linux-kernel - Re: scheduler scalability - cgroups, cpusets and load-balancing

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Message-Id: <20080129060355.7d38338f.pj@sgi.com>
Date:	Tue, 29 Jan 2008 06:03:55 -0600
From:	Paul Jackson <pj@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	vatsa@...ux.vnet.ibm.com, dhaval@...ux.vnet.ibm.com,
	nickpiggin@...oo.com.au, ebiederm@...ssion.com,
	akpm@...ux-foundation.org, sgrubb@...hat.com, rostedt@...dmis.org,
	ghaskins@...ell.com, dmitry.adamushko@...il.com,
	tong.n.li@...el.com, tglx@...utronix.de, menage@...gle.com,
	rientjes@...gle.com
Subject: Re: scheduler scalability - cgroups, cpusets and load-balancing

Paul, responding to Peter:
> > We now have a per-cpuset Boolean flag file called 'sched_load_balance'.
> 
> SD_LOAD_BALANCE, right?

No.  SD_LOAD_BALANCE is some attribute of sched domains.

The 'sched_load_balance' flag is an attribute of cpusets.

The mapping of cpusets to sched domains required several pages of 'fun
to write' code, which had to go through a couple of years of fixing and
one major rewrite before it (knock on wood) worked correctly.  It's not
a one-to-one relation, in other words.  See my earlier messages for
further explanation of how this works.

I'm not sure what SD_LOAD_BALANCE does ... I guess from a quick
read that it just optimizes the recognition of singleton sched
domains for which load balancing would be a wasted effort.

> > This 'sched_load_balance' flag is, thus far, "the" cpuset hook
> > supporting realtime.  One can use it to configure a system so that
> > the kernel does not do normal load balancing on select CPUs, such
> > as those CPUs dedicated to realtime use.
> 
> Ah, here I disagree, it is possible to do (hard) realtime scheduling
> over multiple cpus, the only draw back is that it requires a very strong
> load-balancer, making it unsuitable for large number of cpus.

I don't think we are disagreeing.  I was speaking of "normal"
load balancing (what the mainline kernel/sched.c code normally
does).  You're speaking of hard realtime load balancing.

I think we agree that these both exist, and require different
load balancing code, the latter 'very strong.'

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.940.382.4214
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