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virtio-scsi: Optimize virtio_scsi_init_req 

The VirtQueueElement is a very big structure (>48k!), since it will be
initialzed by virtqueue_pop, we can save the expensive zeroing here.

This saves a few microseconds per request in my test:

[fio-test]      rw         bs         iodepth    jobs       bw         iops       latency
--------------------------------------------------------------------------------------------
Before          read       4k         1          1          110        28269      34
After           read       4k         1          1          131        33745      28

Whereas,

virtio-blk      read       4k         1          1          217        55673      16

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Fam Zheng 2014-09-16 15:20:18 +08:00 committed by Paolo Bonzini
parent 61e68b3fbd
commit faf1e1fb4c
1 changed files with 17 additions and 7 deletions
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24
hw/scsi/virtio-scsi.c
View File

@ -24,12 +24,19 @@
typedef struct VirtIOSCSIReq {
VirtIOSCSI *dev;
VirtQueue *vq;
VirtQueueElement elem;
QEMUSGList qsgl;
QEMUIOVector resp_iov;
/* Note:
* - fields before elem are initialized by virtio_scsi_init_req;
* - elem is uninitialized at the time of allocation.
* - fields after elem are zeroed by virtio_scsi_init_req.
* */
VirtQueueElement elem;
SCSIRequest *sreq;
size_t resp_size;
enum SCSIXferMode mode;
QEMUIOVector resp_iov;
union {
VirtIOSCSICmdResp cmd;
VirtIOSCSICtrlTMFResp tmf;
@ -68,23 +75,26 @@ static inline SCSIDevice *virtio_scsi_device_find(VirtIOSCSI *s, uint8_t *lun)
static VirtIOSCSIReq *virtio_scsi_init_req(VirtIOSCSI *s, VirtQueue *vq)
{
VirtIOSCSIReq *req;
VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
req = g_malloc0(sizeof(*req) + vs->cdb_size);
VirtIOSCSICommon *vs = (VirtIOSCSICommon *)s;
const size_t zero_skip = offsetof(VirtIOSCSIReq, elem)
+ sizeof(VirtQueueElement);
req = g_slice_alloc(sizeof(*req) + vs->cdb_size);
req->vq = vq;
req->dev = s;
req->sreq = NULL;
qemu_sglist_init(&req->qsgl, DEVICE(s), 8, &address_space_memory);
qemu_iovec_init(&req->resp_iov, 1);
memset((uint8_t *)req + zero_skip, 0, sizeof(*req) - zero_skip);
return req;
}
static void virtio_scsi_free_req(VirtIOSCSIReq *req)
{
VirtIOSCSICommon *vs = (VirtIOSCSICommon *)req->dev;
qemu_iovec_destroy(&req->resp_iov);
qemu_sglist_destroy(&req->qsgl);
g_free(req);
g_slice_free1(sizeof(*req) + vs->cdb_size, req);
}
static void virtio_scsi_complete_req(VirtIOSCSIReq *req)