sstable: avoid caching meta blocks

When opening a sstable for the first time, we read two 'meta' blocks: one
describes the layout of sstable, encoding block handles for the index block,
properties block, etc. The other contains table properties. These blocks are
decoded, and the necessary state is copied onto the heap, and then blocks are
released. In typical configurations with sufficiently large table caches, these
blocks are never read again or only much later after the table has been evicted
from the table cache.

This commit uses a BufferPool to hold these temporary blocks, and refrains from
adding these blocks to the block cache. This reduces contention on the block
cache mutexes (#1997).

sstable/buffer_pool.go

@@ -66,6 +66,15 @@ func (p *BufferPool) Init(initialSize int) {
 	}
 }
 
+// initPreallocated is like Init but for internal sstable package use in
+// instances where a pre-allocated slice of []allocedBuffer already exists. It's
+// used to avoid an extra allocation initializing BufferPool.pool.
+func (p *BufferPool) initPreallocated(pool []allocedBuffer) {
+	*p = BufferPool{
+		pool: pool[:0],
+	}
+}
+
 // Release releases all buffers held by the pool and resets the pool to an
 // uninitialized state.
 func (p *BufferPool) Release() {

sstable/reader.go

@@ -3071,6 +3071,14 @@ type Reader struct {
 	rawTombstones bool
 	mergerOK      bool
 	checksumType  ChecksumType
+	// metaBufferPool is a buffer pool used exclusively when opening a table and
+	// loading its meta blocks. metaBufferPoolAlloc is used to batch-allocate
+	// the BufferPool.pool slice as a part of the Reader allocation. It's
+	// capacity 3 to accommodate the meta block (1), and both the compressed
+	// properties block (1) and decompressed properties block (1)
+	// simultaneously.
+	metaBufferPool      BufferPool
+	metaBufferPoolAlloc [3]allocedBuffer
 }
 
 // Close implements DB.Close, as documented in the pebble package.
@@ -3522,8 +3530,21 @@ func (r *Reader) transformRangeDelV1(b []byte) ([]byte, error) {
 }
 
 func (r *Reader) readMetaindex(metaindexBH BlockHandle) error {
+	// We use a BufferPool when reading metaindex blocks in order to avoid
+	// populating the block cache with these blocks. In heavy-write workloads,
+	// especially with high compaction concurrency, new tables may be created
+	// frequently. Populating the block cache with these metaindex blocks adds
+	// additional contention on the block cache mutexes (see #1997).
+	// Additionally, these blocks are exceedingly unlikely to be read again
+	// while they're still in the block cache except in misconfigurations with
+	// excessive sstables counts or a table cache that's far too small.
+	r.metaBufferPool.initPreallocated(r.metaBufferPoolAlloc[:0])
+	// When we're finished, release the buffers we've allocated back to memory
+	// allocator. We don't expect to use metaBufferPool again.
+	defer r.metaBufferPool.Release()
+
 	b, err := r.readBlock(
-		context.Background(), metaindexBH, nil /* transform */, nil /* readHandle */, nil /* stats */, nil /* buffer pool */)
+		context.Background(), metaindexBH, nil /* transform */, nil /* readHandle */, nil /* stats */, &r.metaBufferPool)
 	if err != nil {
 		return err
 	}

testdata/event_listener

@@ -284,8 +284,8 @@ compact         1   2.0 K     0 B       0                          (size == esti
  memtbl         1   256 K
 zmemtbl         0     0 B
    ztbl         0     0 B
- bcache         8   1.2 K   11.1%  (score == hit-rate)
- tcache         1   680 B   40.0%  (score == hit-rate)
+ bcache         6   1.1 K   11.1%  (score == hit-rate)
+ tcache         1   800 B   40.0%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         0
  filter         -       -    0.0%  (score == utility)
@@ -378,8 +378,8 @@ compact         1   4.0 K     0 B       0                          (size == esti
  memtbl         1   512 K
 zmemtbl         0     0 B
    ztbl         0     0 B
- bcache        16   2.5 K   14.3%  (score == hit-rate)
- tcache         1   680 B   50.0%  (score == hit-rate)
+ bcache        12   2.3 K   14.3%  (score == hit-rate)
+ tcache         1   800 B   50.0%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         0
  filter         -       -    0.0%  (score == utility)

testdata/ingest

@@ -47,8 +47,8 @@ compact         0     0 B     0 B       0                          (size == esti
  memtbl         1   256 K
 zmemtbl         0     0 B
    ztbl         0     0 B
- bcache         8   1.2 K   42.9%  (score == hit-rate)
- tcache         1   680 B   50.0%  (score == hit-rate)
+ bcache         6   1.2 K   35.7%  (score == hit-rate)
+ tcache         1   800 B   50.0%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         0
  filter         -       -    0.0%  (score == utility)

testdata/metrics

@@ -33,8 +33,8 @@ compact         0     0 B     0 B       0                          (size == esti
  memtbl         1   256 K
 zmemtbl         1   256 K
    ztbl         0     0 B
- bcache         4   560 B    0.0%  (score == hit-rate)
- tcache         1   680 B    0.0%  (score == hit-rate)
+ bcache         3   528 B    0.0%  (score == hit-rate)
+ tcache         1   800 B    0.0%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         1
  filter         -       -    0.0%  (score == utility)
@@ -82,8 +82,8 @@ compact         1     0 B     0 B       0                          (size == esti
  memtbl         1   256 K
 zmemtbl         2   512 K
    ztbl         2   1.2 K
- bcache         7   1.1 K   42.9%  (score == hit-rate)
- tcache         2   1.3 K   66.7%  (score == hit-rate)
+ bcache         5   1.0 K   42.9%  (score == hit-rate)
+ tcache         2   1.6 K   66.7%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         2
  filter         -       -    0.0%  (score == utility)
@@ -116,8 +116,8 @@ compact         1     0 B     0 B       0                          (size == esti
  memtbl         1   256 K
 zmemtbl         1   256 K
    ztbl         2   1.2 K
- bcache         7   1.1 K   42.9%  (score == hit-rate)
- tcache         2   1.3 K   66.7%  (score == hit-rate)
+ bcache         5   1.0 K   42.9%  (score == hit-rate)
+ tcache         2   1.6 K   66.7%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         2
  filter         -       -    0.0%  (score == utility)
@@ -147,8 +147,8 @@ compact         1     0 B     0 B       0                          (size == esti
  memtbl         1   256 K
 zmemtbl         1   256 K
    ztbl         1   633 B
- bcache         4   560 B   42.9%  (score == hit-rate)
- tcache         1   680 B   66.7%  (score == hit-rate)
+ bcache         3   528 B   42.9%  (score == hit-rate)
+ tcache         1   800 B   66.7%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         1
  filter         -       -    0.0%  (score == utility)
@@ -375,8 +375,8 @@ compact         2   4.8 K     0 B       0                          (size == esti
  memtbl         1   1.0 M
 zmemtbl         0     0 B
    ztbl         0     0 B
- bcache        16   2.4 K   34.4%  (score == hit-rate)
- tcache         3   2.0 K   57.9%  (score == hit-rate)
+ bcache        12   2.3 K   31.1%  (score == hit-rate)
+ tcache         3   2.3 K   57.9%  (score == hit-rate)
   snaps         0       -       0  (score == earliest seq num)
  titers         0
  filter         -       -    0.0%  (score == utility)