NVMe APST high latency power states being skipped

[Christoph Hellwig]

On Thu, Jun 01, 2017 at 04:19:26PM +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.

That doesn't explain why we need more than a minute more for the
compile.  How reproducible are these numbers, btw?