Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[Nick Piggin]

On Thursday 13 September 2007 09:06, Christoph Lameter wrote:
> On Wed, 12 Sep 2007, Nick Piggin wrote:

> > So lumpy reclaim does not change my formula nor significantly help
> > against a fragmentation attack. AFAIKS.
>
> Lumpy reclaim improves the situation significantly because the
> overwhelming majority of allocation during the lifetime of a systems are
> movable and thus it is able to opportunistically restore the availability
> of higher order pages by reclaiming neighboring pages.

I'm talking about non movable allocations.


> > [*] ok, this isn't quite true because if you can actually put a hard
> > limit on unmovable allocations then anti-frag will fundamentally help --
> > get back to me on that when you get patches to move most of the obvious 
> > ones.
>
> We have this hard limit using ZONE_MOVABLE in 2.6.23.

So we're back to 2nd class support.


> > Sure, and I pointed out the theoretical figure for 64K pages as well. Is
> > that figure not problematic to you? Where do you draw the limit for what
> > is acceptable? Why? What happens with tiny memory machines where a
> > reserve or even the anti-frag patches may not be acceptable and/or work
> > very well? When do you require reserve pools? Why are reserve pools
> > acceptable for first-class support of filesystems when it has been very
> > loudly been made a known policy decision by Linus in the past (and for
> > some valid reasons) that we should not put limits on the sizes of caches
> > in the kernel.
>
> 64K pages may problematic because it is above the PAGE_ORDER_COSTLY in
> 2.6.23. 32K is currently much safer because lumpy reclaim can restore
> these and does so on my systems. I expect the situation for 64K pages to
> improve when more of Mel's patches go in. We have long term experience
> with 32k sized allocation through Andrew's tree.
>
> Reserve pools as handled (by the not yet available) large page pool
> patches (which again has altogether another purpose) are not a limit. The
> reserve pools are used to provide a mininum of higher order pages that is
> not broken down in order to insure that a mininum number of the desired
> order of pages is even available in your worst case scenario. Mainly I
> think that is needed during the period when memory defragmentation is
> still under development.

fsblock doesn't need any of those hacks, of course.
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