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author | Gautham R. Shenoy <ego@linux.vnet.ibm.com> | 2017-08-23 21:01:40 +0530 |
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committer | Stewart Smith <stewart@linux.vnet.ibm.com> | 2017-08-24 18:05:10 +1000 |
commit | 51974cab7c89504a4ceb6706437665d5e8d2a533 (patch) | |
tree | 5bde244249e8222a52ef8699b0ecf28568782b50 /hw/p8-i2c.c | |
parent | 01af87cd8953de0a0c9056dc45d0c11232ba5849 (diff) | |
download | skiboot-51974cab7c89504a4ceb6706437665d5e8d2a533.zip skiboot-51974cab7c89504a4ceb6706437665d5e8d2a533.tar.gz skiboot-51974cab7c89504a4ceb6706437665d5e8d2a533.tar.bz2 |
slw: Modify the power9 stop0_lite latency & residency
Currently skiboot exposes the exit-latency for stop0_lite as 200ns and
the target-residency to be 2us.
However, the kernel cpu-idle infrastructure rounds up the latency to
microseconds and lists the stop0_lite latency as 0us, putting it on
par with snooze state. As a result, when the predicted latency is
small (< 1us), cpuidle will select stop0_lite instead of snooze. The
difference between these states is that snooze doesn't require an
interrupt to exit from the state, but stop0_lite does. And the value
200ns doesn't include the interrupt latency.
This shows up in the context_switch2 benchmark
(http://ozlabs.org/~anton/junkcode/context_switch2.c) where the number
of context switches per second with the stop0_lite disabled is found
to be roughly 30% more than with stop0_lite enabled.
===============================================================================
x latency_200ns_residency_2us
+ latency_200ns_residency_2us_stop0_lite_disabled
N Min Max Median Avg Stddev
x 100 222784 473466 294510 302295.26 45380.6
+ 100 205316 609420 385198 396338.72 78135.648
Difference at 99.0% confidence
94043.5 +/- 23276.2
31.1098% +/- 7.69983%
(Student's t, pooled s = 63892.8)
===============================================================================
This can be correlated with the number of times cpuidle enters
stop0_lite compared to snooze.
===================================================================
latency=200ns, residency=2us
stop0_lite enabled.
* snooze usage = 7
* stop0 lite usage = 3200324
* stop1 lite usage = 6
stop0_lite disabled
* snooze usage: 287846
* stop0_lite usage: 0
* stop1_lite usage: 0
==================================================================
Hence, bump up the exit latency of stop0_lite to 1us. Since the target
residency is chosen to be 10 times the exit latency, set the target
residency to 10us.
With these values, we see a 50% improvement in the number of context
switches:
=====================================================================
x latency_200ns_residency_2us
+ latency_1us_residency_10us
N Min Max Median Avg Stddev
x 100 222784 473466 294510 302295.26 45380.6
+ 100 281790 710784 514878 510224.62 85163.252
Difference at 99.0% confidence
207929 +/- 24858.3
68.7835% +/- 8.22319%
(Student's t, pooled s = 68235.5)
=====================================================================
The cpuidle usage statistics show that we choose stop0_lite less often
in such cases.
latency = 1us, residency = 10us
stop0_lite enabled
* snooze usage = 536808
* stop0 lite usage = 3
* stop1 lite usage = 7
Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Diffstat (limited to 'hw/p8-i2c.c')
0 files changed, 0 insertions, 0 deletions