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db: do not cache compaction block reads
During compactions, avoid populating the block cache with input files' blocks. These files will soon be removed from the LSM, making it less likely any iterator will need to read these blocks. While Pebble uses a scan-resistant block cache algorithm (ClockPRO), the act of inserting the blocks into the cache increases contention on the block cache mutexes (cockroachdb#1997). This contention has been observed to significantly contribute to tail latencies, both for reads and for writes during memtable reservation. Additionally, although these blocks may be soon replaced with more useful blocks due to ClockPRO's scan resistance, they may be freed by a different thread inducing excessive TLB shootdowns (cockroachdb#2693). A compaction only requires a relatively small working set of buffers during its scan across input sstables. In this commit, we introduce a per-compaction BufferPool that is used to allocate buffers during cache misses. Buffers are reused throughout the compaction and only freed to the memory allocator when they're too small or the compaction is finished. This reduces pressure on the memory allocator and the block cache.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,140 @@ | ||
| // Copyright 2023 The LevelDB-Go and Pebble Authors. All rights reserved. Use | ||
| // of this source code is governed by a BSD-style license that can be found in | ||
| // the LICENSE file. | ||
|
|
||
| package sstable | ||
|
|
||
| import ( | ||
| "github.com/cockroachdb/errors" | ||
| "github.com/cockroachdb/pebble/internal/cache" | ||
| ) | ||
|
|
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| // A bufferHandle is a handle to manually-managed memory. The handle may point | ||
| // to a block in the block cache (h.Get() != nil), or a buffer that exists | ||
| // outside the block cache allocated from a BufferPool (b.Valid()). | ||
| type bufferHandle struct { | ||
| h cache.Handle | ||
| b Buf | ||
| } | ||
|
|
||
| // Get retrieves the underlying buffer referenced by the handle. | ||
| func (bh bufferHandle) Get() []byte { | ||
| if v := bh.h.Get(); v != nil { | ||
| return v | ||
| } else if bh.b.p != nil { | ||
| return bh.b.p.pool[bh.b.i].b | ||
| } | ||
| return nil | ||
| } | ||
|
|
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| // Release releases the buffer, either back to the block cache or BufferPool. | ||
| func (bh bufferHandle) Release() { | ||
| bh.h.Release() | ||
| bh.b.Release() | ||
| } | ||
|
|
||
| // A BufferPool holds a pool of buffers for holding sstable blocks. An initial | ||
| // size of the pool is provided on Init, but a BufferPool will grow to meet the | ||
| // largest working set size. It'll never shrink. When a buffer is released, the | ||
| // BufferPool recycles the buffer for future allocations. | ||
| // | ||
| // A BufferPool should only be used for short-lived allocations with | ||
| // well-understood working set sizes to avoid excessive memory consumption. | ||
| // | ||
| // BufferPool is not thread-safe. | ||
| type BufferPool struct { | ||
| cache *cache.Cache | ||
| // pool contains all the buffers held by the pool, including buffers that | ||
| // are in-use. | ||
| pool []allocedBuffer | ||
| } | ||
|
|
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| type allocedBuffer struct { | ||
| v *cache.Value | ||
| // b holds the current byte slice. It's backed by v, but may be a subslice | ||
| // of v's memory while the buffer is in-use [ len(b) ≤ len(v.Buf()) ]. | ||
| // | ||
| // If the buffer is not currently in-use, b is nil. When being recycled, the | ||
| // BufferPool.Alloc will reset b to be a subslice of v.Buf(). | ||
| b []byte | ||
| } | ||
|
|
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| // Init initializes the pool to use the provided cache for allocations, and | ||
| // with an initial working set buffer size of `initialSize`. | ||
| func (p *BufferPool) Init(cache *cache.Cache, initialSize int) { | ||
| *p = BufferPool{ | ||
| cache: cache, | ||
| pool: make([]allocedBuffer, 0, initialSize), | ||
| } | ||
| } | ||
|
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| // Release releases all buffers held by the pool and resets the pool to an | ||
| // uninitialized state. | ||
| func (p *BufferPool) Release() { | ||
| for i := range p.pool { | ||
| if p.pool[i].b != nil { | ||
| panic(errors.AssertionFailedf("Release called on a BufferPool with in-use buffers")) | ||
| } | ||
| p.cache.Free(p.pool[i].v) | ||
| } | ||
| *p = BufferPool{} | ||
| } | ||
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| // Alloc allocates a new buffer of size n. If the pool already holds a buffer at | ||
| // least as large as n, the pooled buffer is used instead. | ||
| // | ||
| // Alloc is O(MAX(N,M)) where N is the largest number of concurrently in-use | ||
| // buffers allocated and M is the initialSize passed to Init. | ||
| func (p *BufferPool) Alloc(n int) Buf { | ||
| unusableBufferIdx := -1 | ||
| for i := 0; i < len(p.pool); i++ { | ||
| if p.pool[i].b == nil { | ||
| if len(p.pool[i].v.Buf()) >= n { | ||
| p.pool[i].b = p.pool[i].v.Buf()[:n] | ||
| return Buf{p: p, i: i} | ||
| } | ||
| unusableBufferIdx = i | ||
| } | ||
| } | ||
|
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| // If we would need to grow the size of the pool to allocate another buffer, | ||
| // but there was a slot available occupied by a buffer that's just too | ||
| // small, replace the too-small buffer. | ||
| if len(p.pool) == cap(p.pool) && unusableBufferIdx >= 0 { | ||
| i := unusableBufferIdx | ||
| p.cache.Free(p.pool[i].v) | ||
| p.pool[i].v = p.cache.Alloc(n) | ||
| p.pool[i].b = p.pool[i].v.Buf() | ||
| return Buf{p: p, i: i} | ||
| } | ||
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| // Allocate a new buffer. | ||
| v := p.cache.Alloc(n) | ||
| p.pool = append(p.pool, allocedBuffer{v: v, b: v.Buf()[:n]}) | ||
| return Buf{p: p, i: len(p.pool) - 1} | ||
| } | ||
|
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| // A Buf holds a reference to a manually-managed, pooled byte buffer. | ||
| type Buf struct { | ||
| p *BufferPool | ||
| // i holds the index into p.pool where the buffer may be found. This scheme | ||
| // avoids needing to allocate the handle to the buffer on the heap at the | ||
| // cost of copying two words instead of one. | ||
| i int | ||
| } | ||
|
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| // Valid returns true if the buf holds a valid buffer. | ||
| func (b Buf) Valid() bool { | ||
| return b.p != nil | ||
| } | ||
|
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| // Release releases the buffer back to the pool. | ||
| func (b Buf) Release() { | ||
| if b.p == nil { | ||
| return | ||
| } | ||
| // Clear the allocedBuffer's byte slice. This signals the allocated buffer | ||
| // is no longer in use and a future call to BufferPool.Alloc may reuse this | ||
| // buffer. | ||
| b.p.pool[b.i].b = nil | ||
| } |
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