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

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Message-Id: <200707241147.05888.rjw@sisk.pl>
Date:	Tue, 24 Jul 2007 11:47:05 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	nigel@...pend2.net
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 0/5] Dynamically allocated pageflags.

On Tuesday, 24 July 2007 00:27, Nigel Cunningham wrote:
> 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.

Well, that's what I'm really interested in.

> Do you see advantages to snapshot.c that I might have missed?

Not at the moment, but I need to have a closer look.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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