Subject: Re: newidle balancing in NUMA domain?
From: Peter Zijlstra <peterz@infradead.org>
To: Nick Piggin <npiggin@suse.de>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>
In-Reply-To: <20091123112228.GA2287@wotan.suse.de>
References: <20091123112228.GA2287@wotan.suse.de>
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Date: Mon, 23 Nov 2009 12:36:15 +0100
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On Mon, 2009-11-23 at 12:22 +0100, Nick Piggin wrote:
> Hi,
> 
> I wonder why it was decided to do newidle balancing in the NUMA
> domain? And with newidle_idx == 0 at that.
> 
> This means that every time the CPU goes idle, every CPU in the
> system gets a remote cacheline or two hit. Not very nice O(n^2)
> behaviour on the interconnect. Not to mention trashing our
> NUMA locality.
> 
> And then I see some proposal to do ratelimiting of newidle
> balancing :( Seems like hack upon hack making behaviour much more
> complex.
> 
> One "symptom" of bad mutex contention can be that increasing the
> balancing rate can help a bit to reduce idle time (because it
> can get the woken thread which is holding a semaphore to run ASAP
> after we run out of runnable tasks in the system due to them 
> hitting contention on that semaphore).
> 
> I really hope this change wasn't done in order to help -rt or
> something sad like sysbench on MySQL.

IIRC this was kbuild and other spreading workloads that want this.

the newidle_idx=0 thing is because I frequently saw it make funny
balance decisions based on old load numbers, like f_b_g() selecting a
group that didn't even have tasks in anymore.

We went without newidle for a while, but then people started complaining
about that kbuild time, and there is a x264 encoder thing that looses
tons of throughput.

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