Re: [PATCH v1 2/4] virtio: increase virtuqueue size for virtio-scsi and virtio-blk

[Denis Plotnikov]

On 30.01.2020 17:58, Stefan Hajnoczi wrote:
> 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 <address@hidden>
> > Signed-off-by: Denis Plotnikov <address@hidden>
> > ---
> >   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. ;-)
So in this case the line:

{ "vhost-blk-device", "seg_max_adjust", "off"},

introduced by my patch:

commit 1bf8a989a566b2ba41c197004ec2a02562a766a4
Author: Denis Plotnikov <address@hidden>
Date:   Fri Dec 20 17:09:04 2019 +0300

    virtio: make seg_max virtqueue size dependent

is also wrong. It should be:

{ "vhost-scsi-device", "seg_max_adjust", "off"},

Am I right?

> 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.
It seems to be so. We also have the test checking those settings:
tests/acceptance/virtio_seg_max_adjust.py
For now it checks virtio-scsi-pci and virtio-blk-pci.
I'm going to extend it for the virtqueue size checking.
If I change vhost-user-blk, vhost-user-scsi and vhost-scsi it's worth
to check those devices too. But I don't know how to form a command line
for that 3 devices since they should involve some third party components as
backends (kernel modules, DPDK, etc.) and they seems to be not available 
in the
qemu git.
Is there any way to do it with some qit.qemu available stubs or 
something else?
If so, could you please point out the proper way to do it?

Thanks!
Denis