On Wednesday, February 24, 2016 09:03:01 AM Mel Gorman wrote:

On Tue, Feb 23, 2016 at 01:50:34PM -0800, Srinivas Pandruvada wrote:

quoted

On Tue, 2016-02-23 at 14:29 +0000, Mel Gorman wrote:

quoted

Added a suggested change from Doug Smythies and can add a Signed-off-
by
if Doug is ok with that.

Changelog since v1
o Remove divide that is likely unnecessary			(ds
mythies)
o Rebase on top of linux-pm/linux-next

The PID relies on samples of equal time but this does not apply for
deferrable timers when the CPU is idle. intel_pstate checks if the
actual
duration between samples is large and if so, the "busyness" of the
CPU
is scaled.

This assumes the delay was a deferred timer but a workload may simply
have
been idle for a short time if it's context switching between a server
and
client or waiting very briefly on IO. It's compounded by the problem
that
server/clients migrate between CPUs due to wake-affine trying to
maximise
hot cache usage. In such cases, the cores are not considered busy and
the
frequency is dropped prematurely.

This patch increases the hold-off value before the busyness is
scaled. It
was selected based simply on testing until the desired result was
found.
Tests were conducted with workloads that are either client/server
based
or short-lived IO.

Attached specpower comparison for Haswell EP Grantley server. 

So this looks like a bust in terms of specpower. It is incredibly
unfortunate though. There are basic workloads that are simply performing
way below what the CPU is capable of unless the user is either willing
to tune power management or pin tasks to CPUs and hope for the best.
Ideally we want to reduce those forum postings that suggest disabling
intel_pstate entirely or setting performance.

Given that I'm very weak in the intel_pstate driver in general and was
relying on bisection to find problem commits, are there any others with
"have your cake and eat it twice" options? Ideally it would restore
performance to simple client/server workloads and ones that idle briefly
on IO without getting red flagged by specpower.

Srinivas is working on using utilization data from the scheduler in
intel_pstate, which I think is the way to go to improve performance.

For example, we may react to increases in utilization reported by the
scheduler by ramping up the P-state more aggressivly and similar.  Since
we're now going to get the utilization numbers as soon as they become
available, we should be able to react changes in them right away.

Thanks,
Rafael