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Subject[PATCH v2 0/3] Increase resolution of load weights
Date
Hi All,

Please find attached v2 of the patchset series to increase load resolution.
Based on discussions with Ingo and Peter, this version drops all the
unsigned long -> u64 conversions which greatly simplifies the patchset. We also
only scale load when BITS_PER_LONG > 32 bits. Based on experiments with the
previous patchset, the benefits for 32-bit systems were limited and did not
justify the increased performance penalties.
Major changes from v1:
- Dropped unsigned long -> u64 conversions
- Cleaned up set_load_weight()
- Scale load only when BITS_PER_LONG > 32
- Rebased patchset to -tip instead of v2.6.39-rcX
Previous versions:
- v1: http://thread.gmane.org/gmane.linux.kernel/1133978
- v0: http://thread.gmane.org/gmane.linux.kernel/1129232

The patchset applies cleanly to -tip. Please note that this does not apply
cleanly to v2.6.39-rc8. I can post a patchset against v2.6.39-rc8 if needed.

Below is some analysis of the performance costs/improvements of this patchset.

1. Micro-arch performance costs:

Experiment was to run Ingo's pipe_test_100k about 200 times and measure
instructions, cycles, stalled cycles, branches, branch-misses and other
micro-arch costs as needed.

-tip (baseline):

# taskset 4 perf stat -e instructions -e cycles -e stalled-cycles-backend -e stalled-cycles-frontend --repeat=200 ./pipe-test-100k

Performance counter stats for '/root/load-scale/pipe-test-100k' (200 runs):

964,991,769 instructions # 0.82 insns per cycle
# 0.33 stalled cycles per insn
# ( +- 0.05% )
1,171,186,635 cycles # 0.000 GHz ( +- 0.08% )
306,373,664 stalled-cycles-backend # 26.16% backend cycles idle ( +- 0.28% )
314,933,621 stalled-cycles-frontend # 26.89% frontend cycles idle ( +- 0.34% )

1.122405684 seconds time elapsed ( +- 0.05% )


-tip+patches:

# taskset 4 perf stat -e instructions -e cycles -e stalled-cycles-backend -e stalled-cycles-frontend --repeat=200 ./pipe-test-100k

Performance counter stats for './load-scale/pipe-test-100k' (200 runs):

963,624,821 instructions # 0.82 insns per cycle
# 0.33 stalled cycles per insn
# ( +- 0.04% )
1,175,215,649 cycles # 0.000 GHz ( +- 0.08% )
315,321,126 stalled-cycles-backend # 26.83% backend cycles idle ( +- 0.28% )
316,835,873 stalled-cycles-frontend # 26.96% frontend cycles idle ( +- 0.29% )

1.122238659 seconds time elapsed ( +- 0.06% )


With this version of the patchset, looks like instructions decreases by ~0.10%
and cycles increases by 0.27%. This doesn't look statistically significant. As
expected, number of stalled cycles in the backend increased from 26.16% to
26.83%. This can be attributed to the shifts we do in c_d_m() and other places.
The fraction of stalled cycles in the frontend remains about the same, at 26.96%
compared to 26.89% in -tip.

For comparison sake, I modified the patch to remove the shifts in c_d_m() and
scale down the inverse weight for tasks.

Performance counter stats for './data/pipe-test-100k' (200 runs):

956,993,064 instructions # 0.81 insns per cycle
# 0.34 stalled cycles per insn
# ( +- 0.05% )
1,181,506,294 cycles # 0.000 GHz ( +- 0.10% )
304,271,160 stalled-cycles-backend # 25.75% backend cycles idle ( +- 0.36% )
325,099,601 stalled-cycles-frontend # 27.52% frontend cycles idle ( +- 0.37% )

1.129208596 seconds time elapsed ( +- 0.06% )

The number of instructions decreases by about 0.8% and cycles increase about
0.81%. The fraction of cycles stalled in the backend drops to 25.72% and
fraction of cycles in the frontend increases to 27.52% (increase of 2.61%). This
is probably because we take the path marked unlikely in c_d_m() more often
because we overflow the 64-bit mult.

I tried a few variations of this alternative; i.e. do not scale weights in
c_d_m() and replace the unlikely() compiler hint with (i). no compiler hint,
and (ii). a likley() compiler hint. Neither of these variations resulted in
better performance compared to scaling down weights (as currently done).

(i). -tip+patches(alt) + no compiler hint in c_d_m()

# taskset 4 perf stat -e instructions -e cycles -e stalled-cycles-backend -e stalled-cycles-frontend --repeat=200 ./pipe-test-100k

Performance counter stats for './load-scale/pipe-test-100k' (200 runs):

958,280,690 instructions # 0.80 insns per cycle
# 0.34 stalled cycles per insn
# ( +- 0.07% )
1,191,992,203 cycles # 0.000 GHz ( +- 0.10% )
314,905,306 stalled-cycles-backend # 26.42% backend cycles idle ( +- 0.36% )
324,940,653 stalled-cycles-frontend # 27.26% frontend cycles idle ( +- 0.38% )

1.136591328 seconds time elapsed ( +- 0.08% )


(ii). -tip+patches(alt) + likely() compiler hint

# taskset 4 perf stat -e instructions -e cycles -e stalled-cycles-backend -e stalled-cycles-frontend --repeat=200 ./pipe-test-100k

Performance counter stats for './load-scale/pipe-test-100k' (200 runs):

956,711,525 instructions # 0.81 insns per cycle
# 0.34 stalled cycles per insn
# ( +- 0.07% )
1,184,777,377 cycles # 0.000 GHz ( +- 0.09% )
308,259,625 stalled-cycles-backend # 26.02% backend cycles idle ( +- 0.32% )
325,105,968 stalled-cycles-frontend # 27.44% frontend cycles idle ( +- 0.38% )

1.131130981 seconds time elapsed ( +- 0.06% )


2. Balancing low-weight task groups

Test setup: run 50 tasks with random sleep/busy times (biased around 100ms) in
a low weight container (with cpu.shares = 2). Measure %idle as reported by
mpstat over a 10s window.

-tip (baseline):

06:47:48 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %idle intr/s
06:47:49 PM all 94.32 0.00 0.06 0.00 0.00 0.00 0.00 0.00 5.62 15888.00
06:47:50 PM all 94.57 0.00 0.62 0.00 0.00 0.00 0.00 0.00 4.81 16180.00
06:47:51 PM all 94.69 0.00 0.06 0.00 0.00 0.00 0.00 0.00 5.25 15966.00
06:47:52 PM all 95.81 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4.19 16053.00
06:47:53 PM all 94.88 0.06 0.00 0.00 0.00 0.00 0.00 0.00 5.06 15984.00
06:47:54 PM all 93.31 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6.69 15806.00
06:47:55 PM all 94.19 0.00 0.06 0.00 0.00 0.00 0.00 0.00 5.75 15896.00
06:47:56 PM all 92.87 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.13 15716.00
06:47:57 PM all 94.88 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5.12 15982.00
06:47:58 PM all 95.44 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4.56 16075.00
Average: all 94.49 0.01 0.08 0.00 0.00 0.00 0.00 0.00 5.42 15954.60

-tip+patches:

06:47:03 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %idle intr/s
06:47:04 PM all 100.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16630.00
06:47:05 PM all 99.69 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.31 16580.20
06:47:06 PM all 99.69 0.00 0.06 0.00 0.00 0.00 0.00 0.00 0.25 16596.00
06:47:07 PM all 99.20 0.00 0.74 0.00 0.00 0.06 0.00 0.00 0.00 17838.61
06:47:08 PM all 100.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16540.00
06:47:09 PM all 100.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16575.00
06:47:10 PM all 100.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16614.00
06:47:11 PM all 99.94 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 16588.00
06:47:12 PM all 99.94 0.00 0.06 0.00 0.00 0.00 0.00 0.00 0.00 16593.00
06:47:13 PM all 99.94 0.00 0.06 0.00 0.00 0.00 0.00 0.00 0.00 16551.00
Average: all 99.84 0.00 0.09 0.00 0.00 0.01 0.00 0.00 0.06 16711.58

On a related note, compared to v2.6.39-rc7, -tip seems to have regressed in
this test. A bisection points to this merge commit:

$ git show 493bd3180190511770876ed251d75dab8595d322
commit 493bd3180190511770876ed251d75dab8595d322
Merge: 0cc5bc3 d906f0e
Author: Ingo Molnar <mingo@elte.hu>
Date: Fri Jan 7 14:09:55 2011 +0100

Merge branch 'x86/numa'

Is there a way to bisect into this merge commit? I don't have much experience
with git bisect and I've been manually inspecting commits :-( Full bisection
log below.

git bisect start
# good: [693d92a1bbc9e42681c42ed190bd42b636ca876f] Linux 2.6.39-rc7
git bisect good 693d92a1bbc9e42681c42ed190bd42b636ca876f
# bad: [6639653af57eb8f48971cc8662318dfacd192929] Merge branch 'perf/urgent'
git bisect bad 6639653af57eb8f48971cc8662318dfacd192929
# bad: [1ebfeec0895bf5afc59d135deab2664ad4d71e82] Merge branch 'x86/urgent'
git bisect bad 1ebfeec0895bf5afc59d135deab2664ad4d71e82
# good: [0d51ef74badd470c0f84ee4eff502e06bdb7e06b] Merge branch 'perf/core'
git bisect good 0d51ef74badd470c0f84ee4eff502e06bdb7e06b
# good: [4c637fc4944b153dc98735cc79dcd776c1a47303] Merge branch 'perf/core'
git bisect good 4c637fc4944b153dc98735cc79dcd776c1a47303
# good: [959620395dcef68a93288ac6055f6010fbefa243] Merge branch 'perf/core'
git bisect good 959620395dcef68a93288ac6055f6010fbefa243
# good: [6a8aeb83d3d7e5682df4542e6080cd4037a1f6d7] Merge branch 'out-of-tree'
git bisect good 6a8aeb83d3d7e5682df4542e6080cd4037a1f6d7
# good: [106f3c7499db2d4094e2d3a669c2f2c6f08ab093] Merge branch 'perf/core'
git bisect good 106f3c7499db2d4094e2d3a669c2f2c6f08ab093
# good: [0de7032fa49c88b06a492a88da7ea0c5118cedad] Merge branch 'linus'
git bisect good 0de7032fa49c88b06a492a88da7ea0c5118cedad
# good: [5e02063f418c58fb756b1ac972a7d1805127f299] Merge branch 'perf/core'
git bisect good 5e02063f418c58fb756b1ac972a7d1805127f299
# bad: [f9a3e42e48b14d6cb698cf8a5380ea32c316c949] Merge branch 'sched/urgent'
git bisect bad f9a3e42e48b14d6cb698cf8a5380ea32c316c949
# good: [0cc5bc39098229cf4192c3639f0f6108afe4932b] Merge branch 'x86/urgent'
git bisect good 0cc5bc39098229cf4192c3639f0f6108afe4932b
# bad: [1f01b669d560f33ba388360aa6070fbdef9f8f44] Merge branch 'perf/core'
git bisect bad 1f01b669d560f33ba388360aa6070fbdef9f8f44
# bad: [493bd3180190511770876ed251d75dab8595d322] Merge branch 'x86/numa'
git bisect bad 493bd3180190511770876ed251d75dab8595d322
# bad: [493bd3180190511770876ed251d75dab8595d322] Merge branch 'x86/numa'
git bisect bad 493bd3180190511770876ed251d75dab8595d322

-Thanks,
Nikhil

Nikhil Rao (3):
sched: cleanup set_load_weight()
sched: introduce SCHED_POWER_SCALE to scale cpu_power calculations
sched: increase SCHED_LOAD_SCALE resolution

include/linux/sched.h | 25 +++++++++++++++++-----
kernel/sched.c | 38 ++++++++++++++++++++++++-----------
kernel/sched_fair.c | 52 +++++++++++++++++++++++++-----------------------
3 files changed, 72 insertions(+), 43 deletions(-)
--
1.7.3.1



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