linux-kernel - Re: [PATCH v3] PM/Memory-hotplug: Avoid task freezing failures

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Date:	Mon, 21 Nov 2011 22:42:29 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>, pavel@....cz, lenb@...nel.org,
	ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3] PM/Memory-hotplug: Avoid task freezing failures

On 11/21/2011 10:17 PM, Tejun Heo wrote:
> Hello, Rafael.
> 
> On Sat, Nov 19, 2011 at 10:57:19PM +0100, Rafael J. Wysocki wrote:
>>> +	while (!mutex_trylock(&pm_mutex)) {
>>> +		try_to_freeze();
>>> +		msleep(10);
>>
>> The number here seems to be somewhat arbitrary.  Is there any reason not to
>> use 100 or any other number?
> 
> This is a bit moot at this point but, at least for me, yeah, it's a
> number I pulled out of my ass.  That said, I think it's a good number
> to pull out of ass for userland visible retry delays for the following
> reasons.
> 
> * It's a good number - 10! which happens to match the number of
>   fingers I have!  Isn't that just weird? @.@
> 
> * For modern hardware of most classes, repeating not-so-complex stuff
>   every 10ms for a while isn't taxing (or even noticeable) at all.
> 
> * Sub 10ms delays usually aren't noticeable to human beings even when
>   several of them are staggered.  This is very different when you get
>   to 100ms range.
> 
> ie. going from 1ms to 10ms doesn't cost you too much in terms of human
> noticeable latency (for this type of situations anyway) but going from
> 10ms to 100ms does.  In terms of computational cost, the reverse is
> somewhat true too.  So, yeah, I think 10ms is a good out-of-ass number
> for this type of delays.
> 

My God! I had absolutely no idea you had cooked up that number just like
that ;-) Look at how creative I was when defending that number :P
Your justification is not bad either ;-)

[ Well, seriously, I had given a fair amount of thought before incorporating
that number in my patch, by looking at the freezer re-try latency and so on,
which I explained in my reply earlier.]

Anyways, nice one :-)

Thanks,
Srivatsa S. Bhat

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