[PATCH] qemu-kvm: clear only essential parts of VirtIOBlockReq on object allocation

Saul Tamari
2009-11-04T17:41:03Z

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

On every virtio-blk IO 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 <st...@gmail.com>
---

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 2630b99..fd44371 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -79,12 +79,12 @@ 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;
+ VirtQueueElement elem;
} VirtIOBlockReq;

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