NVMe APST high latency power states being skipped

[Kai-Heng Feng]

On Fri, May 26, 2017 at 5:25 PM, Christoph Hellwig <hch at infradead.org> wrote:
> On Thu, May 25, 2017 at 04:21:07PM +0800, Kai-Heng Feng wrote:
>> I did some benchmark on the high latency SanDisk A400:
>> Kernel compilation, no PS3/PS4:
>> real    23m36.466s
>> user    115m49.944s
>> sys     10m58.352s
>>
>> Kernel compilation, allow PS3/PS4:
>> real    24m40.308s
>> user    116m12.600s
>> sys     11m47.484s
>
> That's quite a bit of a slow down.  Can we play a bit with the
> entry latency (maybe just for PS4) so that we still get into the
> modes, but not as quickly?

I changed the "Idle Time Prior to Transition" from 55000ms to 110000ms,
The time to compile kernel source is roughly the same.

My guess is that since the filesystem is constantly doing I/O while
compiling kernel, nvme never (or rarely) hits PS4.

Probably need to think a better scenario for power saving latency...