On Thu, Feb 13, 2020 at 12:28:25PM +0300, Denis Plotnikov wrote:
> 
> 
> On 13.02.2020 12:08, Stefan Hajnoczi wrote:
> > On Thu, Feb 13, 2020 at 11:08:35AM +0300, Denis Plotnikov wrote:
> > > 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 <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.
> > > 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?
> > Argh!  Good point :-).
> > 
> > How about a seg_max_default property that is initialized to 254 for
> > modern machines and 126 to old machines?
> Hmm, but we'll achieve the same but with more code changes, don't we?
> 254 is because the queue-size is 256. We gonna leave 128-2 for older machine
> types
> just for not breaking anything. All other seg_max adjustment is provided by
> seg_max_adjust which is "on" by default in modern machine types.
> 
> to summarize:
> 
> modern mt defaults:
> seg_max_adjust = on
> queue_size = 256
> 
> => default seg_max = 254
> => changing queue-size will change seg_max = queue_size - 2
> 
> old mt defaults:
> seg_max_adjust = off
> queue_size = 128
> 
> => default seg_max = 126
> => changing queue-size won't change seg_max, it's always = 126 like it was
> before

You're right!  The only strange case is a modern machine type with
seg_max_adjust=off, where queue_size will be 256 but seg_max will be
126.  But no user would want to disable seg_max_adjust, so it's okay.

I agree with you that the line of code can remain unchanged:

  /*
   * Only old machine types use seg_max_adjust=off and there the default
   * value of queue_size is 128.
   */
  virtio_stl_p(vdev, &blkcfg.seg_max,
               s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 128 - 2);

Stefan