Re: OOM fixes 2/5

[Nick Piggin]

On Thu, 2005-01-20 at 22:46 -0800, Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:

> > It does turn on lowmem protection by default. We never reached
> > an agreement about doing this though, but Andrea has shown that
> > it fixes trivial OOM cases.
> > 
> > I think it should be turned on by default. I can't recall what
> > your reservations were...?
> > 
> 
> Just that it throws away a bunch of potentially usable memory.  In three
> years I've seen zero reports of any problems which would have been solved
> by increasing the protection ratio.
> 
> Thus empirically, it appears that the number of machines which need a
> non-zero protection ratio is exceedingly small.  Why change the setting on
> all machines for the benefit of the tiny few?  Seems weird.  Especially
> when this problem could be solved with a few-line initscript.  Ho hum.


That is true, but it should not reserve a great deal of memory on
small memory machines. ZONE_NORMAL reservation may not even be too
noticeable as you'll usually have ZONE_NORMAL allocations during
the course of normal running.

Although it is true that there haven't been many problems attributed
to this, one example I can remember is when we fixed the __alloc_pages
watermark code, we fixed a bug that was reserving much more ZONE_DMA
than it was supposed to. This cased all those page allocation failure
problems. So we raised the atomic reserve, but that didn't bring
ZONE_DMA reservation back to its previous levels.

"So the buffer between GFP_KERNEL and GFP_ATOMIC allocations is:

2.6.8      | 465 dma, 117 norm, 582 tot = 2328K
2.6.10-rc  |   2 dma, 146 norm, 148 tot =  592K
patch      |  12 dma, 500 norm, 512 tot = 2048K"

So we were still seeing GFP_DMA allocation failures in the sound code.
You recently had to make that NOWARN to shut it up.

OK this is a fairly lame example... but the current code is more or
less just lucky that ZONE_DMA doesn't usually fill up with pinned mem
on machines that need explicit ZONE_DMA allocations.



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