[2/8] virtio-blk: Avoid zeroing every request structure

Message ID 1274361043-25862-3-git-send-email-kwolf@redhat.com
State New
Headers show

Commit Message

Kevin Wolf May 20, 2010, 1:10 p.m.
From: 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>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
 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;