Re: O_DIRECT question

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On 1/11/07, Nick Piggin <[email protected]> wrote:
Andrew Morton wrote:
> On Thu, 11 Jan 2007 14:45:12 +0800
> Aubrey <[email protected]> wrote:
>
>
>>>In the interim you could do the old "echo 3 > /proc/sys/vm/drop_caches"
>>>thing, but that's terribly crude - drop_caches is really only for debugging
>>>and benchmarking.
>>>
>>
>>Yes. This method can drop caches, but will fragment memory.
>
>
> That's what page reclaim will do as well.
>
> What you want is Mel's antifragmentation work, or lumpy reclaim.
>
>
>>This is
>>not what I want. I want cache is limited to a tunable value of the
>>whole memory. For example, if total memory is 128M, is there a way to
>>trigger reclaim when cache size > 16M?
>
>
> If there was, it'd "fragment memory" as well.
>
> You might get a little benefit from increasing /proc/sys/vm/min_free_kbytes,
> but not much.  Some page allocation tweaks would aid that.
>
> But basically, to do this well, serious work is needed.

OTOH, the antifragmentation stuff can also break down eventually,
especially if higher order allocations are actually in common use.

That's right. When VFS cache eat up almost all of the memory, I think
no memory algorithm can help the case, including Mei's anti-fragment
patch.


What you _really_ want to do is avoid large mallocs after boot, or use
a CPU with an mmu. I don't think nommu linux was ever intended to be a
simple drop in replacement for a normal unix kernel.

Is there a position available working on mmu CPU? Joking, :)
Yes, some problems are serious on nommu linux. But I think we should
try to fix them not avoid them.

-Aubrey
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