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

[Jens Axboe <axboe@fb.com>]

On 05/04/2017 03:27 PM, Shaohua Li wrote:
> 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. 

It's probably a vm issue, I bet the timed sleep are just too slow to be
useful in a vm.

-- 
Jens Axboe

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