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Subject[RFC PATCH 0/3] Fix ebizzy performance regression on IvyBridge due to X86 TLB range flush
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I found that ebizzy regressed between 3.4 and 3.10 while testing on a new
machine. Bisection initially found at least two problems of which the
first was commit 611ae8e3 (x86/tlb: enable tlb flush range support for
x86). The second was related to ACPI cpufreq and so it was disabled for
the purposes of this series.

The intent of the TLB range flush patch appeared to be to preserve
existing TLB entries which makes sense. The decision on whether to do a
full mm flush or a number of single page flushes depends on the size of the
relevant TLB and the CPU which is presuably taking the cost of a TLB refill.
It is a gamble because the cost of the per-page flushes must be offset by a
reduced TLB miss count. There are no indications what the cost of calling
invlpg are if there are no TLB entries and it's also not taking into
account how many CPUs it may have to execute these single TLB flushes on.

Ebizzy sees very little benefit as it discards newly allocated memory very
quickly which is why it appeared to regress so badly. While I'm wary of
optimising for such a benchmark, it's commonly tested and the defaults for
Ivybridge may need to be re-examined.

The following small series restores ebizzy to 3.4-era performance. Is there a
better way of doing this? Bear in mind that I'm testing on a single IvyBridge
machine and there is no guarantee the gain is universal or even relevant.

kernel build was tested but it's uninteresting as TLB range is unimportant
to it. A page fault benchmark was also tested but it does not hit the same paths
impacted by commit 611ae8e3.

ebizzy
3.13.0-rc3 3.4.69 3.13.0-rc3
vanilla vanilla newdefault-v1
Mean 1 7353.60 ( 0.00%) 6782.00 ( -7.77%) 7836.20 ( 6.56%)
Mean 2 8120.40 ( 0.00%) 8278.80 ( 1.95%) 9520.60 ( 17.24%)
Mean 3 8087.80 ( 0.00%) 8083.60 ( -0.05%) 9003.80 ( 11.33%)
Mean 4 7919.20 ( 0.00%) 7842.60 ( -0.97%) 8680.60 ( 9.61%)
Mean 5 7310.60 ( 0.00%) 7740.60 ( 5.88%) 8273.20 ( 13.17%)
Mean 6 6798.00 ( 0.00%) 7720.20 ( 13.57%) 8033.20 ( 18.17%)
Mean 7 6759.40 ( 0.00%) 7644.00 ( 13.09%) 7643.80 ( 13.08%)
Mean 8 6501.80 ( 0.00%) 7666.40 ( 17.91%) 6944.40 ( 6.81%)
Mean 12 6606.00 ( 0.00%) 7523.20 ( 13.88%) 7035.80 ( 6.51%)
Mean 16 6655.40 ( 0.00%) 7287.40 ( 9.50%) 7099.20 ( 6.67%)
Mean 20 6703.80 ( 0.00%) 7152.20 ( 6.69%) 7116.60 ( 6.16%)
Mean 24 6705.80 ( 0.00%) 7014.80 ( 4.61%) 7113.60 ( 6.08%)
Mean 28 6706.60 ( 0.00%) 6940.40 ( 3.49%) 7115.20 ( 6.09%)
Mean 32 6727.20 ( 0.00%) 6878.80 ( 2.25%) 7110.80 ( 5.70%)
Stddev 1 42.71 ( 0.00%) 53.16 (-24.46%) 39.80 ( 6.82%)
Stddev 2 250.26 ( 0.00%) 150.57 ( 39.84%) 31.94 ( 87.24%)
Stddev 3 71.67 ( 0.00%) 69.38 ( 3.19%) 84.13 (-17.39%)
Stddev 4 30.25 ( 0.00%) 87.06 (-187.82%) 31.80 ( -5.14%)
Stddev 5 71.18 ( 0.00%) 25.68 ( 63.92%) 125.24 (-75.95%)
Stddev 6 34.22 ( 0.00%) 23.35 ( 31.75%) 124.40 (-263.57%)
Stddev 7 100.59 ( 0.00%) 112.83 (-12.17%) 65.07 ( 35.31%)
Stddev 8 20.26 ( 0.00%) 43.43 (-114.32%) 48.26 (-138.16%)
Stddev 12 19.43 ( 0.00%) 19.73 ( -1.55%) 23.25 (-19.65%)
Stddev 16 14.47 ( 0.00%) 26.42 (-82.54%) 17.71 (-22.40%)
Stddev 20 21.37 ( 0.00%) 15.97 ( 25.27%) 14.87 ( 30.42%)
Stddev 24 12.87 ( 0.00%) 28.12 (-118.44%) 10.46 ( 18.75%)
Stddev 28 13.89 ( 0.00%) 17.97 (-29.36%) 12.22 ( 12.04%)
Stddev 32 18.14 ( 0.00%) 20.37 (-12.31%) 16.40 ( 9.58%)

3.13.0-rc3 3.4.69 3.13.0-rc3
vanilla vanilla newdefault-v1
User 900.27 995.20 947.33
System 1583.41 1680.76 1533.17
Elapsed 2100.78 2100.81 2100.76

This shows the ebizzy comparison between 3.13-rc3, 3.4.69-stable and this series.
The series is not a universal win against 3.4 but the figure are generally better
and system CPU usage is reduced.

arch/x86/kernel/cpu/intel.c | 2 +-
arch/x86/mm/tlb.c | 15 +++++++++------
2 files changed, 10 insertions(+), 7 deletions(-)

--
1.8.4



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