linux-kernel - Re: workqueue thing

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Message-ID: <20091223060229.GA14805@elte.hu>
Date:	Wed, 23 Dec 2009 07:02:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org
Subject: Re: workqueue thing


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Workqueues generally aren't about heavy CPU usage, although some workqueue 
> usage has scalability issues. And the most common scalability problem is not 
> "I need more than one CPU", but often "I need to run even though another 
> workqueue entry is blocked on IO" - iow, it's not about lacking CPU power, 
> it's about in-fighting with other workqueue users.
> 
> That said, if we can improve on this further, I'd be all for it. I'd love to 
> have some generic async model that really works. So far, our async models 
> have tended to not really work out well, whether they be softirq's or kernel 
> threads (many of the same issues: some subsystems start tons of kernel 
> threads just because one kernel thread blocks, not because you need 
> multi-processor CPU usage per se). And AIO/threadlets never got anywhere etc 
> etc.

Not from lack of trying though ;-)

One key thing i havent seen in this discussion are actual measurements. I 
think a lot could be decided by simply testing this patch-set, by looking at 
the hard numbers: how much faster (or slower) did a particular key workload 
get before/after these patches.

Likewise, if there's a reduction in complexity, that is a tangible metric as 
well: lets do a few conversions as part of the patch-set and see how much 
simpler things have become as a result of it.

We really are not forced to the space of Gedankenexperiments here.

	Ingo
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