linux-kernel - Re: [PATCH] fastboot: keep at least one thread per cpu during boot

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Message-ID: <20090209133428.GA4705@nowhere>
Date:	Mon, 9 Feb 2009 14:34:29 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Cornelia Huck <cornelia.huck@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fastboot: keep at least one thread per cpu during boot

On Sun, Feb 08, 2009 at 09:27:48PM -0800, Arjan van de Ven wrote:
> On Mon, 9 Feb 2009 04:48:27 +0100
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > Async threads are created and destroyed depending on the number of
> > jobs in queue. It means that several async threads can be created for
> > a specific batch of work, then the threads will die after the
> > completion of this batch, but they could be needed just after this
> > completion for another batch of work. During the boot, such
> > repetitive thread creations can be wasteful, that's why this patch
> > proposes to keep at least one thread per cpu (if they already have
> > been created once). Such a threshold of threads kept alive will
> > prevent from a part of the thread creation overhead. This threshold
> > will be dropped one the system_state switches from SYSTEM_BOOTING to
> > SYSTEM_RUNNING.
> 
> I'm not very fond of this to be honest;
> at least during boot there's enough activity, and the time is so short
> (that's the point of the parallel stuff!) that this will not kick in to
> make a difference; specifically, every boot I've seen the number of
> threads is highest near the end, and also the total kernel boot time is
> below 1.5 seconds or so, not long enough for the threads to die.


My boot takes more time (about 5 seconds before modules loading).

 
> Creating a thread is *CHEAP*. Really really cheap. You can do 100
> thousands/second on even a modest CPU. If you have a high frequency of
> events, you don't want this, sure, and that is why there is a one
> second delay to give opportunity for reuse... but really....


Ok. And that's a problem with my patch. I did not have a suitable
testcase to produce a relevant benchmark: the aync insertions were to close
in time to capture something interesting.
If I saw the result of a testcase, I would have seen probably no different :-)

I guess you're right, this would have added new code to maintain for only micro optimizations...


> 
> Now, if async function calls get used more, I can see the point of
> always keeping one thread alive, just for both performance and VM low
> memory issues; but that's not what your patch is doing.


Ok.
Perhaps the testcase would be suitable under embedeed systems.
I will perhaps test one day :-)

Thanks!

> -- 
> Arjan van de Ven 	Intel Open Source Technology Centre
> For development, discussion and tips for power savings, 
> visit http://www.lesswatts.org

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