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From: Tejun Heo <tj@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Don <joshdon@google.com>, Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Paul Turner <pjt@google.com>,
	David Rientjes <rientjes@google.com>,
	Oleg Rombakh <olegrom@google.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Steve Sistare <steven.sistare@oracle.com>,
	linux-kernel@vger.kernel.org, Rik van Riel <riel@surriel.com>
Subject: Re: [PATCH] sched: cgroup SCHED_IDLE support
Date: Sat, 26 Jun 2021 06:06:32 -0400	[thread overview]
Message-ID: <YNb8KI5zww8Pweat@mtj.duckdns.org> (raw)
In-Reply-To: <YNWPBDH/mQdojEu9@hirez.programming.kicks-ass.net>

Hello, Peter.

On Fri, Jun 25, 2021 at 10:08:36AM +0200, Peter Zijlstra wrote:
> It's a direct concequence of the hierarchical requirement. The approach
> is the only valid one. The other relative controllers that don't do
> this, are simply broken.
> 
> Absolute controllers have it easier, they can be trivially flattened.

That's too strong a claim. e.g. iocost controller, while in a different
domain, is a weight controller which takes different trade-offs to achieve
hierarchical weight based distribution at negligible nesting overhead. There
usually are more than one way to skin a cat.

> > There are several practical challenges with the current implementation
> > caused by the full nesting - e.g. nesting levels are expensive for context
> > switch heavy applicaitons often going over >1% per level,
> 
> Yeah, and there's numerical problems you run into as well due to
> limitied precision.

Another issue is per-queue level heuristics like boosting after idle
nesting not in quite optimal ways.

> Just don't do deep hierarchies.
>
> AFAICT it's a simple matter of conflicting requirements, on the one hand
> the hierarchical thing is required, on the other hand people seem to
> think all this crap is 'free' and create super deep hierarchies and then
> complain shit don't work right.

The problem is that the overhead is significant enough even at pretty
shallow levels. Even at just two/three levels, the cost is already
significant enough for some large-scale applications.

Thanks.

-- 
tejun

  reply	other threads:[~2021-06-26 10:06 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-08 23:11 [PATCH] sched: cgroup SCHED_IDLE support Josh Don
2021-06-10 12:53 ` Dietmar Eggemann
2021-06-10 19:14   ` Josh Don
2021-06-11 16:43     ` Dietmar Eggemann
2021-06-11 23:34       ` Josh Don
2021-06-15 10:06         ` Dietmar Eggemann
2021-06-15 23:30           ` Josh Don
2021-06-25  9:24           ` Peter Zijlstra
2021-06-16 15:42 ` Tejun Heo
2021-06-17  1:01   ` Josh Don
2021-06-26  9:57     ` Tejun Heo
2021-06-29  4:57       ` Josh Don
2021-06-25  8:08   ` Peter Zijlstra
2021-06-26 10:06     ` Tejun Heo [this message]
2021-06-26 11:42     ` Rik van Riel
2021-06-25  8:14 ` Peter Zijlstra
2021-06-26  0:18   ` Josh Don
2021-06-25  8:20 ` Peter Zijlstra
2021-06-26  0:35   ` Josh Don

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