[PATCH v2] virtio: increase virtuqueue size for virtio-scsi and virtio-blk

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

The goal is to reduce the amount of requests issued by a guest on
1M reads/writes. This rises the performance up to 4% on that kind of
disk access pattern.

The maximum chunk size to be used for the guest disk accessing is
limited with seg_max parameter, which represents the max amount of
pices in the scatter-geather list in one guest disk request.

Since seg_max is virqueue_size dependent, increasing the virtqueue
size increases seg_max, which, in turn, increases the maximum size
of data to be read/write from a guest disk.

More details in the original problem statment:
https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg03721.html

Suggested-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 hw/block/virtio-blk.c | 4 ++--
 hw/core/machine.c     | 2 ++
 hw/scsi/virtio-scsi.c | 4 ++--
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 09f46ed85f..6df3a7a6df 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -914,7 +914,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
     memset(&blkcfg, 0, sizeof(blkcfg));
     virtio_stq_p(vdev, &blkcfg.capacity, capacity);
     virtio_stl_p(vdev, &blkcfg.seg_max,
-                 s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 128 - 2);
+                 s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 256 - 2);
     virtio_stw_p(vdev, &blkcfg.geometry.cylinders, conf->cyls);
     virtio_stl_p(vdev, &blkcfg.blk_size, blk_size);
     virtio_stw_p(vdev, &blkcfg.min_io_size, conf->min_io_size / blk_size);
@@ -1272,7 +1272,7 @@ static Property virtio_blk_properties[] = {
     DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
                     true),
     DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
-    DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 128),
+    DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 256),
     DEFINE_PROP_BOOL("seg-max-adjust", VirtIOBlock, conf.seg_max_adjust, true),
     DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
                      IOThread *),
diff --git a/hw/core/machine.c b/hw/core/machine.c
index 2501b540ec..3427d6cf4c 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -28,6 +28,8 @@
 #include "hw/mem/nvdimm.h"
 
 GlobalProperty hw_compat_4_2[] = {
+    { "virtio-blk-device", "queue-size", "128"},
+    { "virtio-scsi-device", "virtqueue_size", "128"},
     { "virtio-blk-device", "x-enable-wce-if-config-wce", "off" },
     { "virtio-blk-device", "seg-max-adjust", "off"},
     { "virtio-scsi-device", "seg_max_adjust", "off"},
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 3b61563609..b38f50a429 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -660,7 +660,7 @@ static void virtio_scsi_get_config(VirtIODevice *vdev,
 
     virtio_stl_p(vdev, &scsiconf->num_queues, s->conf.num_queues);
     virtio_stl_p(vdev, &scsiconf->seg_max,
-                 s->conf.seg_max_adjust ? s->conf.virtqueue_size - 2 : 128 - 2);
+                 s->conf.seg_max_adjust ? s->conf.virtqueue_size - 2 : 256 - 2);
     virtio_stl_p(vdev, &scsiconf->max_sectors, s->conf.max_sectors);
     virtio_stl_p(vdev, &scsiconf->cmd_per_lun, s->conf.cmd_per_lun);
     virtio_stl_p(vdev, &scsiconf->event_info_size, sizeof(VirtIOSCSIEvent));
@@ -965,7 +965,7 @@ static void virtio_scsi_device_unrealize(DeviceState *dev, Error **errp)
 static Property virtio_scsi_properties[] = {
     DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
     DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
-                                         parent_obj.conf.virtqueue_size, 128),
+                                         parent_obj.conf.virtqueue_size, 256),
     DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSI,
                       parent_obj.conf.seg_max_adjust, true),
     DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
-- 
2.17.0