On Wed, Jan 29, 2020 at 05:07:00PM +0300, Denis Plotnikov wrote:
> 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"},

vhost-blk-device?!  Who has this?  It's not in qemu.git so please omit
this line. ;-)

On the other hand, do you want to do this for the vhost-user-blk,
vhost-user-scsi, and vhost-scsi devices that exist in qemu.git?  Those
devices would benefit from better performance too.