Re: [RFC] Limit the size of the pagecache

[Aubrey Li]

On 1/24/07, Christoph Lameter <clameter@xxxxxxx> wrote:
> On Wed, 24 Jan 2007, Nick Piggin wrote:
>
> > > 1. Insure that anonymous pages that may contain performance
> > >    critical data is never subject to swap.
> > >
> > > 2. Insure rapid turnaround of pages in the cache.
> >
> > So if these two aren't working properly at 100%, then I want to know the
> > reason why. Or at least see what the workload and the numbers look like.
>
> The reason for the anonymous page may be because data is rarely touched
> but for some reason the pages must stay in memory. Rapid turnaround is
> just one of the reason that I vaguely recall but I never really
> understood what the purpose was.
>
> > > 3. Reserve memory for other uses? (Aubrey?)
> >
> > Maybe. This is still a bad hack, and I don't like to legitimise such use
> > though. I hope Aubrey isn't relying on this alone for his device to work
> > because his customers might end up hitting fragmentation problems sooner
> > or later.
>
> I surely wish that Aubrey would give us some more clarity on
> how this should work. Maybe the others who want this feature could also
> speak up? I am not that clear on its purpose.
Sorry for the delay. Somehow this thread was put into the spam folder
of my gmail box. :(
The patch I posted several days ago works properly on my side. I'm
working on blackfin-uclinux platform. So I'm not sure it works 100% on
the other arch platform. From O_DIRECT threads, I know different
people suffer from VFS pagecache issue for different reason. So I
really hope the patch can be improved.

On my side, When VFS pagecache eat up all of the available memory,
applications who want to allocate the largeish block(order =4 ?) will
fail. So the logic is as follows:

if request pagecache
     watermark =  min + reserved_pagecache.
else
     watermark =  min.

Here, assume min=123 pages, reserved_pagecache = 200 pages. That means
when VFS pagecache eat up its all of available memory, there are still
200 pages available for the allocation of the application. Does that
make sense?

> I hope Aubrey isn't relying on this alone for his device to work
> because his customers might end up hitting fragmentation problems sooner
> or later.

That's true. I wrote a replacement of buddy system, it's here:
http://lkml.org/lkml/2006/12/30/36.

That can improve the fragmentation problems on our platform.

Christoph - I can't find your original patch, Can you send me again?
it would be great if you merged all of the  enhancement.

Thanks,
-Aubrey
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