mem: Add faster tiered buffer pool (#8775)

This change adds a new tiered buffer pool that uses power-of-2 tier
sizes. It reduces the lookup time for the relevant sizedBufferPool from
$O(\log n)$ to $O(1)$, where n is the number of tiers. This creates
constant-time lookups independent of the tier count, allowing users to
add more tiers without performance overhead.

## Benchmarks

Micro-benchmark that measures only the pool query performance, ignoring
the allocation time:
```go
func BenchmarkSearch(b *testing.B) {
	defaultBufferPoolSizes := make([]int, len(defaultBufferPoolSizeExponents))
	for i, exp := range defaultBufferPoolSizeExponents {
		defaultBufferPoolSizes[i] = 1 << exp
	}
	b.Run("pool=Tiered", func(b *testing.B) {
		p := NewTieredBufferPool(defaultBufferPoolSizes...).(*tieredBufferPool)
		for b.Loop() {
			for size := range 1 << 19 {
				// One for get, one for put.
				_ = p.getPool(size)
				_ = p.getPool(size)
			}
		}
	})

	b.Run("pool=BinaryTiered", func(b *testing.B) {
		p := NewBinaryTieredBufferPool(defaultBufferPoolSizeExponents...).(*binaryTieredBufferPool)
		for b.Loop() {
			for size := range 1 << 19 {
				_ = p.poolForGet(size)
				_ = p.poolForPut(size)
			}
		}
	})
}
```
With 5 tiers:
```sh
go test -bench=BenchmarkSearch -count=10 -benchmem | benchstat -col '/pool' -
goos: linux
goarch: amd64
pkg: google.golang.org/grpc/mem
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
          │   Tiered    │            BinaryTiered             │
          │   sec/op    │   sec/op     vs base                │
Search-48   5.353m ± 2%   2.036m ± 0%  -61.97% (p=0.000 n=10)

          │   Tiered   │          BinaryTiered          │
          │    B/op    │    B/op     vs base            │
Search-48   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

          │   Tiered   │          BinaryTiered          │
          │ allocs/op  │ allocs/op   vs base            │
Search-48   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal
```
With 9 tiers:
```sh
go test -bench=BenchmarkSearch -count=10 -benchmem | benchstat -col '/pool' -
goos: linux
goarch: amd64
pkg: google.golang.org/grpc/mem
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
          │   Tiered    │            BinaryTiered             │
          │   sec/op    │   sec/op     vs base                │
Search-48   5.659m ± 0%   2.035m ± 0%  -64.04% (p=0.000 n=10)

          │   Tiered   │          BinaryTiered          │
          │    B/op    │    B/op     vs base            │
Search-48   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

          │   Tiered   │          BinaryTiered          │
          │ allocs/op  │ allocs/op   vs base            │
Search-48   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal
```

RELEASE NOTES:
* mem: Add faster tiered buffer pool. Use `NewBinaryTieredBufferPool` to
create such pools.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: arjan-bal

mem/buffer_pool.go

@@ -19,6 +19,9 @@
 package mem
 
 import (
+	"fmt"
+	"math/bits"
+	"slices"
 	"sort"
 	"sync"
 
@@ -38,20 +41,29 @@ type BufferPool interface {
 	Put(*[]byte)
 }
 
-const goPageSize = 4 << 10 // 4KiB. N.B. this must be a power of 2.
-
-var defaultBufferPoolSizes = []int{
-	256,
-	goPageSize,
-	16 << 10, // 16KB (max HTTP/2 frame size used by gRPC)
-	32 << 10, // 32KB (default buffer size for io.Copy)
-	1 << 20,  // 1MB
-}
+const (
+	goPageSizeExponent = 12
+	goPageSize         = 1 << goPageSizeExponent // 4KiB. N.B. this must be a power of 2.
+)
 
-var defaultBufferPool BufferPool
+var (
+	defaultBufferPoolSizeExponents = []uint8{
+		8,
+		goPageSizeExponent,
+		14, // 16KB (max HTTP/2 frame size used by gRPC)
+		15, // 32KB (default buffer size for io.Copy)
+		20, // 1MB
+	}
+	defaultBufferPool BufferPool
+	uintSize          = bits.UintSize // use a variable for mocking during tests.
+)
 
 func init() {
-	defaultBufferPool = NewTieredBufferPool(defaultBufferPoolSizes...)
+	var err error
+	defaultBufferPool, err = NewBinaryTieredBufferPool(defaultBufferPoolSizeExponents...)
+	if err != nil {
+		panic(fmt.Sprintf("Failed to create default buffer pool: %v", err))
+	}
 
 	internal.SetDefaultBufferPool = func(pool BufferPool) {
 		defaultBufferPool = pool
@@ -109,6 +121,135 @@ func (p *tieredBufferPool) getPool(size int) BufferPool {
 	return p.sizedPools[poolIdx]
 }
 
+type binaryTieredBufferPool struct {
+	// exponentToNextLargestPoolMap maps a power-of-two exponent (e.g., 12 for
+	// 4KB) to the index of the next largest sizedBufferPool. This is used by
+	// Get() to find the smallest pool that can satisfy a request for a given
+	// size.
+	exponentToNextLargestPoolMap []int
+	// exponentToPreviousLargestPoolMap maps a power-of-two exponent to the
+	// index of the previous largest sizedBufferPool. This is used by Put()
+	// to return a buffer to the most appropriate pool based on its capacity.
+	exponentToPreviousLargestPoolMap []int
+	sizedPools                       []*sizedBufferPool
+	fallbackPool                     simpleBufferPool
+	maxPoolCap                       int // Optimization: Cache max capacity
+}
+
+// NewBinaryTieredBufferPool returns a BufferPool backed by multiple sub-pools.
+// This structure enables O(1) lookup time for Get and Put operations.
+//
+// The arguments provided are the exponents for the buffer capacities (powers
+// of 2), not the raw byte sizes. For example, to create a pool of 16KB buffers
+// (2^14 bytes), pass 14 as the argument.
+func NewBinaryTieredBufferPool(powerOfTwoExponents ...uint8) (BufferPool, error) {
+	slices.Sort(powerOfTwoExponents)
+	powerOfTwoExponents = slices.Compact(powerOfTwoExponents)
+
+	// Determine the maximum exponent we need to support. This depends on the
+	// word size (32-bit vs 64-bit).
+	maxExponent := uintSize - 1
+	indexOfNextLargestBit := slices.Repeat([]int{-1}, maxExponent+1)
+	indexOfPreviousLargestBit := slices.Repeat([]int{-1}, maxExponent+1)
+
+	maxTier := 0
+	pools := make([]*sizedBufferPool, 0, len(powerOfTwoExponents))
+
+	for i, exp := range powerOfTwoExponents {
+		// Allocating slices of size > 2^maxExponent isn't possible on
+		// maxExponent-bit machines.
+		if int(exp) > maxExponent {
+			return nil, fmt.Errorf("mem: allocating slice of size 2^%d is not possible", exp)
+		}
+		tierSize := 1 << exp
+		pools = append(pools, newSizedBufferPool(tierSize))
+		maxTier = max(maxTier, tierSize)
+
+		// Map the exact power of 2 to this pool index.
+		indexOfNextLargestBit[exp] = i
+		indexOfPreviousLargestBit[exp] = i
+	}
+
+	// Fill gaps for Get() (Next Largest)
+	// We iterate backwards. If current is empty, take the value from the right (larger).
+	for i := maxExponent - 1; i >= 0; i-- {
+		if indexOfNextLargestBit[i] == -1 {
+			indexOfNextLargestBit[i] = indexOfNextLargestBit[i+1]
+		}
+	}
+
+	// Fill gaps for Put() (Previous Largest)
+	// We iterate forwards. If current is empty, take the value from the left (smaller).
+	for i := 1; i <= maxExponent; i++ {
+		if indexOfPreviousLargestBit[i] == -1 {
+			indexOfPreviousLargestBit[i] = indexOfPreviousLargestBit[i-1]
+		}
+	}
+
+	return &binaryTieredBufferPool{
+		exponentToNextLargestPoolMap:     indexOfNextLargestBit,
+		exponentToPreviousLargestPoolMap: indexOfPreviousLargestBit,
+		sizedPools:                       pools,
+		maxPoolCap:                       maxTier,
+	}, nil
+}
+
+func (b *binaryTieredBufferPool) Get(size int) *[]byte {
+	return b.poolForGet(size).Get(size)
+}
+
+func (b *binaryTieredBufferPool) poolForGet(size int) BufferPool {
+	if size == 0 || size > b.maxPoolCap {
+		return &b.fallbackPool
+	}
+
+	// Calculate the exponent of the smallest power of 2 >= size.
+	// We subtract 1 from size to handle exact powers of 2 correctly.
+	//
+	// Examples:
+	// size=16 (0b10000) -> size-1=15 (0b01111) -> bits.Len=4 -> Pool for 2^4
+	// size=17 (0b10001) -> size-1=16 (0b10000) -> bits.Len=5 -> Pool for 2^5
+	querySize := uint(size - 1)
+	poolIdx := b.exponentToNextLargestPoolMap[bits.Len(querySize)]
+
+	return b.sizedPools[poolIdx]
+}
+
+func (b *binaryTieredBufferPool) Put(buf *[]byte) {
+	// We pass the capacity of the buffer, and not the size of the buffer here.
+	// If we did the latter, all buffers would eventually move to the smallest
+	// pool.
+	b.poolForPut(cap(*buf)).Put(buf)
+}
+
+func (b *binaryTieredBufferPool) poolForPut(bCap int) BufferPool {
+	if bCap == 0 {
+		return NopBufferPool{}
+	}
+	if bCap > b.maxPoolCap {
+		return &b.fallbackPool
+	}
+	// Find the pool with the largest capacity <= bCap.
+	//
+	// We calculate the exponent of the largest power of 2 <= bCap.
+	// bits.Len(x) returns the minimum number of bits required to represent x;
+	// i.e. the number of bits up to and including the most significant bit.
+	// Subtracting 1 gives the 0-based index of the most significant bit,
+	// which is the exponent of the largest power of 2 <= bCap.
+	//
+	// Examples:
+	// cap=16 (0b10000) -> Len=5 -> 5-1=4 -> 2^4
+	// cap=15 (0b01111) -> Len=4 -> 4-1=3 -> 2^3
+	largestPowerOfTwo := bits.Len(uint(bCap)) - 1
+	poolIdx := b.exponentToPreviousLargestPoolMap[largestPowerOfTwo]
+	// The buffer is smaller than the smallest power of 2, discard it.
+	if poolIdx == -1 {
+		// Buffer is smaller than our smallest pool bucket.
+		return NopBufferPool{}
+	}
+	return b.sizedPools[poolIdx]
+}
+
 // sizedBufferPool is a BufferPool implementation that is optimized for specific
 // buffer sizes. For example, HTTP/2 frames within gRPC have a default max size
 // of 16kb and a sizedBufferPool can be configured to only return buffers with a

mem/buffer_pool_internal_test.go

@@ -0,0 +1,126 @@
+/*
+ *
+ * Copyright 2026 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package mem
+
+import (
+	"testing"
+)
+
+func TestNewBinaryTieredBufferPool_WordSize(t *testing.T) {
+	origUintSize := uintSize
+	defer func() { uintSize = origUintSize }()
+
+	tests := []struct {
+		name      string
+		wordSize  int
+		exponents []uint8
+		wantErr   bool
+	}{
+		{
+			name:      "32-bit_valid_exponent",
+			wordSize:  32,
+			exponents: []uint8{31},
+			wantErr:   false,
+		},
+		{
+			name:      "32-bit_invalid_exponent",
+			wordSize:  32,
+			exponents: []uint8{32},
+			wantErr:   true,
+		},
+		{
+			name:      "64-bit_valid_exponent",
+			wordSize:  64,
+			exponents: []uint8{63},
+			wantErr:   false,
+		},
+		{
+			name:      "64-bit_invalid_exponent",
+			wordSize:  64,
+			exponents: []uint8{64},
+			wantErr:   true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			uintSize = tt.wordSize
+			pool, err := NewBinaryTieredBufferPool(tt.exponents...)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("NewBinaryTieredBufferPool() error = %t, wantErr %t", err, tt.wantErr)
+			}
+			if err != nil {
+				return
+			}
+			bp := pool.(*binaryTieredBufferPool)
+			if len(bp.exponentToNextLargestPoolMap) != tt.wordSize {
+				t.Errorf("exponentToNextLargestPoolMap length = %d, want %d", len(bp.exponentToNextLargestPoolMap), tt.wordSize)
+			}
+			if len(bp.exponentToPreviousLargestPoolMap) != tt.wordSize {
+				t.Errorf("exponentToPreviousLargestPoolMap length = %d, want %d", len(bp.exponentToPreviousLargestPoolMap), tt.wordSize)
+			}
+		})
+	}
+}
+
+// BenchmarkTieredPool benchmarks the performance of the tiered buffer pool
+// implementations, specifically focusing on the overhead of selecting the
+// correct bucket for a given size.
+func BenchmarkTieredPool(b *testing.B) {
+	defaultBufferPoolSizes := make([]int, len(defaultBufferPoolSizeExponents))
+	for i, exp := range defaultBufferPoolSizeExponents {
+		defaultBufferPoolSizes[i] = 1 << exp
+	}
+	b.Run("pool=Tiered", func(b *testing.B) {
+		p := NewTieredBufferPool(defaultBufferPoolSizes...).(*tieredBufferPool)
+		for b.Loop() {
+			for size := range 1 << 19 {
+				// One for get, one for put.
+				_ = p.getPool(size)
+				_ = p.getPool(size)
+			}
+		}
+	})
+
+	b.Run("pool=BinaryTiered", func(b *testing.B) {
+		pool, err := NewBinaryTieredBufferPool(defaultBufferPoolSizeExponents...)
+		if err != nil {
+			b.Fatalf("Failed to create buffer pool: %v", err)
+		}
+		p := pool.(*binaryTieredBufferPool)
+		for b.Loop() {
+			for size := range 1 << 19 {
+				_ = p.poolForGet(size)
+				_ = p.poolForPut(size)
+			}
+		}
+	})
+}
+
+func TestNewBinaryTieredBufferPool_Duplicates(t *testing.T) {
+	exponents := []uint8{1, 2, 3, 4, 5, 6, 6, 5, 4, 3, 2, 1}
+	pool, err := NewBinaryTieredBufferPool(exponents...)
+	if err != nil {
+		t.Fatalf("NewBinaryTieredBufferPool() error = %v", err)
+	}
+	binaryPool := pool.(*binaryTieredBufferPool)
+	if len(binaryPool.sizedPools) != 6 {
+		t.Errorf("sized buffer pool count = %d, want %d", len(binaryPool.sizedPools), 6)
+	}
+}

mem/buffer_pool_test.go

@@ -20,6 +20,7 @@ package mem_test
 
 import (
 	"bytes"
+	"fmt"
 	"testing"
 	"unsafe"
 
@@ -105,3 +106,43 @@ func (s) TestBufferPoolIgnoresShortBuffers(t *testing.T) {
 	// pool, it could cause a panic.
 	pool.Get(10)
 }
+
+func TestBinaryBufferPool(t *testing.T) {
+	poolSizes := []uint8{0, 2, 3, 4, 2, 3, 4} // duplicates will be ignored.
+
+	testCases := []struct {
+		requestSize  int
+		wantCapacity int
+	}{
+		{requestSize: 0, wantCapacity: 0},
+		{requestSize: 1, wantCapacity: 1},
+		{requestSize: 2, wantCapacity: 4},
+		{requestSize: 3, wantCapacity: 4},
+		{requestSize: 4, wantCapacity: 4},
+		{requestSize: 5, wantCapacity: 8},
+		{requestSize: 6, wantCapacity: 8},
+		{requestSize: 7, wantCapacity: 8},
+		{requestSize: 8, wantCapacity: 8},
+		{requestSize: 9, wantCapacity: 16},
+		{requestSize: 15, wantCapacity: 16},
+		{requestSize: 16, wantCapacity: 16},
+		{requestSize: 17, wantCapacity: 4096}, // fallback pool returns sizes in multiples of 4096.
+	}
+
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("requestSize=%d", tc.requestSize), func(t *testing.T) {
+			pool, err := mem.NewBinaryTieredBufferPool(poolSizes...)
+			if err != nil {
+				t.Fatalf("Failed to create buffer pool: %v", err)
+			}
+			buf := pool.Get(tc.requestSize)
+			if cap(*buf) != tc.wantCapacity {
+				t.Errorf("Get(%d) returned buffer with capacity: %d, want %d", tc.requestSize, cap(*buf), tc.wantCapacity)
+			}
+			if len(*buf) != tc.requestSize {
+				t.Errorf("Get(%d) returned buffer with length: %d, want %d", tc.requestSize, len(*buf), tc.requestSize)
+			}
+			pool.Put(buf)
+		})
+	}
+}