Performance measuring & improvements for arena

README.md

@@ -14,7 +14,7 @@ Also available:
 
 * `DefaultAllocator` - calls cgo for Malloc and Free
 * `ThresholdAllocator` - If the malloc size is <= a provided value, use one allocator.  Otherwise, use the other.  Allocations made above the threshold size are stored in a map to enable `Free`. You can use this with a `FixedBlockAllocator` to use the default allocator for large requests.  You could also use several to set up a multi-tiered FBA, I suppose. 
-* `ArenaAllocator` - sits on top of another allocator.  Exposes a FreeAll method which will free all memory allocated through the ArenaAllocator.
+* `ArenaAllocator` - sits on top of another allocator.  Exposes a FreeAll method which will free all memory allocated through the ArenaAllocator.  ArenaAllocator is optimized for `FreeAll` and ordinary frees have a cost of O(N)
 
 ### Are these thread-safe?
 
@@ -24,15 +24,26 @@ The DefaultAllocator is! And as slow as cgo is, it's still far faster than any l
 
 In terms of memory overhead, it's kind of bad! I use a lot of maps and slices to track allocated-but-not-freed data.  In terms of speed:
 
+Default cgo
 ```
 BenchmarkDefaultTemporaryData
-BenchmarkDefaultTemporaryData-16    	12688918	        96.51 ns/op
-BenchmarkFBATemporaryData
-BenchmarkFBATemporaryData-16        	100000000	        11.07 ns/op
+BenchmarkDefaultTemporaryData-16    	12792590	        94.58 ns/op
 BenchmarkDefaultGrowShrink
-BenchmarkDefaultGrowShrink-16       	11640133	       105.1 ns/op
+BenchmarkDefaultGrowShrink-16       	11286946	       104.7 ns/op
+```
+
+Fixed Buffer
+```
+BenchmarkFBATemporaryData
+BenchmarkFBATemporaryData-16        	123561244	         9.714 ns/op
 BenchmarkFBAGrowShrink
-BenchmarkFBAGrowShrink-16           	61445820	        34.88 ns/op
+BenchmarkFBAGrowShrink-16           	64682006	        34.83 ns/op
+```
+
+Arena
+```
+BenchmarkArenaTemporaryData
+BenchmarkArenaTemporaryData-16      	40963460	        29.24 ns/op
 ```
 
 "It's fine!"

arena.go

@@ -7,38 +7,47 @@ import (
 type ArenaAllocator struct {
 	inner Allocator
 
-	allocations map[unsafe.Pointer]bool
+	allocations []unsafe.Pointer
 }
 
 func CreateArenaAllocator(inner Allocator) *ArenaAllocator {
 	return &ArenaAllocator{
 		inner: inner,
-
-		allocations: make(map[unsafe.Pointer]bool),
+		allocations: make([]unsafe.Pointer, 0, 1),
 	}
 }
 
 func (a *ArenaAllocator) Malloc(size int) unsafe.Pointer {
 	alloc := a.inner.Malloc(size)
-	a.allocations[alloc] = true
+	a.allocations = append(a.allocations, alloc)
 	return alloc
 }
 
 func (a *ArenaAllocator) Free(ptr unsafe.Pointer) {
-	_, ok := a.allocations[ptr]
-	if !ok {
+	allocIndex := -1
+	for i := 0; i < len(a.allocations); i++ {
+		if a.allocations[i] == ptr {
+			allocIndex = i
+			break
+		}
+	}
+
+	if allocIndex < 0 {
 		panic("arenaallocator: attempted to free a pointer which had not been allocated with this allocator")
 	}
-	delete(a.allocations, ptr)
+
+	newEnd := len(a.allocations)-1
+	a.allocations[allocIndex] = a.allocations[newEnd]
+	a.allocations = a.allocations[:newEnd]
 
 	a.inner.Free(ptr)
 }
 
 func (a *ArenaAllocator) FreeAll() {
-	for ptr := range a.allocations {
-		a.inner.Free(ptr)
+	for i := 0; i < len(a.allocations); i++ {
+		a.inner.Free(a.allocations[i])
 	}
-	a.allocations = make(map[unsafe.Pointer]bool)
+	a.allocations = nil
 }
 
 func (a *ArenaAllocator) Destroy() {

bench_test.go

@@ -28,6 +28,21 @@ func BenchmarkFBATemporaryData(b *testing.B) {
 	}
 }
 
+func BenchmarkArenaTemporaryData(b *testing.B) {
+	alloc, err := CreateFixedBlockAllocator(&DefaultAllocator{}, 4096, 64, 8)
+	if err != nil {
+		b.FailNow()
+	}
+	defer alloc.Destroy()
+
+	for i := 0; i < b.N/2; i++ {
+		arena := CreateArenaAllocator(alloc)
+		_ = arena.Malloc(64)
+		_ = arena.Malloc(64)
+		arena.FreeAll()
+	}
+}
+
 func BenchmarkDefaultGrowShrink(b *testing.B) {
 	alloc := &DefaultAllocator{}
 	defer alloc.Destroy()