[Devel] [PATCH 0/2] Add group awareness to CFS - v2

[Srivatsa Vaddagiri]

Hi Ingo,
	Here's an update for the group-aware CFS scheduler that I have been
working on.

(For those reading these patches for the first time:)

The basic idea is to reuse CFS core and other pieces of scheduler like
smpnice-driven load balance for driving fairness between 'schedulable entities'
other than tasks, for ex: users or containers.

The time-sorted rb-tree and nanosecond accurate accounting aspects of
CFS are "repeated" for schedulable entities other than tasks. 

For ex: there could be N task-level rb-trees for N users (which stores
tasks) and a single user-level rb-tree which stores user-level entities.
CFS operations on each user's task-level rb-tree drives fairness between tasks
of that user, while CFS operations on user-level rb-tree drives
fairness between users.

v17 CFS introduced basic changes in CFS to support group scheduling.
The two patches to follow build upon them as follows:

Patch 1	=> introduces a notion of scheduler hierarchy (of entities) and
	   applies CFS operations at all levels of this hierarchy.

Patch 2 => hooks up the cpu scheduler with task grouping feature in mm tree
	   (CONFIG_CONTAINERS) as an interface to task-grouping functionality.

A single config option CONFIG_FAIR_GROUP_SCHED allows the group-scheduling
feature to be turned on/off at compile time.

I have tried my best to ensure there is no impact to existing CFS performance
when CONFIG_FAIR_GROUP_SCHED is disabled. Some results in this regard are 
provided at the end.

One noticeable change in functionality may be the /proc/sched_debug output
(I had to rearrange that code a bit to dump group cfs_rq information also).

Changes since last version:

	- Fixed some bugs in SMP load balance (pointed by Dmitry)
	- Modified sched_debug.c to dump all cfs_rq stats

Todo:
	- Weighted fair-share
		Currently all groups get "equal" cpu bandwidth. I plan
		to support weighted fair-sharing on the lines of task
		niceness.
	- Separate out tunable
		Right now tunable are same for all layers of scheduling.
		I strongly think we will need to separate them, esp
                sysctl_sched_runtime_limit.
        - Optimization
		- reduce frequency of timer tick processing at higher levels
        	- during load balance, pick cache-cold tasks first to migrate
	- hierarchy flattening
		Experiment with this (to reduce number of hierarchical levels)
		as per http://lkml.org/lkml/2007/5/26/81
		

Some results follows. Legends used in them are:

cfs        = base cfs performance (sched-cfs-v2.6.22-rc4-mm2-v18.patch)
cfsgrpdi   = base cfs + patches 1-2 applied (CONFIG_FAIR_GROUP_SCHED disabled)
cfsgrpdi   = base cfs + patches 1-2 applied (CONFIG_FAIR_GROUP_SCHED enabled)

All tests run on a 4-cpu Intel Xeon (x86_64) box:


A. Overhead Test

lat_ctx (from lmbench)
======================

Context switching - times in microseconds - smaller is better
-------------------------------------------------------------------------
Host                 OS  2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
                         ctxsw  ctxsw  ctxsw ctxsw  ctxsw   ctxsw   ctxsw
--------- ------------- ------ ------ ------ ------ ------ ------- -------
cfs       Linux 2.6.22- 6.7400 7.8200 8.0100 8.7900  10.90 8.20000    19.88
cfsgrpdi  Linux 2.6.22- 6.7000 7.6700 8.0700 9.0100  11.54 9.34000    18.71
cfsgrpen  Linux 2.6.22- 7.8600 7.8700 8.6500 9.4600  10.27 9.44000    19.74


hackbench -pipe 100
===================
Average of 10 runs was taken. Smaller numbers are better.

cfs		4.0171
cfsgrpdi	4.154
cfsgrpen	4.7749


B. UP Group fairness test
	These tests were forced to run on a single CPU by making using
	of exclusive cpusets.


hackbench
=========
	
The two user's shell were put in different groups (as explained in Patch 2/2).
Each user then ran this script:

i=0
while [ $i -lt 10 ]
do
./hackbench -pipe 100 >> log
i=`expr $i + 1`
done

Time taken to complete this script was measured as follows (note that
both the scripts were made to run simultaneously on /same/ cpu).

vatsa		103.51 s (real)
guest		103.37 s (real)

Inference: Both users completed the same amount of work in (nearly) same time.

kernel compilation
==================

Again the two user's shell were put in different groups.

User vatsa ran "make -s -j4 bzImage", while
User guest ran "make -s -j20 bzImage"

Both are compiling the same sources (and hence should effectively be
doing the same amount of work). Time taken to complete kernel-compile by
both users:

vatsa	777.46 s (real)
guest 	778.30 s (real)

Inference: Both users completed the same amount of work in nearly same
time, even though one had higher number of threads dedicated to the job.


C. SMP Fairness test
====================

I used a simple cpu-intensive program which measures how much CPU time it got 
(using getrusage) over a minute. N (=4*NUM_CPUS) such tasks were spawned with 
N/2 in one group and N/2 in another group. Total CPU time obtained by one group 
was compared with total cpu time obtained by another group. While the test
was running, I observed distribution of all tasks across CPUs. I am
quite happy with the results obtained and with the load distribution. I
can share the sources/results of the program/script upon request.


Looking forward to your feedback on these patches!

[P.S : Since I am travelling this weekend, I may not respond promptly ]


-- 
Regards,
vatsa
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