Re: tx_queue_size with tap device

[Michael S. Tsirkin]

On Thu, Jul 28, 2022 at 10:51:22AM +0200, Markus Frank wrote:
> Hello,
> 
> I have a few questions concerning these commits.
> 
> commit 9b02e1618cf26aa52cf786f215d757506dda14f8
> commit 2eef278b9e6326707410eed23be40e57f6c331b7
> commit 0ea5778f066ea5c5e73246a4c11f0773edc4c45d
> 
> Therefore I added the developers in CC.
> 
> Do backends other than vhost-user or vhost-vdpa really not support the
> maximum tx_queue_size and why?
> Example: When using tap as backend and adding NET_CLIENT_DRIVER_TAP as a case
> to the switch statement, so that it can use the maximum size, ethtool also 
> shows
> the configured tx_queue_size of 1024.
> 
> In the commit 9b02e1618cf2 is stated, that the maximum tx_queue_size for other
> backends is 512. Is this still the case? If yes shouldn't it be possible to
> default to that (512), if something higher (1024) is configured, instead of 
> 256?
> 
> Also is there another way to check/test the tx_queue_size than ethtool?
> 
> Thanks in advance.
> 
> Markus

This is qemu virtio queue size which limits two things at the same time:
1- how many packets in queue
2- how long can a scatter gather list be

2 is what we are trying to limit here and so it has nothing to do
with ethtool.

That did not change.

In practical terms linux guests will not push scatter gather lists
larger than 512 even if queue size is larger, so you most likely
can increase the tx queue size and things will still work,
they are just not guaranteed by the spec.

As for changing defaults whether that is a gain or a loss will
depend on the workload.

-- 
MST