aio: use g_slice_alloc() for AIOCB pooling

AIO control blocks are frequently acquired and released because each aio
request involves at least one AIOCB.  Therefore, we pool them to avoid
heap allocation overhead.

The problem with the freelist approach in AIOPool is thread-safety.  If
we want BlockDriverStates to associate with AioContexts that execute in
multiple threads, then a global freelist becomes a problem.

This patch drops the freelist and instead uses g_slice_alloc() which is
tuned for per-thread fixed-size object pools.  qemu_aio_get() and
qemu_aio_release() are now thread-safe.

Note that the change from g_malloc0() to g_slice_alloc() should be safe
since the freelist reuse case doesn't zero the AIOCB either.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block.c b/block.c
index da1fdca0e00d9f3e19bc7fc3faec8eb5232715e8..ea0f7d8367ddc4216f9e8050d52dcbe94e1b1cf9 100644 (file)
--- a/block.c
+++ b/block.c
@@ -3909,13 +3909,8 @@ void *qemu_aio_get(AIOPool *pool, BlockDriverState *bs,
 {
     BlockDriverAIOCB *acb;
 
-    if (pool->free_aiocb) {
-        acb = pool->free_aiocb;
-        pool->free_aiocb = acb->next;
-    } else {
-        acb = g_malloc0(pool->aiocb_size);
-        acb->pool = pool;
-    }
+    acb = g_slice_alloc(pool->aiocb_size);
+    acb->pool = pool;
     acb->bs = bs;
     acb->cb = cb;
     acb->opaque = opaque;
@@ -3924,10 +3919,8 @@ void *qemu_aio_get(AIOPool *pool, BlockDriverState *bs,
 
 void qemu_aio_release(void *p)
 {
-    BlockDriverAIOCB *acb = (BlockDriverAIOCB *)p;
-    AIOPool *pool = acb->pool;
-    acb->next = pool->free_aiocb;
-    pool->free_aiocb = acb;
+    BlockDriverAIOCB *acb = p;
+    g_slice_free1(acb->pool->aiocb_size, acb);
 }
 
 /**************************************************************/

diff --git a/qemu-aio.h b/qemu-aio.h
index 111b0b3f0a72de10cc2e6a5558b01dcf6ba1a65d..b29c509f2760956de5d210c8a1bd1b18a20281ee 100644 (file)
--- a/qemu-aio.h
+++ b/qemu-aio.h
@@ -24,7 +24,6 @@ typedef void BlockDriverCompletionFunc(void *opaque, int ret);
 typedef struct AIOPool {
     void (*cancel)(BlockDriverAIOCB *acb);
     size_t aiocb_size;
-    BlockDriverAIOCB *free_aiocb;
 } AIOPool;
 
 struct BlockDriverAIOCB {
@@ -32,7 +31,6 @@ struct BlockDriverAIOCB {
     BlockDriverState *bs;
     BlockDriverCompletionFunc *cb;
     void *opaque;
-    BlockDriverAIOCB *next;
 };
 
 void *qemu_aio_get(AIOPool *pool, BlockDriverState *bs,