linux-kernel - Re: [PATCH 34/35] async: use workqueue for worker pool

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Message-ID: <20100629155228.GK5318@nowhere>
Date:	Tue, 29 Jun 2010 17:52:29 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, oleg@...hat.com, axboe@...nel.dk,
	dwalker@...eaurora.org, stefanr@...6.in-berlin.de,
	florian@...kler.org, andi@...stfloor.org, mst@...hat.com,
	randy.dunlap@...cle.com, Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 34/35] async: use workqueue for worker pool

On Tue, Jun 29, 2010 at 05:46:32PM +0200, Tejun Heo wrote:
> Hello,
> 
> On 06/29/2010 02:18 PM, Frederic Weisbecker wrote:
> >> Yeah, well, that's kind of the whole point of cmwq.  It would try to
> >> minimize the number of used workers but the provided concurrency will
> >> still be enough.  No async probe will be stalled due to lack of
> >> execution context and the timings should be about the same between the
> >> original async implemetnation and cmwq based one.
> > 
> > Right. I just don't know what is supposed to be slow on boot that
> > needs to use async.  Is that because reading some ports is slow or
> > because we need to do something and wait for some times to get the
> > result.
> 
> It's things like ATA bus resetting and probing.  They're usually
> composed of short CPU activities and rather long sleeps.


Ok.


 
> > If there is a question of slow ports to probe, then cmwq wouldn't seem the
> > right thing here, as it only forks when we go to sleep.
> 
> I lost you here.  If something during boot has to burn cpu cycles
> (which it shouldn't, really), it has to burn cpu cycles and having
> multiple concurent threads won't help anything.



It would on SMP.



> If something doesn't
> burn cpu cycles but takes long, it gotta sleep and cmwq will start a
> new thread immediately.  So, can you please elaborate why cmwq would
> be problematic?


No in this case it's not problematic, as far as the things that were using
async have a small cpu burn and long sleep waiting, it looks like cmwq
fits :)

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