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

Chen, Tim C
2017-05-04T23:23:19Z

>-----Original Message-----
>From: Jens Axboe [mailto:axboe@fb.com]
>Sent: Thursday, May 04, 2017 2:29 PM
>To: Shaohua Li
>Cc: linux-mm@kvack.org; Andrew Morton; Kernel-team@fb.com; Chen, Tim C;
>Huang, Ying
>Subject: Re: [PATCH] swap: add block io poll in swapin path
>
>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.
>

The speedup is quite nice.
The high CPU utilization is somewhat of a concern. But this is directly
proportional to the poll time or latency of the drive's response. The latest generation of
SSD drive's latency is a factor of 7 or more compared to the previous one, so the poll time
could go down quite a bit, depending on what drive you were using in your test.
What is the latency and the kind of drive you're using?

Tim