Re: RFC: control virtqueue size by the vdpa tool

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 30, 2022 at 02:04:55PM -0700, Si-Wei Liu wrote:
> 
> 
> On 8/30/2022 12:58 PM, Michael S. Tsirkin wrote:
> > On Tue, Aug 30, 2022 at 06:22:31AM +0000, Eli Cohen wrote:
> > > 
> > > Hi,
> > > 
> > > 
> > > I have been experimenting with different queue sizes with mlx5_vdpa and noticed
> > > that queue size can affect performance.
> > > 
> > > I would like to propose an extension to vdpa tool to allow to specify the queue
> > > size. Valid values will conform to the max of 32768 specified by the spec.
> > > 
> > > 
> > > “vdpa mgmtdev show” will have another line specifying the valid range for a
> > > management device which could be narrower than the spec allows. This range will
> > > be valid for data queues only (not for control VQ).
> > > 
> > > Another line will display the default queue size
> > > 
> > > 
> > > Example:
> > > 
> > > $ vdpa mgmtdev show
> > > 
> > > auxiliary/mlx5_core.sf.6:
> > > 
> > >    supported_classes net
> > > 
> > >    max_supported_vqs 65
> > > 
> > >    dev_features CSUM GUEST_CSUM MTU HOST_TSO4 HOST_TSO6 STATUS CTRL_VQ CTRL_VLAN
> > > MQ CTRL_MAC_ADDR VERSION_1 ACCESS_PLATFORM
> > > 
> > >    data queue range 256-4096
> > > 
> > >    default queue size 256
> > > 
> > > 
> > > When you create the vdpa device you can specify the requested value:
> > > 
> > > $ vdpa dev add name vdpa-a mgmtdev auxiliary/mlx5_core.sf.6 max_vqp 1 mtu 9000
> > > queue_size 1024
> > > 
> > 
> > A follow up question: isn't it enough to control the size
> > from qemu? do we need ability to control it at the kernel level?
> > 
> Right, I think today we can optionally control the queue size from qemu via
> rx_queue_size or tx_queue_size, but it has a limit of 1024 (btw why it has
> such limit, which is relatively lower in my opinion). I think what was
> missing for QEMU is to query the max number of queue size that the hardware
> can support from the backend.
> 
> -Siwei
> 

okay sure. my question is how important is it to control it in the
kernel?

-- 
MST

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