Re: Page allocator: Single Zone optimizations

[Mel Gorman <mel@csn.ul.ie>]

On Tue, 7 Nov 2006, Christoph Lameter wrote:

> On Tue, 7 Nov 2006, Mel Gorman wrote:
>
>> Hence, I'm still convinced that slab pages for caches like inode and
>> short-lived allocations need to be clustered separetly.
>
> So the problem seems to be that some slab of "reclaimable" slabs are
> not reclaimable at all even with the most aggressive approach?
>

Right. You may be able to shrink the slab cache considerably, but still 
not empty it. By clustering the pages together, shrinking all the caches 
has a chance of freeing up high order pages but there is no guarantee of 
course.

> Then we have a fundamental issue that we are unable to categorize
> pages correctly. EasyReclaimable pages may be unreclaimable because they
> are mlocked.

They are migratable though. In the patchset I am currently working on, I 
identify pages as Movable, Reclaimable and Unmovable. The redefinitions 
are a bit more logical (especially for mlock) and move away from the idea 
of page reclaim being the only way of getting high order allocations to 
succeed.

> Reclaimable (such as slab pages) may turn out to be not
> reclaimable because some entries are pinned.
>

yep. That will hurt hugepage allocations in those blocks but it should 
help allocations required for network cards with large MTUs for example.

> I think we will run into the same issues for EasyReclaim once an
> application generates a sufficient amount of mlocked pages that are
> placed all over the memory of interest.
>

Yep, I agree. At that point, migration will be required but the clustering 
will be in place so that moving all the "movable" pages will result in 
large contiguous free pages.

> Could it be that the only reason that the current approach works is that
> we have not tested with an application that behaves this way?
>

Probably. The applications I currently test are not mlocking. The tests 
currently run workloads that are known to leave the system in a fragmented 
state when they complete. In this situation, higher-order allocations fail 
even when nothing is running and there are no mlocked() pages on the 
standard allocator.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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