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

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Message-ID: <20140721155540.GO30979@8bytes.org>
Date:	Mon, 21 Jul 2014 17:55:40 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Pavel Machek <pavel@....cz>
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] PM / Hibernate: Memory bitmap scalability
 improvements

Hi,

On Mon, Jul 21, 2014 at 04:39:05PM +0200, Pavel Machek wrote:
> So you reduced O(n) to O(n/2 log(n)) ... that's still O(n log(n)), right?

Radix trees have a memory complexity of ~O(n + log(n)). We have an
additional log(n) space requirement for the tree itself, but we linearly
safe memory for each node compared to the old implementation. Since n
beats log(n) at some point there is a point where we need less pages
with the new code than with the old. That break-even point is somewhere
around 32GB of RAM, for smaller sizes we need 2 additional pages at
most.

> 
> > You can also modify the above tool to give you some data to plot, then
> > you will also SEE that there is still a linear relationship between RAM
> > size and memory required by the old and new memory bitmap
> > implementation.
> 
> I believe radix tree is O(log(N)) per operation, but its memory is
> certainly not linear.

Numbers beat believes.


	Joerg

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