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virtio-blk: Avoid zeroing every request structure 

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>
This commit is contained in:
Stefan Hajnoczi 2010-05-14 22:52:30 +01:00 committed by Kevin Wolf
parent c53a7285b4
commit de6c8042ec
1 changed files with 3 additions and 1 deletions
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4
hw/virtio-blk.c
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@ -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;
}