Re: [RFC/RFT] [PATCH v3 0/4] Intel_pstate: HWP Dynamic performance boost

[Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>]

Hi Giovanni,

On Fri, 2018-06-01 at 13:32 +0200, Giovanni Gherdovich wrote:
> 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.
Thanks. 


-Srinivas
> 
> 
> 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
> > 
> >