Date:   Fri, 1 Jun 2018 13:32:09 +0200
From:   Giovanni Gherdovich <ggherdovich@...e.cz>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     lenb@...nel.org, rjw@...ysocki.net, peterz@...radead.org,
        mgorman@...hsingularity.net, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com,
        viresh.kumar@...aro.org
Subject: Re: [RFC/RFT] [PATCH v3 0/4] Intel_pstate: HWP Dynamic performance
 boost

On Thu, May 31, 2018 at 03:51:39PM -0700, Srinivas Pandruvada wrote:
> v3
> - Removed atomic bit operation as suggested.
> - Added description of contention with user space.
> - Removed hwp cache, boost utililty function patch and merged with util callback
>   patch. This way any value set is used somewhere.
> 
> Waiting for test results from Mel Gorman, who is the original reporter.
> 

Hello Srinivas,

thanks for this series. I'm testing it on behalf of Mel; while I'm waiting for
more benchmarks to finish, an initial report I've got from dbench on ext4
looks very promising. Good!
Later when I have it all I'll post my detailed results.


Giovanni Gherdovich
SUSE Labs


> v2
> This is a much simpler version than the previous one and only consider IO
> boost, using the existing mechanism. There is no change in this series
> beyond intel_pstate driver.
> 
> Once PeterZ finishes his work on frequency invariant, I will revisit
> thread migration optimization in HWP mode.
> 
> Other changes:
> - Gradual boost instead of single step as suggested by PeterZ.
> - Cross CPU synchronization concerns identified by Rafael.
> - Split the patch for HWP MSR value caching as suggested by PeterZ.
> 
> Not changed as suggested:
> There is no architecture way to identify platform with Per-core
> P-states, so still have to enable feature based on CPU model.
> 
> -----------
> v1
> 
> This series tries to address some concern in performance particularly with IO
> workloads (Reported by Mel Gorman), when HWP is using intel_pstate powersave
> policy.
> 
> Background
> HWP performance can be controlled by user space using sysfs interface for
> max/min frequency limits and energy performance preference settings. Based on
> workload characteristics these can be adjusted from user space. These limits
> are not changed dynamically by kernel based on workload.
> 
> By default HWP defaults to energy performance preference value of 0x80 on
> majority of platforms(Scale is 0-255, 0 is max performance and 255 is min).
> This value offers best performance/watt and for majority of server workloads
> performance doesn't suffer. Also users always have option to use performance
> policy of intel_pstate, to get best performance. But user tend to run with
> out of box configuration, which is powersave policy on most of the distros.
> 
> In some case it is possible to dynamically adjust performance, for example,
> when a CPU is woken up due to IO completion or thread migrate to a new CPU. In
> this case HWP algorithm will take some time to build utilization and ramp up
> P-states. So this may results in lower performance for some IO workloads and
> workloads which tend to migrate. The idea of this patch series is to
> temporarily boost performance dynamically in these cases. This is only
> applicable only when user is using powersave policy, not in performance policy.
> 
> Results on a Skylake server:
> 
> Benchmark                       Improvement %
> ----------------------------------------------------------------------
> dbench                          50.36
> thread IO bench (tiobench)      10.35
> File IO                         9.81
> sqlite                          15.76
> X264 -104 cores                 9.75
> 
> Spec Power                      (Negligible impact 7382 Vs. 7378)
> Idle Power                      No change observed
> -----------------------------------------------------------------------
> 
> HWP brings in best performace/watt at EPP=0x80. Since we are boosting
> EPP here to 0, the performance/watt drops upto 10%. So there is a power
> penalty of these changes.
> 
> Also Mel Gorman provided test results on a prior patchset, which shows
> benifits of this series.
> 
> Srinivas Pandruvada (4):
>   cpufreq: intel_pstate: Add HWP boost utility and sched util hooks
>   cpufreq: intel_pstate: HWP boost performance on IO wakeup
>   cpufreq: intel_pstate: New sysfs entry to control HWP boost
>   cpufreq: intel_pstate: enable boost for SKX
> 
>  drivers/cpufreq/intel_pstate.c | 177 ++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 173 insertions(+), 4 deletions(-)
> 
> -- 
> 2.13.6
> 
> 

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