linux-kernel - Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and SPECjvm2008 benchmarks against 4.6 kernel

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Message-ID: <20160623184318.GX30927@twins.programming.kicks-ass.net>
Date:	Thu, 23 Jun 2016 20:43:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jirka Hladky <jhladky@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Kamil Kolakowski <kkolakow@...hat.com>
Subject: Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and
 SPECjvm2008 benchmarks against 4.6 kernel

On Thu, Jun 23, 2016 at 08:33:18PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 17, 2016 at 01:04:23AM +0200, Jirka Hladky wrote:
> 
> > > What kind of config and userspace setup? Do you run this cruft in a
> > > cgroup of sorts?
> > 
> >  No, we don't do any special setup except to control the number of threads.
> 
> OK, so I'm fairly certain you _do_ run in a cgroup, because its made
> almost impossible not to these days.
> 
> Run:
> 
> 	grep java /proc/sched_debug
> 
> while the thing is running. That'll show you the actual cgroup the stuff
> is running in.

That'll end up looking something like:

root@...-ep:/usr/src/linux-2.6# grep java /proc/sched_debug
            java  2714     18270.634925        89   120         0.000000         1.490023         0.000000 0 0 /user.slice/user-0.slice/session-2.scope
            java  2666     18643.629673         2   120         0.000000         0.063129         0.000000 0 0 /user.slice/user-0.slice/session-2.scope
            java  2676     18655.652878         3   120         0.000000         0.077127         0.000000 0 0 /user.slice/user-0.slice/session-2.scope
            java  2680     18655.683384         3   120         0.000000         0.082993         0.000000 0 0 /user.slice/user-0.slice/session-2.scope

which shows a 3 deep hierarchy. Clearly these people haven't the
faintest clue about the cost of what they're doing. This stuff ain't
free.