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

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Message-Id: <1201609804.28547.137.camel@lappy>
Date:	Tue, 29 Jan 2008 13:30:04 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Jackson <pj@....com>
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


On Tue, 2008-01-29 at 06:03 -0600, Paul Jackson wrote:
> 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.

Ok, I'll take a stab at understanding that code. Perhaps it seems to me
a lot of confusion could be solved by getting a more level playing
ground :-)

> > > 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.'

Great :-)



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