runtime: manage a pool of spanSetBlocks and free them eagerly

This change adds a global pool of spanSetBlocks to the spanSet data
structure and adds support for eagerly freeing these blocks back to the
pool if the block goes empty.

This change prepares us to use this data structure in more places in the
runtime by allowing reuse of spanSetBlock.

Updates golang#37487.

Change-Id: I0752226e3667a9e3e1d87c9b66edaedeae1ac23f
Reviewed-on: https://go-review.googlesource.com/c/go/+/221180
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mspanset.go

@@ -27,10 +27,11 @@ type spanSet struct {
 	//
 	// The spine and all blocks are allocated off-heap, which
 	// allows this to be used in the memory manager and avoids the
-	// need for write barriers on all of these. We never release
-	// this memory because there could be concurrent lock-free
-	// access and we're likely to reuse it anyway. (In principle,
-	// we could do this during STW.)
+	// need for write barriers on all of these. spanSetBlocks are
+	// managed in a pool, though never freed back to the operating
+	// system. We never release spine memory because there could be
+	// concurrent lock-free access and we're likely to reuse it
+	// anyway. (In principle, we could do this during STW.)
 
 	spineLock mutex
 	spine     unsafe.Pointer // *[N]*spanSetBlock, accessed atomically
@@ -48,6 +49,17 @@ const (
 )
 
 type spanSetBlock struct {
+	// Free spanSetBlocks are managed via a lock-free stack.
+	lfnode
+
+	// used represents the number of slots in the spans array which are
+	// currently in use. This number is used to help determine when a
+	// block may be safely recycled.
+	//
+	// Accessed and updated atomically.
+	used uint32
+
+	// spans is the set of spans in this block.
 	spans [spanSetBlockEntries]*mspan
 }
 
@@ -102,8 +114,10 @@ retry:
 			// during STW.
 		}
 
-		// Allocate a new block and add it to the spine.
-		block = (*spanSetBlock)(persistentalloc(unsafe.Sizeof(spanSetBlock{}), cpu.CacheLineSize, &memstats.gc_sys))
+		// Allocate a new block from the pool.
+		block = spanSetBlockPool.alloc()
+
+		// Add it to the spine.
 		blockp := add(b.spine, sys.PtrSize*top)
 		// Blocks are allocated off-heap, so no write barrier.
 		atomic.StorepNoWB(blockp, unsafe.Pointer(block))
@@ -113,6 +127,7 @@ retry:
 
 	// We have a block. Insert the span atomically, since there may be
 	// concurrent readers via the block API.
+	atomic.Xadd(&block.used, 1)
 	atomic.StorepNoWB(unsafe.Pointer(&block.spans[bottom]), unsafe.Pointer(s))
 }
 
@@ -134,6 +149,16 @@ func (b *spanSet) pop() *mspan {
 	s := block.spans[bottom]
 	// Clear the pointer for block(i).
 	block.spans[bottom] = nil
+
+	// If we're the last popper in the block, free the block.
+	if used := atomic.Xadd(&block.used, -1); used == 0 {
+		// Decrement spine length and clear the block's pointer.
+		atomic.Xadduintptr(&b.spineLen, ^uintptr(0) /* -1 */)
+		atomic.StorepNoWB(add(b.spine, sys.PtrSize*uintptr(top)), nil)
+
+		// Return the block to the block pool.
+		spanSetBlockPool.free(block)
+	}
 	return s
 }
 
@@ -174,3 +199,25 @@ func (b *spanSet) block(i int) []*mspan {
 	}
 	return spans
 }
+
+// spanSetBlockPool is a global pool of spanSetBlocks.
+var spanSetBlockPool spanSetBlockAlloc
+
+// spanSetBlockAlloc represents a concurrent pool of spanSetBlocks.
+type spanSetBlockAlloc struct {
+	stack lfstack
+}
+
+// alloc tries to grab a spanSetBlock out of the pool, and if it fails
+// persistentallocs a new one and returns it.
+func (p *spanSetBlockAlloc) alloc() *spanSetBlock {
+	if s := (*spanSetBlock)(p.stack.pop()); s != nil {
+		return s
+	}
+	return (*spanSetBlock)(persistentalloc(unsafe.Sizeof(spanSetBlock{}), cpu.CacheLineSize, &memstats.gc_sys))
+}
+
+// free returns a spanSetBlock back to the pool.
+func (p *spanSetBlockAlloc) free(block *spanSetBlock) {
+	p.stack.push(&block.lfnode)
+}