Re: SLUB: Reduce antifrag max order (fwd)

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andy Whitcroft wrote:
> Mel Gorman wrote:
> 
>>Sorry for resend, I didn't add Andy to the cc as intended.
>>
>>On Sat, 5 May 2007, Christoph Lameter wrote:
>>
>>
>>>My test systems fails to obtain order 4 allocs after prolonged use.
>>>So the Antifragmentation patches are unable to guarantee order 4
>>>blocks after a while (straight compile, edit load).
>>>
>>
>>Anti-frag still depends on reclaim to take place and I imagine you have
>>not altered min_free_kbytes to keep pages free. Also, I don't think
>>kswapd is currently making any effort to keep blocks free at a known
>>desired order although I'm cc'ing Andy Whitcroft to confirm. As the
>>kernel gives up easily when order > PAGE_ALLOC_COSTLY_ORDER, prehaps you
>>should be using PAGE_ALLOC_COSTLY_ORDER instead of
>>DEFAULT_ANTIFRAG_MAX_ORDER for SLUB.
> 
> 
> kswapd only reactively uses orders above 0.  If allocations are pushing
> below the high water marks those will trigger kswapd to reclaim at their
> highest order.  No attempt overall is made to keep "some" higher order
> pages free.

It does try, if you have a look at zone_watermark_ok. But it doesn't
check for pages of a higher order than are being allocated (ie. so
an order-0 alloc could split the last free order-3 page).

This is intentional, because if your workload isn't doing any higher
order allocations, it should not be trying to keep any free.

-- 
SUSE Labs, Novell Inc.

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