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

[Denis Plotnikov]

On 12.02.2020 18:43, Stefan Hajnoczi wrote:
> 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 <address@hidden>
> > Signed-off-by: Denis Plotnikov <address@hidden>
> > ---
> >   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.
Yes, that's true, but ..
> 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?
If we set seg-max-adjust "on" in older machine types, the setups using 
them and having queue_sizes set , for example, 1024 will also set 
seg_max to 1024 - 2 which isn't the expected behavior: older mt didn't 
change seg_max in that case and stuck with 128 - 2.
So, should we, instead, leave the default 128 - 2, for seg_max?

Denis
> >       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