[Qemu-devel] [PATCH 0/2] virtio-scsi: Optimizing request allocation

[Fam Zheng <famz@redhat.com>]

Zeroing is relatively expensive since we have big request structures.
VirtQueueElement (> 4k) and sense_buf (256 bytes) are two points to look at.

This visibly reduces overhead of request handling when testing with the
unmerged "null" driver and virtio-scsi dataplane. Before, the issue is very
obvious with perf top:

perf top -G -p `pidof qemu-system-x86_64`
-----------------------------------------
+  16.50%  libc-2.17.so             [.] __memset_sse2
+   2.28%  libc-2.17.so             [.] _int_malloc
+   2.25%  [vdso]                   [.] 0x0000000000000cd1
+   2.02%  [kernel]                 [k] _raw_spin_lock_irqsave
+   1.97%  libpthread-2.17.so       [.] pthread_mutex_lock
+   1.87%  libpthread-2.17.so       [.] pthread_mutex_unlock
+   1.81%  [kernel]                 [k] fget_light
+   1.70%  libc-2.17.so             [.] malloc

After, the high __memset_sse2 and _int_malloc is gone:

perf top -G -p `pidof qemu-system-x86_64`
-----------------------------------------
+   4.20%  [kernel]                 [k] vcpu_enter_guest
+   3.97%  [kernel]                 [k] vmx_vcpu_run
+   2.63%  [kernel]                 [k] _raw_spin_lock_irqsave
+   1.72%  [kernel]                 [k] native_read_msr_safe
+   1.65%  [kernel]                 [k] __srcu_read_lock
+   1.64%  [kernel]                 [k] _raw_spin_unlock_irqrestore
+   1.57%  [vdso]                   [.] 0x00000000000008d8
+   1.49%  libc-2.17.so             [.] _int_malloc
+   1.29%  libpthread-2.17.so       [.] pthread_mutex_unlock
+   1.26%  [kernel]                 [k] native_write_msr_safe

See the commit message of patch 2 for some fio test data.

Thanks,
Fam

Fam Zheng (2):
  scsi: Optimize scsi_req_alloc
  virtio-scsi: Optimize virtio_scsi_init_req

 hw/scsi/scsi-bus.c              |  7 +++++--
 hw/scsi/virtio-scsi.c           | 17 ++++++++++-------
 include/hw/scsi/scsi.h          | 21 ++++++++++++++-------
 include/hw/virtio/virtio-scsi.h |  1 +
 4 files changed, 30 insertions(+), 16 deletions(-)

-- 
1.9.3