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
Signed-off-by: Denis Plotnikov
---
v3:
* typos fixed
v2:
* seg_max default value changing removed
---
hw/block/virtio-blk.c | 2 +-
hw/core/machine.c | 2 ++
hw/scsi/virtio-scsi.c | 2 +-
3 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 09f46ed85f..142863a3b2 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -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..472bbd233b 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -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
