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

[Stefan Hajnoczi]

On Tue, Feb 04, 2020 at 12:59:04PM +0300, Denis Plotnikov wrote:
> 
> 
> 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?

It's just called "vhost-scsi":

include/hw/virtio/vhost-scsi.h:#define TYPE_VHOST_SCSI "vhost-scsi"

> > 
> > 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?

qemu.git has contrib/vhost-user-blk/ and contrib/vhost-user-scsi/ if
you need to test those vhost-user devices without external dependencies.

Stefan

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