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SubjectRe: [PATCH] allocation looping + kswapd CPU cycles

On Tue, 8 May 2001, David S. Miller wrote:
> Actually, the change was made because it is illogical to try only
> once on multi-order pages. Especially because we depend upon order
> 1 pages so much (every task struct allocated). We depend upon them
> even more so on sparc64 (certain kinds of page tables need to be
> allocated as 1 order pages).
> The old code failed _far_ too easily, it was unacceptable.
> Why put some strange limit in there? Whatever number you pick
> is arbitrary, and I can probably piece together an allocation
> state where the choosen limit is too small.

Agreed, but some allocations of non-zero orders can fall back to other
schemes (such as an emergency buffer, or using vmalloc for a temp
buffer) and don't want to be trapped in __alloc_pages() for too long.

Could introduce another allocation flag (__GFP_FAIL?) which is or'ed
with a __GFP_WAIT to limit the looping?

> So instead, you could test for the condition that prevents any
> possible forward progress, no?

Yes, it is possible to trap when kswapd might not make any useful
progress for a failing non-zero ordered allocation, and to set a global
"force" flag (kswapd_force) to ensure it does something useful.
For order-1 allocations, that would work.

For order-2 (and above) it becomes much more difficult as the page
'reap' routines release/process pages based upon age and do not factor in
whether a page may/will buddy (now or in the near future). This 'blind'
processing of pages can wipe a significant percentage of the page cache
when trying to build a buddy at a high order.

Of course, no one should be doing really large order allocations and
expecting them to succeed. But, if they are doing this, the allocation
should at least fail.


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