On Sunday den 2 September 2001 03:57, Daniel Phillips wrote:
> On September 1, 2001 08:28 pm, Stephan von Krawczynski wrote:
>
> The next part of the theory says that the higher order allocations are
> failing because of fragmentation.  I put considerable thought into this
> today while wandering around in a dungeon in Berlin watching bats (really)
> and I will post an article to lkml tomorrow with my findings.  To summarize
> briefly here: a Linux system in steady state operation *is* going to show
> physical fragmentation so that the chance of a higher order allocation
> succeeding becomes very small.  The chance of failure increases
> exponentially (or worse) with a) the allocation order and b) the ratio of
> allocated to free memory.  The second of these you can control: the higher
> you set zone->pages_min the better chance your higher order allocations
> will have to succeed.  Do you want a patch for that, to see if this works
> in practice?
>

You beat me to it, with some minutes...
(I sent a email to Stephan...)

> Of course it would be much better if we had some positive mechanism for
> defragging physical memory instead of just relying on chance and hoping
> for the best the way we do now.  IMHO, such a mechanism can be built
> practically and I'm willing to give it a try.  Basically, kswapd would try
> to restore a depleted zone order list by scanning mem_map to find buddy
> partners for free blocks of the next lower order.  This strategy, together
> with the one used in the patch above, could largly eliminate atomic
> allocation failures.  (Although as I mentioned some time ago, getting rid
> of them entirely is an impossible problem.)
>
> The question remains why we suddenly started seeing more atomic allocation
> failures in the recent Linus trees.  I'll guess that the changes in
> scanning strategy have caused the system to spend more time close to the
> zone->pages_min amount of free memory.  This idea seems to be supported by
> your memstat listings.  In some sense, it's been good to have the issue
> forced so that we must come up with ways to make atomic and higher order
> allocations less fragile.

It might be that the elevator works now... :-)

You will only see it once there are no remaining free pages of an even higher
order left - then you will start to fail...

Two things are required:
1) You have lots of memory.
2) You have used it all at some point.

Another thing to do could be to add a order parameter to free.
The pages allocated has to be freed sometime... if we make sure that
they are freed together it could simplify things - no chance that the
first part gets allocated directly...

Or/and we could remove the sources of higher order allocs, as an example:
why is the SCSI layer allowed to allocate order 3 allocs (32 kB) several
times per second? Will we really get a performance hit by not allowing
higher order allocs in active driver code?

/RogerL
-- 
Roger Larsson
Skellefteå
Sweden
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