[Qemu-devel] [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device

[Vadim Rozenfeld <vrozenfe@redhat.com>]

The following patch allows us to improve Windows virtio 
block driver performance on small size requests. 
Additionally, it leads to reducing of cpu usage on write IOs  

repository: /home/vadimr/work/win7/qemu
branch: master
commit 68290c4e9c96f345d544ca5d2b89f27a1e67e27a
Author: Vadim Rozenfeld <vadimr@localhost.localdomain>
Date:   Mon Jan 11 09:00:21 2010 +0200

[RFC][PATCH] small IOs performance for windows guests, running on top of
virtio block device

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index a2f0639..0e3a8d5 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -28,6 +28,7 @@ typedef struct VirtIOBlock
     char serial_str[BLOCK_SERIAL_STRLEN + 1];
     QEMUBH *bh;
     size_t config_size;
+    unsigned int pending;
 } VirtIOBlock;
 
 static VirtIOBlock *to_virtio_blk(VirtIODevice *vdev)
@@ -87,6 +88,8 @@ typedef struct VirtIOBlockReq
     struct VirtIOBlockReq *next;
 } VirtIOBlockReq;
 
+static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue
*vq);
+
 static void virtio_blk_req_complete(VirtIOBlockReq *req, int status)
 {
     VirtIOBlock *s = req->dev;
@@ -95,6 +98,11 @@ static void virtio_blk_req_complete(VirtIOBlockReq
*req, int status)
     virtqueue_push(s->vq, &req->elem, req->qiov.size +
sizeof(*req->in));
     virtio_notify(&s->vdev, s->vq);
 
+    if(--s->pending == 0) { 
+        virtio_queue_set_notification(s->vq, 1);
+        virtio_blk_handle_output(&s->vdev, s->vq);
+    }
+
     qemu_free(req);
 }
 
@@ -340,6 +348,9 @@ static void virtio_blk_handle_output(VirtIODevice
*vdev, VirtQueue *vq)
             exit(1);
         }
 
+        if(++s->pending == 1)
+            virtio_queue_set_notification(s->vq, 0);
+
         req->out = (void *)req->elem.out_sg[0].iov_base;
         req->in = (void *)req->elem.in_sg[req->elem.in_num -
1].iov_base;