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Message-ID: <20081115011452.GA28135@google.com>
Date: Fri, 14 Nov 2008 17:14:53 -0800
From: Ken Chen <kenchen@...gle.com>
To: Ingo Molnar <mingo@...e.hu>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: busted CFS group load balancer?
It appears that the fair-group load balancer in 2.6.27 does not work
properly. It broke down severely with real-world application workload
here at google. We are trying to use CFS cgroup scheduler as a
proportional scheduler to control CPU cycle allocation for concurrent
groups of tasks on the same server machine. For example, we have one
cgroup named 'bee' which has a cfs weight of 1024, and another cgroup
named 'ant' that has a cfs weight of 2. Ideally, I would expect 99.8%
of CPU cycles goes into 'bee' cgroup while the remaining 0.2% goes into
'ant'. However, when evaluating 2.6.27, the result deviates significantly
from the expectation. It appears that the fair-group load balancer is sub
par and not doing its job properly.
Attached is a snapshot of /proc/sched_debug. Two CPUs were dominated by
tasks of a very low weight 'ant' cgroup, while multiple tasks in higher
weight 'bee' cgroup landed on the same CPU and competing among themselves.
I think the load balancer does its job poorly. Have you seen similar l-b
deficiency?
- Ken
Sched Debug Version: v0.07, 2.6.27 #1
now at 62931690.049344 msecs
.sysctl_sched_latency : 80.000000
.sysctl_sched_min_granularity : 16.000000
.sysctl_sched_wakeup_granularity : 20.000000
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.sysctl_sched_features : 7935
cpu#0, 2333.417 MHz
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cfs_rq[0]:/ant
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cfs_rq[0]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4202 60790257.707167 5421910 120 60790257.707167 53371613.637974 0.000000 /ant
R hpro 12721 10434396.891434 425169 120 10434396.891434 2351102.444642 1015320.568226 /bee
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cfs_rq[1]:/ant
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cfs_rq[1]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4200 98877044.002500 14797192 120 98877044.002500 50553231.092595 0.000000 /ant
R hpro 12727 14039095.103079 422222 120 14039095.103079 2347785.641546 1017814.287309 /bee
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cfs_rq[2]:/ant
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cfs_rq[2]:/bee
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cfs_rq[2]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4205 62505737.284348 1919921 120 62505737.284348 53264991.605391 0.000000 /ant
hpro 12726 10838701.944779 423772 120 10838701.944779 2344927.648792 1017455.081051 /bee
R hpro 12728 10838780.058709 421699 120 10838780.058709 2345123.009711 1019061.435473 /bee
cpu#3, 2333.417 MHz
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cfs_rq[3]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
R hog 4204 61265712.229407 1590630 120 61265712.229407 53314966.642964 0.000000 /ant
cpu#4, 2333.417 MHz
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cfs_rq[4]:/bee
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cfs_rq[4]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4199 61439338.118634 2252113 120 61439338.118634 57463477.501906 0.000000 /ant
R hpro 12722 5520893.049620 423940 120 5520893.049620 2353601.970167 1016247.261338 /bee
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task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4201 97702539.108896 15313291 120 97702539.108896 50479803.305704 0.000000 /ant
R hpro 12724 13553228.752335 428968 120 13553228.752335 2359267.848257 1021475.119277 /bee
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cfs_rq[6]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4198 60648319.346763 2155112 120 60648319.346763 57412425.922637 0.000000 /ant
R cat 18280 5564927.823046 3 120 5564927.823046 1.995011 0.012258 /bee
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cfs_rq[7]:/ant
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cfs_rq[7]:/
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runnable tasks:
task PID tree-key switches prio exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
hog 4203 61941066.080427 2636297 120 61941066.080427 57417288.830170 0.000000 /ant
R hpro 12723 5580819.679520 424397 120 5580819.679520 2359524.675698 1020120.649702 /bee
--
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