Re: [PATCH] Separate global/perzone inactive/free shortage

[Mike Galbraith <mikeg@wen-online.de>]

On Mon, 16 Jul 2001, Stephen C. Tweedie wrote:

> Hi,
>
> On Mon, Jul 16, 2001 at 05:44:17PM +0200, Mike Galbraith wrote:
>
> > > Why not just round-robin between the eligible zones when allocating,
> > > biasing each zone based on size?
>
> > What prevents this from happening, and lets make ZONE_DINKY _really_
> > dinky just for the sake of argument.  ZONE_DINKY will have say 4 pages,
> > one for active, dirty, clean and free.  Balanced is 2 dirty and 2 free,
> > or 1 free, 1 clean and 1 dirty.  2 tasks are running, and both are giant
> > economy size, with very nearly 2gig of vm allocated each.
> >
> > ZONE_DINKY, ZONE_BIG, and ZONE_MONDO are all fully engaged and under
> > pressure.  ZONE_DINKY gets aged/laundered such that it is in balance.
> > Task A is using 1 ZONE_DINKY page.  Task B requests a page to do pagein,
> > and reclaims a page from ZONE_DINKY because there's only 1 free page.
> > We are back to inactive shortage instantly, so we have to walk 4gig of
> > vm looking for one ZONE_DINKY page to activate/age/deactivate.  During
> > the aging process, any other in use page from that zone is fair game.
>
> Agreed, but in that sort of case, if we have (say) close 1GB in
> ZONE_NORMAL and 16MB in ZONE_DMA, then only one allocation in 64 will
> even _try_ to allocate from the DMA zone.  Replace the DMA zone with a
> hypothetical DINKY 4-page zone and it goes down to one allocation in
> 65536.  You don't reduce the cost of a DINKY allocation, but you
> reduce the change that such an allocation will happen.
>
> The balanced round-robin still seems like a helpful next step here
> even if it doesn't cure all the balance problems immediately.

Yes, this should mitigate the effect.  I think something will still
end up having to be done about the search time though.  Dirk's case
seems to be the pathalogical one.

	-Mike

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