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[Qemu-devel] [PATCH] qemu-kvm: clear only essential parts of VirtIOBlock

From: Saul Tamari
Subject: [Qemu-devel] [PATCH] qemu-kvm: clear only essential parts of VirtIOBlockReq on object allocation - RESUBMIT
Date: Mon, 9 Nov 2009 20:04:34 +0200

This patch reduces the size of memory being cleared on every virtio-blk IO.

Improve number of IOPS when using avirtio-blk device.

On every virtio-blk IO command passed to QEMU, virtio_blk_alloc_request()
allocates and clears (with qemu_mallocz()) a VirtIOBlockReq object.
The sizeof(VirtIOBlockReq) equals 41040 bytes on my x86-64 machine.
By moving the 'elem' variable to the end of VirtIOBlockReq and
clearing only upto the address of the 'elem.in_addr' field, the
memset() call now clears only 80 bytes.

Signed-off-by: Saul Tamari <address@hidden>

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 2630b99..de74b00 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -79,12 +79,13 @@ static inline void virtio_identify_template(struct
virtio_blk_config *bc)
 typedef struct VirtIOBlockReq
     VirtIOBlock *dev;
-    VirtQueueElement elem;
     struct virtio_blk_inhdr *in;
     struct virtio_blk_outhdr *out;
     struct virtio_scsi_inhdr *scsi;
     QEMUIOVector qiov;
     struct VirtIOBlockReq *next;
+    /* Members that need clearing, must be added prior to elem */
+    VirtQueueElement elem;
 } VirtIOBlockReq;

 static void virtio_blk_req_complete(VirtIOBlockReq *req, int status)
@@ -139,7 +140,8 @@ 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));
+    memset(req, 0, offsetof(VirtIOBlockReq, elem.in_addr[0]));
     req->dev = s;
     return req;

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