On Tue, Feb 11, 2020 at 05:14:14PM +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 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);

This value must not change on older machine types.  So does this patch
need to turn seg-max-adjust *on* in hw_compat_4_2 so that old machine
types get 126 instead of 254?

>      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
> 
>