linux-kernel - Re: High priority tasks break SMP balancer?

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Message-ID: <b647ffbd0711270121v691ec06bjf62166c83929d844@mail.gmail.com>
Date:	Tue, 27 Nov 2007 10:21:12 +0100
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Micah Dowty" <micah@...are.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Christoph Lameter" <clameter@....com>,
	"Kyle Moffett" <mrmacman_g4@....com>,
	"Cyrus Massoumi" <cyrusm@....net>,
	"LKML Kernel" <linux-kernel@...r.kernel.org>,
	"Andrew Morton" <akpm@...l.org>, "Mike Galbraith" <efault@....de>,
	"Paul Menage" <menage@...gle.com>,
	"Peter Williams" <pwil3058@...pond.net.au>
Subject: Re: High priority tasks break SMP balancer?

On 26/11/2007, Micah Dowty <micah@...are.com> wrote:
>
> The application doesn't really depend on the load-balancer's decisions
> per se, it just happens that this behaviour I'm seeing on NUMA systems
> is extremely bad for performance.
>
> In this context, the application is a virtual machine runtime which is
> executing either an SMP VM or it's executing a guest which has a
> virtual GPU. In either case, there are at least three threads:
>
>   - Two virtual CPU/GPU threads, which are nice(0) and often CPU-bound
>   - A low-latency event handling thread, at nice(-10)
>
> The event handling thread performs periodic tasks like delivering
> timer interrupts and completing I/O operations.

Are I/O operations initiated by these "virtual CPU/GPU threads"?

If so, would it make sense to have per-CPU event handling threads
(instead of one global)? They would handle I/O operations initiated
from their respective CPUs to (hopefully) achieve better data locality
(esp. if the most part of the involved data is per-CPU).

Then let the load balancer to evenly distribute the "virtual CPU/GPU
threads" or even (at least, as an experiment) fix them to different
CPUs as well?

sure, the scenario is highly dependent on a nature of those
'events'... and I can just speculate here :-) (but I'd imagine
situations when such a scenario would scale better).


>
> Thank you again,
> --Micah
>

-- 
Best regards,
Dmitry Adamushko
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