Re: [RPC][PATCH 2.6.20-rc5] limit total vfs page cache

["Aubrey Li" <aubreylee@gmail.com>]

On 1/20/07, Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> Aubrey Li wrote:
>
> > So what's the right way to limit pagecache?
>
> Probably something a lot more complicated... if you can say there
> is a "right way".
>
> >> Secondly, your patch isn't actually very good. It unconditionally
> >> shrinks memory to below the given % mark each time a pagecache alloc
> >> occurs, regardless of how much pagecache is in the system. Effectively
> >> that seems to just reduce the amount of memory available to the system.
> >
> >
> > It doesn't reduce the amount of memory available to the system. It
> > just reduce the amount of memory available to the page cache. So that
> > page cache is limited and the reserved memory can be allocated by the
> > application.
>
> But the patch doesn't do that, as I explained.

I'm not sure you read the correct patch. Let me explain the logic again.

assume:
min = 123pages
pagecache_reserved = 200 pages

if( alloc_flags & ALLOC_PAGECACHE)
        watermark = min + pagecache_reserved ( 323 pages)
else
        watermark = min ( 123 pages)

So if request pagecache, when free pages < 323 pages, reclaim is triggered.
But at this time if request memory not pagecache, reclaim will be
triggered when free pages < 123 as the present reclaimer does.

I verified it on my side, why do you think it doesn't work properly?

>
> >> Luckily, there are actually good, robust solutions for your higher
> >> order allocation problem. Do higher order allocations at boot time,
> >> modifiy userspace applications, or set up otherwise-unused, or easily
> >> reclaimable reserve pools for higher order allocations. I don't
> >> understand why you are so resistant to all of these approaches?
> >>
> >
> > I think we have explained the reason too much. We are working on
> > no-mmu arch and provide a platform running linux to our customer. They
> > are doing very good things like mplayer, asterisk, ip camera, etc on
> > our platform, some applications was migrated from mmu arch. I think
> > that means in some cases no-mmu arch is somewhat better than mmu arch.
> > So we are taking effort to make the migration smooth or make no-mmu
> > linux stronger.
> > It's no way to let our customer modify their applications, we also
> > unwilling to do it. And we have not an existing mechanism to set up a
> > pools for the complex applications. So I'm trying to do some coding
> > hack in the kernel to satisfy these kinds of requirement.
>
> Oh, maybe you misunderstand the reserve pools idea: that is an entirely
> kernel based solution where you can preallocate a large, contiguous
> pool of memory at boot time which you can use to satisfy your nommu
> higher order anonymous memory allocations.
>
> This is something that will not get fragmented by pagecache, nor will
> it get fragmented by any other page allocation, slab allocation. Tt is
> a pretty good solution provided that you size the pool correctly for
> your application's needs.
>

So if application malloc(1M), how does kernel know to allocate
reserved pool not from buddy system? I didn't see any special code
about this. Is there any doc or example?

-Aubrey

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