linux-kernel - Re: [RFC] [PATCH 0/5] Dynamically allocated pageflags.

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Date:	Tue, 24 Jul 2007 08:27:18 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 0/5] Dynamically allocated pageflags.

Hi.

On Tuesday 24 July 2007 08:05:21 Rafael J. Wysocki wrote:
> Hi,
> 
> On Monday, 23 July 2007 15:05, Nigel Cunningham wrote:
> > Hi all.
> > 
> > As we all know, pageflags have been a scarce resource for a while now. 
These 
> > patches seek to help address that issue by adding support for a new type 
> > of 'dynamically allocated' pageflag.
> > 
> > The basic idea is that we use per node & zone bitmaps built out of order 
zero 
> > allocations, to replace bits in page->flags. Bitmaps can be sparse, being 
> > populated when a bit on the page is set, and returning zero for all bits 
in 
> > sparse pages. Untested hotplug support is included.
> > 
> > This method of storing the data does of course come with a performance 
hit. 
> > I've included some simple timing loops in #ifdef'd code that help quantify 
> > that.
> > 
> > Interestingly, the new implementation is actually quicker under some 
> > circumstances. In cases where the usage pattern involves operating on the 
> > flags for a number of pages in succession, the hit involved in getting the 
> > struct pages from main memory appears to be greater than that involved in 
> > calculating which unsigned long and bit to test.
> > 
> > Tested only on UP (x86_64) so far.
> 
> How does it compare to the memory bitmaps used by swsusp, defined in
> kernel/power/snapshot.c?

Looking through kernel/power/snapshot.c, I'd say this implementation has 
advantages in having support for memory hotplugging, sparseness and random 
access to the flags in the bitmap. Like the snapshot.c implementation, it 
uses per-zone bitmaps and has a method that can be used to iterate over the 
contents of a bitmap. Having per-node support might also be useful. I haven't 
looked at the speed of the snapshot.c implementation. Do you see advantages 
to snapshot.c that I might have missed?

Regards,

Nigel
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