Re: io_uring zero-copy send test results

[vitalif@yourcmc.ru]

> fwiw, -z1 -b1 is the default, i.e. zc and fixed buffers

Yes, I know. :-) that's why I re-ran tests with -b 0 the second time.

> Sounds like another case of iommu being painfully slow. The difference
> is that while copying normal sends coalesce data into nice big contig
> buffers, but zerocopy has to deal with whatever pages it's given. That's
> 32KB vs 4KB, and the worst case scenario you get 8x more frags (and skbs)
> and 8x iommu mappings for zerocopy.

Problem is that on EPYC it's slow even with 64k buffers. Being slow is rather expectable with 4k buffers, but 64k...

> Try huge pages and see if it helps, it's -l1 in the benchmark. I can
> also take a look at adding pre-mapped buffers again.
> 
> Perf profiles would also be useful to have if you can grab and post
> them.

I.e. flamegraphs?