sstable: optimize blobReferenceValues allocation

When writing a sstable containing references to blob files, the sstable writer
constructs the blob reference liveness index—an accounting of which values in
which blob files are referenced. Once we've finalized the data for an
individual block within a blob file, we encode the data to a buffer through
appending. These appends show up as 3% of all allocations in heap profiles from
a recent run of the pebble ycsb/D/values=1024 benchmark. This commit adds a
call to slices.Grow to avoid unnecessary intermediate allocations.
Additionally, when constructing state for a new blob file, we allocate an
initial 512B under the expectation that most referenced blob files will require
around 512B to encode all the values that are referenced from the sstable.

Author: jbowens

sstable/blob_reference_index.go

@@ -7,6 +7,7 @@ package sstable
 import (
 	"encoding/binary"
 	"iter"
+	"slices"
 
 	"github.com/cockroachdb/errors"
 	"github.com/cockroachdb/pebble/internal/base"
@@ -55,9 +56,12 @@ func (s *blobReferenceValues) finishCurrentBlock() {
 	if s.currentBlock.valuesSize == 0 {
 		panic(errors.AssertionFailedf("no pending current block"))
 	}
+	// Ensure there's enough space for the bytes we're about to append.
+	bitmapSize := s.currentBlock.bitmap.Size()
+	s.encodedFinishedBlocks = slices.Grow(s.encodedFinishedBlocks, 3*binary.MaxVarintLen64+bitmapSize)
 	s.encodedFinishedBlocks = binary.AppendUvarint(s.encodedFinishedBlocks, uint64(s.currentBlock.blockID))
 	s.encodedFinishedBlocks = binary.AppendUvarint(s.encodedFinishedBlocks, s.currentBlock.valuesSize)
-	s.encodedFinishedBlocks = binary.AppendUvarint(s.encodedFinishedBlocks, uint64(s.currentBlock.bitmap.Size()))
+	s.encodedFinishedBlocks = binary.AppendUvarint(s.encodedFinishedBlocks, uint64(bitmapSize))
 	s.encodedFinishedBlocks = s.currentBlock.bitmap.FinishAndAppend(s.encodedFinishedBlocks)
 }
 
@@ -107,6 +111,7 @@ func (w *blobRefValueLivenessWriter) addLiveValue(
 
 		// We have a new reference.
 		state := blobReferenceValues{}
+		state.encodedFinishedBlocks = slices.Grow(state.encodedFinishedBlocks, 512)
 		state.initNewBlock(blockID)
 		w.refState = append(w.refState, state)
 	}