Re: [dpdk-dev] [PATCH v5 3/4] vhost: support async dequeue for split ring

[David Marchand <david.marchand@redhat.com>]

On Fri, Jul 16, 2021 at 3:45 PM Hu, Jiayu <jiayu.hu@intel.com> wrote:
> > - I don't like this threshold, this is too low level and most users will only see
> > the shiny aspect "better performance" without understanding the
> > consequences.
> > By default, it leaves the door open to a _bad_ behavior, that is packet
> > reordering.
> > At a very minimum, strongly recommend to use 0 in the API.
>
> That's a good point. But there are some reasons of open this value to users:
> - large packets will block small packets, like control packets of TCP.
> - dma efficiency. We usually see 20~30% drops because of offloading 64B copies to
> dma engine.
> - the threshold is not only related to hardware, but also application. The value decides
> which copies are assigned to which worker, the CPU or the DMA. As async vhost works
> in an asynchronous way, the threshold value decides how many works can be done in
> parallel. It's not only about what DMA engine and what platform we use, but also what
> computation the CPU has been assigned. Different users will have different values.
>
> I totally understand the worry about reordering. But simple iperf tests show positive
> results with setting threshold in our lab. We need more careful tests before modifying
> it, IMHO.

If you need more time, then please take it.
The dma generic API could have an impact on this feature too.

Why the rush for merging this now?


-- 
David Marchand