Re: [Lsf-pc] [LSF/MM TOPIC][LSF/MM ATTEND] NAPI polling for block drivers

Johannes Thumshirn
2017-01-12T18:59:48Z

On Thu, Jan 12, 2017 at 04:41:00PM +0200, Sagi Grimberg wrote:
>
> >>**Note: when I ran multiple threads on more cpus the performance
> >>degradation phenomenon disappeared, but I tested on a VM with
> >>qemu emulation backed by null_blk so I figured I had some other
> >>bottleneck somewhere (that's why I asked for some more testing).
> >
> >That could be because of the vmexits as every MMIO access in the guest
> >triggers a vmexit and if you poll with a low budget you do more MMIOs hence
> >you have more vmexits.
> >
> >Did you do testing only in qemu or with real H/W as well?
>
> I tried once. IIRC, I saw the same phenomenons...

JFTR I tried my AHCI irq_poll patch on the Qemu emulation and the read
throughput dropped from ~1GB/s to ~350MB/s. But this can be related to
Qemu's I/O wiredness as well I think. I'll try on real hardware tomorrow.

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