Re: [PATCH] swap: add block io poll in swapin path

[Shaohua Li <shli@fb.com>]

On Thu, May 04, 2017 at 02:53:59PM -0600, Jens Axboe wrote:
> On 05/04/2017 02:42 PM, Shaohua Li wrote:
> > For fast flash disk, async IO could introduce overhead because of
> > context switch. block-mq now supports IO poll, which improves
> > performance and latency a lot. swapin is a good place to use this
> > technique, because the task is waitting for the swapin page to continue
> > execution.
> 
> Nitfy!
> 
> > In my virtual machine, directly read 4k data from a NVMe with iopoll is
> > about 60% better than that without poll. With iopoll support in swapin
> > patch, my microbenchmark (a task does random memory write) is about 10%
> > ~ 25% faster. CPU utilization increases a lot though, 2x and even 3x CPU
> > utilization. This will depend on disk speed though. While iopoll in
> > swapin isn't intended for all usage cases, it's a win for latency
> > sensistive workloads with high speed swap disk. block layer has knob to
> > control poll in runtime. If poll isn't enabled in block layer, there
> > should be no noticeable change in swapin.
> 
> Did you try with hybrid polling enabled? We should be able to achieve
> most of the latency win at much less CPU cost with that.

Hybrid poll is much slower than classic in my test, I tried different settings.
maybe because this is a vm though. 

Thanks,
Shaohua

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