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Subject[PATCH V3 RESEND RFC 0/2] kvm: Improving undercommit scenarios

In some special scenarios like #vcpu <= #pcpu, PLE handler may
prove very costly, because there is no need to iterate over vcpus
and do unsuccessful yield_to burning CPU.

The first patch optimizes all the yield_to by bailing out when there
is no need to continue in yield_to (i.e., when there is only one task
in source and target rq).

Second patch uses that in PLE handler. Further when a yield_to fails
we do not immediately go out of PLE handler instead we try thrice
to have better statistical possibility of false return. Otherwise that
would affect moderate overcommit cases.

Result on 3.7.0-rc6 kernel shows around 140% improvement for ebizzy 1x and
around 51% for dbench 1x with 32 core PLE machine with 32 vcpu guest.


base = 3.7.0-rc6
machine: 32 core mx3850 x5 PLE mc

--+-----------+-----------+-----------+------------+-----------+
ebizzy (rec/sec higher is beter)
--+-----------+-----------+-----------+------------+-----------+
base stdev patched stdev %improve
--+-----------+-----------+-----------+------------+-----------+
1x 2511.3000 21.5409 6051.8000 170.2592 140.98276
2x 2679.4000 332.4482 2692.3000 251.4005 0.48145
3x 2253.5000 266.4243 2192.1667 178.9753 -2.72169
--+-----------+-----------+-----------+------------+-----------+

--+-----------+-----------+-----------+------------+-----------+
dbench (throughput in MB/sec. higher is better)
--+-----------+-----------+-----------+------------+-----------+
base stdev patched stdev %improve
--+-----------+-----------+-----------+------------+-----------+
1x 6677.4080 638.5048 10098.0060 3449.7026 51.22643
2x 2012.6760 64.7642 2019.0440 62.6702 0.31639
3x 1302.0783 40.8336 1292.7517 27.0515 -0.71629
--+-----------+-----------+-----------+------------+-----------+

Here is the refernce of no ple result.
ebizzy-1x_nople 7592.6000 rec/sec
dbench_1x_nople 7853.6960 MB/sec

The result says we can still improve by 60% for ebizzy, but overall we are
getting impressive performance with the patches.

Changes Since V2:
- Dropped global measures usage patch (Peter Zilstra)
- Do not bail out on first failure (Avi Kivity)
- Try thrice for the failure of yield_to to get statistically more correct
behaviour.

Changes since V1:
- Discard the idea of exporting nrrunning and optimize in core scheduler (Peter)
- Use yield() instead of schedule in overcommit scenarios (Rik)
- Use loadavg knowledge to detect undercommit/overcommit

Peter Zijlstra (1):
Bail out of yield_to when source and target runqueue has one task

Raghavendra K T (1):
Handle yield_to failure return for potential undercommit case

Please let me know your comments and suggestions.

Link for the discussion of V3 original:
https://lkml.org/lkml/2012/11/26/166

Link for V2:
https://lkml.org/lkml/2012/10/29/287

Link for V1:
https://lkml.org/lkml/2012/9/21/168

kernel/sched/core.c | 25 +++++++++++++++++++------
virt/kvm/kvm_main.c | 26 ++++++++++++++++----------
2 files changed, 35 insertions(+), 16 deletions(-)



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