Re: [PATCH v2] cpufreq: intel_pstate: Change the calculation of next pstate
From: Dirk Brandewie
Date: Thu May 01 2014 - 21:48:19 EST
On 05/01/2014 04:18 PM, Rafael J. Wysocki wrote:
On Thursday, May 01, 2014 02:30:42 PM Dirk Brandewie wrote:
On 05/01/2014 02:00 PM, Stratos Karafotis wrote:
Currently the driver calculates the next pstate proportional to
core_busy factor, scaled by the ratio max_pstate / current_pstate.
Using the scaled load (core_busy) to calculate the next pstate
is not always correct, because there are cases that the load is
independent from current pstate. For example, a tight 'for' loop
through many sampling intervals will cause a load of 100% in
every pstate.
So, change the above method and calculate the next pstate with
the assumption that the next pstate should not depend on the
current pstate. The next pstate should only be directly
proportional to measured load.
Tested on Intel i7-3770 CPU @ 3.40GHz.
Phoronix benchmark of Linux Kernel Compilation 3.1 test shows an
increase ~1.5% in performance. Below the test results using turbostat
(5 iterations):
Without patch:
Ph. avg Time Total time PkgWatt Total Energy
79.63 266.416 57.74 15382.85984
79.63 265.609 57.87 15370.79283
79.57 266.994 57.54 15362.83476
79.53 265.304 57.83 15342.53032
79.71 265.977 57.76 15362.83152
avg 79.61 266.06 57.74 15364.36985
With patch:
Ph. avg Time Total time PkgWatt Total Energy
78.23 258.826 59.14 15306.96964
78.41 259.110 59.15 15326.35650
78.40 258.530 59.26 15320.48780
78.46 258.673 59.20 15313.44160
78.19 259.075 59.16 15326.87700
avg 78.34 258.842 59.18 15318.82650
The total test time reduced by ~2.6%, while the total energy
consumption during a test iteration reduced by ~0.35%
Signed-off-by: Stratos Karafotis <stratosk@xxxxxxxxxxxx>
---
Changes v1 -> v2
- Enhance change log as Rafael and Viresh suggested
drivers/cpufreq/intel_pstate.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 0999673..8e309db 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -608,28 +608,27 @@ static inline void intel_pstate_set_sample_time(struct cpudata *cpu)
mod_timer_pinned(&cpu->timer, jiffies + delay);
}
-static inline int32_t intel_pstate_get_scaled_busy(struct cpudata *cpu)
+static inline int32_t intel_pstate_get_busy(struct cpudata *cpu)
{
- int32_t core_busy, max_pstate, current_pstate;
+ int32_t core_busy, max_pstate;
core_busy = cpu->sample.core_pct_busy;
max_pstate = int_tofp(cpu->pstate.max_pstate);
- current_pstate = int_tofp(cpu->pstate.current_pstate);
- core_busy = mul_fp(core_busy, div_fp(max_pstate, current_pstate));
+ core_busy = mul_fp(core_busy, max_pstate);
NAK, The goal of this code is to find out how busy the core is at the current
P state. This change will return a value WAY too high.
Assume core_busy is 100 and the max non-turbo P state is 34 (3.4GHz) this code
would return a busy value of 3400. The PID is trying to keep the busy value
at the setpoint any value of ~3% will drive the P state to the highest turbo
P state in this example.
Well, the problem is that the numbers above indicate an improvement in energy
efficiency as a result of this patch and we need to explain that result.
The performance governor is the best option for this workload.
This change will give you the highest trubo for all but very idle work loads.
Lets say you have a processor with max P state of 3.4GHz The current P state
is 1.6 GHz so if the processor was 100% in C0 the core_busy values would be
47% This number scaled would be 100%. With the change above the PID would be
reacting to a load of 1598%. APERF/MPERF give you the percent of entire
core scaling it lets you find out how busy your are within the cureent P state.
--Dirk
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