-id: f16a57e369ed37be5e7e33525ace848d0537c88f
2021-12-16 17:15:13 -08:00

110 lines
3.4 KiB
C++

// 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 "table/persistent_cache_helper.h"
#include "table/block_based/block_based_table_reader.h"
#include "table/format.h"
namespace ROCKSDB_NAMESPACE {
const PersistentCacheOptions PersistentCacheOptions::kEmpty;
void PersistentCacheHelper::InsertRawPage(
const PersistentCacheOptions& cache_options, const BlockHandle& handle,
const char* data, const size_t size) {
assert(cache_options.persistent_cache);
assert(cache_options.persistent_cache->IsCompressed());
CacheKey key =
BlockBasedTable::GetCacheKey(cache_options.base_cache_key, handle);
cache_options.persistent_cache->Insert(key.AsSlice(), data, size)
.PermitUncheckedError();
}
void PersistentCacheHelper::InsertUncompressedPage(
const PersistentCacheOptions& cache_options, const BlockHandle& handle,
const BlockContents& contents) {
assert(cache_options.persistent_cache);
assert(!cache_options.persistent_cache->IsCompressed());
// Precondition:
// (1) content is cacheable
// (2) content is not compressed
CacheKey key =
BlockBasedTable::GetCacheKey(cache_options.base_cache_key, handle);
cache_options.persistent_cache
->Insert(key.AsSlice(), contents.data.data(), contents.data.size())
.PermitUncheckedError();
;
}
Status PersistentCacheHelper::LookupRawPage(
const PersistentCacheOptions& cache_options, const BlockHandle& handle,
std::unique_ptr<char[]>* raw_data, const size_t raw_data_size) {
#ifdef NDEBUG
(void)raw_data_size;
#endif
assert(cache_options.persistent_cache);
assert(cache_options.persistent_cache->IsCompressed());
CacheKey key =
BlockBasedTable::GetCacheKey(cache_options.base_cache_key, handle);
size_t size;
Status s =
cache_options.persistent_cache->Lookup(key.AsSlice(), raw_data, &size);
if (!s.ok()) {
// cache miss
RecordTick(cache_options.statistics, PERSISTENT_CACHE_MISS);
return s;
}
// cache hit
// Block-based table is assumed
assert(raw_data_size == handle.size() + BlockBasedTable::kBlockTrailerSize);
assert(size == raw_data_size);
RecordTick(cache_options.statistics, PERSISTENT_CACHE_HIT);
return Status::OK();
}
Status PersistentCacheHelper::LookupUncompressedPage(
const PersistentCacheOptions& cache_options, const BlockHandle& handle,
BlockContents* contents) {
assert(cache_options.persistent_cache);
assert(!cache_options.persistent_cache->IsCompressed());
if (!contents) {
// We shouldn't lookup in the cache. Either
// (1) Nowhere to store
return Status::NotFound();
}
CacheKey key =
BlockBasedTable::GetCacheKey(cache_options.base_cache_key, handle);
std::unique_ptr<char[]> data;
size_t size;
Status s =
cache_options.persistent_cache->Lookup(key.AsSlice(), &data, &size);
if (!s.ok()) {
// cache miss
RecordTick(cache_options.statistics, PERSISTENT_CACHE_MISS);
return s;
}
// please note we are potentially comparing compressed data size with
// uncompressed data size
assert(handle.size() <= size);
// update stats
RecordTick(cache_options.statistics, PERSISTENT_CACHE_HIT);
// construct result and return
*contents = BlockContents(std::move(data), size);
return Status::OK();
}
} // namespace ROCKSDB_NAMESPACE