[Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

The VirtIOBlockRequest structure is about 40 KB in size.  This patch
avoids zeroing every request by only initializing fields that are read.
The other fields are either written to or may not be used at all.

Oprofile shows about 10% of CPU samples in memset called by
virtio_blk_alloc_request().  The workload is
dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
times.  This patch makes memset disappear to the bottom of the profile.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
This applies to qemu.git and qemu-kvm.git.

A related change would be a pool of requests to avoid malloc/free for every
single request.  That's a separate change and malloc/free do not show up at the
top of the profile, so I am not introducing a pool yet.

 hw/virtio-blk.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index b05d15e..d270225 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -105,8 +105,10 @@ static void virtio_blk_flush_complete(void *opaque, int ret)
 
 static VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
 {
-    VirtIOBlockReq *req = qemu_mallocz(sizeof(*req));
+    VirtIOBlockReq *req = qemu_malloc(sizeof(*req));
     req->dev = s;
+    req->qiov.size = 0;
+    req->next = NULL;
     return req;
 }
 
-- 
1.7.1