linux-kernel - Load balancing behavior for sched autogroup

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Message-ID: <1364599219.27102.56.camel@schen9-DESK>
Date:	Fri, 29 Mar 2013 16:20:19 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Alex Shi <alex.shi@...el.com>,
	Changlong Xie <changlongx.xie@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Load balancing behavior for sched autogroup

During our testings of 3.8 kernel, we noticed that after the patch

Revert "sched: Update_cfs_shares at period edge" (commit 17bc14b7), 

the load between the sockets or larger system can have 
large imbalance.  For example, for a 4 socket Westmere-EX 
(10 cores/socket), we notice the loadings between the sockets
can differ by more than a factor of 4. 

We did a simple experiment that kicks off 29 simple 
processes that execute a tight loop.  We noticed
socket 3 is already starting to schedule on hyperthreaded cpus 
(13 loaded cpus) while socket 1 still have lots of 
idle cores (3 loaded cpus).  Before the patch, the 
load was evenly distributed across sockets.  
If I turn off CONFIG_SCHED_AUTOGROUP,the loads are also 
distributed evenly.

(load on cpus, running on 4)
socket                    0                    1                    2                    3
---------------------------------------------------------------------------------------------
cpu:    0-3               0.00                 0.00                 0.00                99.00
cpu:    4-7               0.00                 0.00                 0.00                99.20
cpu:    8-11              0.00                 0.00                 0.00                99.00
cpu:   12-15             99.20                 0.00                 0.00                 0.00
cpu:   16-19              0.00                 0.00                 0.00                99.00
cpu:   20-23              0.00                 0.00                 0.00                 0.00
cpu:   24-27              0.00                 0.00                 0.00                 0.00
cpu:   28-31              0.00                 0.00                 0.00                 0.00
cpu:   32-35             99.20                 0.00                 0.00                99.00
cpu:   36-39             99.20                99.40                99.20                 0.00
cpu:   40-43              0.00                99.40                99.40                99.20
cpu:   44-47              0.00                99.40                99.40                99.20
cpu:   48-51             99.40                 0.00                99.40                99.20
cpu:   52-55             99.20                 0.00                99.40                99.20
cpu:   56-59              0.00                 0.00                99.40                99.40
cpu:   60-63              0.00                 0.00                 0.00                99.00
cpu:   64-67              0.00                 0.00                 0.00                99.40
cpu:   68-71              0.00                 0.00                 0.00                99.40
cpu:   72-75             99.40                 0.00                 0.00                 0.00
cpu:   76-79             99.40                 0.00                 0.00                 0.00
---------------------------------------------------------------------------------------------
Loaded cpus                  7                    3                    6                   13

Is this the intended behavior of sched autogroup? I'm a bit surprised 
that we are reserving this much cpu bandwidth for very low load 
processes (or interactive processes) in other groups.

So should the sched autogroup config option be turned off by default for server 
system, when we are not concerned about interactivity but want to maximize 
throughput by balancing out the load?

Thanks for clarifying.

Tim


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