[PATCH v1 2/4] 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 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/core/machine.c          | 3 +++
 include/hw/virtio/virtio.h | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 3e288bfceb..8bc401d8b7 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -28,6 +28,9 @@
 #include "hw/mem/nvdimm.h"
 
 GlobalProperty hw_compat_4_2[] = {
+    { "virtio-blk-device", "queue-size", "128"},
+    { "virtio-scsi-device", "virtqueue_size", "128"},
+    { "vhost-blk-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/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index a66ea2368b..16d540e390 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -48,7 +48,7 @@ size_t virtio_feature_get_config_size(VirtIOFeature *features,
 typedef struct VirtQueue VirtQueue;
 
 #define VIRTQUEUE_MAX_SIZE 1024
-#define VIRTQUEUE_DEFAULT_SIZE 128
+#define VIRTQUEUE_DEFAULT_SIZE 256
 
 typedef struct VirtQueueElement
 {
-- 
2.17.0