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

["Stephen C. Tweedie" <sct@redhat.com>]

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.

--Stephen
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