linux-kernel - Re: workqueue thing

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Message-ID: <20091223083705.GA25240@elte.hu>
Date:	Wed, 23 Dec 2009 09:37:05 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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


* Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On 12/23/2009 05:01 PM, Ingo Molnar wrote:
> > At least as far as i'm concerned, i'd like to see actual uses. It's a big 
> > linecount increase all things considered:
> > 
> >    20 files changed, 2783 insertions(+), 660 deletions(-)
> > 
> > and you say it _wont_ help performance/scalability (this aspect
> > wasnt clear to me from previous discussions),
> 
> I'm just not sure how it would turn out. I guess it would be an overall win 
> under loaded situations due to lowered cache footprint but I don't think it 
> will be anything which would stand out.
>
> > so the (yet to be seen) complexity reduction in other code ought to be 
> > worth it.
> 
> Sure, fair enough but there's also a different side.  It'll allow much 
> easier implementation of things like in-kernel media presence polling (I 
> have some code for this but it's still just forming) and per-device.  It 
> gives a much easier tool to extract concurrency and thus opens up new 
> possibilities.
> 
> So, anyways, alright, I'll go try some conversions.

Well, but note that you are again talking performance. Concurrency _IS_ 
performance: either in terms of reduced IO/app/request latency or in terms of 
CPU utilization.

Both metrics can be measured (and there's a massive effort underway to help 
measure such things - see current results under tools/perf/ in your favorite 
kernel repo ;-)

(Plus reduction in driver complexity can be measured as well, in the diffstat 
space.)

So there's no leap of faith needed really, IMHO.

	Ingo
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