Re: [dpdk-dev] [PATCH] vhost: adaptively batch small guest memory copies

[Tiwei Bie <tiwei.bie@intel.com>]

On Fri, Sep 01, 2017 at 11:45:42AM +0200, Maxime Coquelin wrote:
> On 08/24/2017 04:19 AM, Tiwei Bie wrote:
> > This patch adaptively batches the small guest memory copies.
> > By batching the small copies, the efficiency of executing the
> > memory LOAD instructions can be improved greatly, because the
> > memory LOAD latency can be effectively hidden by the pipeline.
> > We saw great performance boosts for small packets PVP test.
> > 
> > This patch improves the performance for small packets, and has
> > distinguished the packets by size. So although the performance
> > for big packets doesn't change, it makes it relatively easy to
> > do some special optimizations for the big packets too.
> 
> Do you mean that if we would batch unconditionnaly whatever the size,
> we see performance drop for larger (>256) packets?
> 

Yeah, you are right.

> Other question is about indirect descriptors, my understanding of the
> patch is that the number of batched copies is limited to the queue size.
> In theory, we could have more than that with indirect descriptors (first
> indirect desc for the vnet header, second one for the packet).
> 
> So in the worst case, we would have the first small copies being
> batched, but not the last ones if there are more than queue size.
> So, I think it works, but I'd like your confirmation.
> 

Yeah, you are right. If the number of small copies is larger than
the queue size, the last ones won't be batched any more.

> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > Signed-off-by: Zhihong Wang <zhihong.wang@intel.com>
> > Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
> > ---
> > This optimization depends on the CPU internal pipeline design.
> > So further tests (e.g. ARM) from the community is appreciated.
> 
> Agree, I think this is important to have it tested on ARM platforms at
> least to ensure it doesn't introduce a regression.
> 
> Adding Santosh, Jerin & Hemant in cc, who might know who could do the
> test.
> 

Thank you very much! :-)

Best regards,
Tiwei Bie