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SubjectRe: [RFC] [PATCH 0/5] Dynamically allocated pageflags.
Date
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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