linux-kernel - Re: [PATCH 0/6 v2] PM / Hibernate: Memory bitmap scalability improvements

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Date:	Tue, 22 Jul 2014 12:55:46 +0200
From:	Pavel Machek <pavel@....cz>
To:	Joerg Roedel <joro@...tes.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6 v2] PM / Hibernate: Memory bitmap scalability
 improvements

On Tue 2014-07-22 12:34:44, Joerg Roedel wrote:
> On Tue, Jul 22, 2014 at 02:41:29AM +0200, Rafael J. Wysocki wrote:
> > It looks like some specific need motivated the Joerg's work, however,
> > so let's just not dismiss the use case lightly without knowing it.
> 
> The motivation was to optimize the data structures for machines with
> large amounts of RAM without penalizing average machines. On a 12TB
> machine you are close to 100000 pages just for one bitmap. Scanning
> through that linearly to find a given bit just doesnt scale anymore in
> this case.

Writing out every single page on 12TB machine to disk does not scale,
either :-).

> I also see how the problem could be solved differently, but what I
> didn't get from the discussion yet is: What is actually *wrong* with
> *this* approach?

It throws complex / tricky to review code at a problem... that is not
a problem in any reasonable configuration.

Now... should I spend half an hour reviewing your changes, or are we
maybe better without them?

> So we save around 50ms (or 62% of time) already on this 16GB machine.

Or about 5 seeks or about 0.000% of total hibernation cycle.
									Pavel
-- 
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