[PATCH 1/1] intel_pstate: Increase hold-off time before samples are scaled v2
From: Mel Gorman
Date: Tue Feb 23 2016 - 09:29:51 EST
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 (dsmythies)
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.
dbench4
4.5.0-rc4 4.5.0-rc4
pmnext-20160219 sample-v2r3
Hmean mb/sec-1 322.84 ( 0.00%) 322.40 ( -0.14%)
Hmean mb/sec-2 604.32 ( 0.00%) 615.03 ( 1.77%)
Hmean mb/sec-4 680.53 ( 0.00%) 707.78 ( 4.00%)
Hmean mb/sec-8 705.40 ( 0.00%) 742.36 ( 5.24%)
4.5.0-rc4 4.5.0-rc4
pmnext-20160219 sample-v2r3
User 1483.79 1393.30
System 3847.87 3652.56
Elapsed 5406.79 5405.82
4.5.0-rc4 4.5.0-rc4
pmnext-20160219 sample-v2r3
Mean %Busy 27.59 26.21
Mean CPU%c1 43.37 44.21
Mean CPU%c3 7.30 7.67
Mean CPU%c6 21.74 21.91
Mean CPU%c7 0.00 0.00
Mean CorWatt 4.69 5.11
Mean PkgWatt 6.92 7.34
There performance boost is marginal but the system CPU usage is much reduced and overall the
impact on power usage is marginal.
iozone for small files and varying block sizes. Format is IOOperation-filessize-recordsize
4.5.0-rc2 4.5.0-rc2
vanilla sample-v1r1
Hmean SeqWrite-200704-1 745153.35 ( 0.00%) 835705.87 ( 12.15%)
Hmean SeqWrite-200704-2 1073584.72 ( 0.00%) 1181464.54 ( 10.05%)
Hmean SeqWrite-200704-4 1470279.09 ( 0.00%) 1800606.95 ( 22.47%)
Hmean SeqWrite-200704-8 1557199.39 ( 0.00%) 1858933.62 ( 19.38%)
Hmean SeqWrite-200704-16 1604615.45 ( 0.00%) 1982299.77 ( 23.54%)
Hmean SeqWrite-200704-32 1651599.28 ( 0.00%) 1896837.26 ( 14.85%)
Hmean SeqWrite-200704-64 1666177.22 ( 0.00%) 2061195.61 ( 23.71%)
Hmean SeqWrite-200704-128 1669019.85 ( 0.00%) 1940620.93 ( 16.27%)
Hmean SeqWrite-200704-256 1657685.15 ( 0.00%) 2054770.87 ( 23.95%)
Hmean SeqWrite-200704-512 1657502.45 ( 0.00%) 2064537.12 ( 24.56%)
Hmean SeqWrite-200704-1024 1658418.19 ( 0.00%) 2065680.07 ( 24.56%)
Hmean SeqWrite-401408-1 823115.74 ( 0.00%) 873454.97 ( 6.12%)
Hmean SeqWrite-401408-2 1175839.58 ( 0.00%) 1380834.12 ( 17.43%)
Hmean SeqWrite-401408-4 1746819.22 ( 0.00%) 1959568.79 ( 12.18%)
Hmean SeqWrite-401408-8 1857904.68 ( 0.00%) 2119305.42 ( 14.07%)
Hmean SeqWrite-401408-16 1883956.56 ( 0.00%) 2263314.65 ( 20.14%)
Hmean SeqWrite-401408-32 1928933.02 ( 0.00%) 2359131.00 ( 22.30%)
Hmean SeqWrite-401408-64 1947503.44 ( 0.00%) 2269170.03 ( 16.52%)
Hmean SeqWrite-401408-128 1963530.81 ( 0.00%) 2360367.91 ( 20.21%)
Hmean SeqWrite-401408-256 1930490.52 ( 0.00%) 2179920.99 ( 12.92%)
Hmean SeqWrite-401408-512 1944400.52 ( 0.00%) 2268039.39 ( 16.64%)
Hmean SeqWrite-401408-1024 1930551.06 ( 0.00%) 2294266.42 ( 18.84%)
Hmean Rewrite-200704-1 1157432.45 ( 0.00%) 1161993.50 ( 0.39%)
Hmean Rewrite-200704-2 1769952.94 ( 0.00%) 1875955.03 ( 5.99%)
Hmean Rewrite-200704-4 2534237.50 ( 0.00%) 2850813.95 ( 12.49%)
Hmean Rewrite-200704-8 2739338.32 ( 0.00%) 3069949.91 ( 12.07%)
Hmean Rewrite-200704-16 2869980.18 ( 0.00%) 3084573.49 ( 7.48%)
Hmean Rewrite-200704-32 2893382.66 ( 0.00%) 3125994.45 ( 8.04%)
Hmean Rewrite-200704-64 2971476.80 ( 0.00%) 3037778.64 ( 2.23%)
Hmean Rewrite-200704-128 2899499.67 ( 0.00%) 3061961.77 ( 5.60%)
Hmean Rewrite-200704-256 2931964.78 ( 0.00%) 3047588.38 ( 3.94%)
Hmean Rewrite-200704-512 2905287.39 ( 0.00%) 2716185.78 ( -6.51%)
Hmean Rewrite-200704-1024 2852964.56 ( 0.00%) 2979784.30 ( 4.45%)
Hmean Rewrite-401408-1 1340119.25 ( 0.00%) 1367559.86 ( 2.05%)
Hmean Rewrite-401408-2 2066152.00 ( 0.00%) 2150180.25 ( 4.07%)
Hmean Rewrite-401408-4 2877697.54 ( 0.00%) 3141556.92 ( 9.17%)
Hmean Rewrite-401408-8 3111565.24 ( 0.00%) 3351724.68 ( 7.72%)
Hmean Rewrite-401408-16 3121552.56 ( 0.00%) 3460645.54 ( 10.86%)
Hmean Rewrite-401408-32 3156754.87 ( 0.00%) 3689350.17 ( 16.87%)
Hmean Rewrite-401408-64 3323557.00 ( 0.00%) 3476782.18 ( 4.61%)
Hmean Rewrite-401408-128 3402701.75 ( 0.00%) 3530951.84 ( 3.77%)
Hmean Rewrite-401408-256 3204914.57 ( 0.00%) 3277704.44 ( 2.27%)
Hmean Rewrite-401408-512 3133442.60 ( 0.00%) 3387768.91 ( 8.12%)
Hmean Rewrite-401408-1024 3143721.63 ( 0.00%) 3341908.51 ( 6.30%)
4.5.0-rc4 4.5.0-rc4
pmnext-20160219 sample-v2r3
Mean %Busy 3.45 3.32
Mean CPU%c1 5.44 6.01
Mean CPU%c3 0.13 0.09
Mean CPU%c6 90.98 90.58
Mean CPU%c7 0.00 0.00
Mean CorWatt 1.75 1.83
Mean PkgWatt 3.92 3.98
Max %Busy 16.46 16.46
Max CPU%c1 17.33 17.60
Max CPU%c3 1.62 1.42
Max CPU%c6 96.10 95.43
Max CPU%c7 0.00 0.00
Max CorWatt 5.47 5.54
Max PkgWatt 7.60 7.63
The other operations are omitted as they showed either no or negligible performance difference.
For sequential writes and rewrites there is a massive gain in throughput
for very small files. The increase in power consumption is negligible.
It is known that the increase is not universal. Larger core machines see
a much smaller benefit so the rate of CPU migrations are a factor.
In all cases, there are some CPU migrations because wakers pull wakees
to nearby CPUs. It could be argued that such workloads should be pinned
but this puts a burden on the user that may not even be possible in all
cases. The scheduler could try keeping processes on the same CPUs but that
would impact cache hotness and cause a different class of issues. It is
inevitable that there will be some conflict between power management and
scheduling decisions but there is some gains from delaying idling slightly
without a severe impact on power consumption.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
drivers/cpufreq/intel_pstate.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index f4d85c2ae7b1..6f3bf1e68f63 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -975,17 +975,15 @@ static inline int32_t get_target_pstate_use_performance(struct cpudata *cpu)
/*
* Since our utilization update callback will not run unless we are
- * in C0, check if the actual elapsed time is significantly greater (3x)
- * than our sample interval. If it is, then we were idle for a long
- * enough period of time to adjust our busyness.
+ * in C0, check if the actual elapsed time is significantly greater (12x)
+ * than our sample interval. If it is, then assume we were idle for a long
+ * enough period of time to adjust our busyness. While the assumption
+ * is not always true, it seems to be good enough.
*/
duration_ns = cpu->sample.time - cpu->last_sample_time;
- if ((s64)duration_ns > pid_params.sample_rate_ns * 3
- && cpu->last_sample_time > 0) {
- sample_ratio = div_fp(int_tofp(pid_params.sample_rate_ns),
- int_tofp(duration_ns));
- core_busy = mul_fp(core_busy, sample_ratio);
- }
+ if ((s64)duration_ns > pid_params.sample_rate_ns * 12
+ && cpu->last_sample_time > 0)
+ core_busy = 0;
cpu->sample.busy_scaled = core_busy;
return cpu->pstate.current_pstate - pid_calc(&cpu->pid, core_busy);
--
2.6.4