linux-kernel - Re: [RFC][PATCH] create workqueue threads only when needed

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Date:	Mon, 02 Feb 2009 20:05:28 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC][PATCH] create workqueue threads only when needed

On Mon, 2009-02-02 at 09:42 +0100, Stefan Richter wrote:

> >  
> > IE. It will be pretty responsive -in general- but can suffer form
> > horrible latencies every now and then.
> 
> Actually it /should/ be the other way around:
> 
> The shared workqueue should only be used for work that sleeps only
> briefly (perhaps with the exception of very unlikely longer sleeps e.g.
> for allocations that cause paging).

I agree, I'm just stating the current situation :-) Hopefully something
like async funcs / slow work / whatever will take over the case of stuff
that wants to be around for longer. I haven't had a chance to look at
the async funcs yet, sounds like they may do the job tho in which case
I'll look at converting a driver or two to use them.

> Work which /may/ sleep longer, for example performs SCSI transactions,
> needs to go into a private workqueue or other kind of context.

Well, it's a bit silly to allocate a private workqueue with all it's
associated per CPU kernel threads for something as rare as resetting
your eth NIC ... or even SCSI error handling in fact.

> OTOH you are right too; work which must not be deferred too long by work
> from another uncooperative/ unfair subsystem is probably also better off
> in an own workqueue...

I suspect the main reason for dedicated work queues is that, plus the
per-CPU affinity.

Cheers,
Ben.

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