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SubjectRe: [PATCH 00/35] Cleanup and optimise the page allocator V3
On Mon, Mar 16, 2009 at 12:02:17PM +0000, Mel Gorman wrote:
> On Mon, Mar 16, 2009 at 12:33:58PM +0100, Nick Piggin wrote:
> > Wheras if you defer this until the point you need a higher order
> > page, the only thing you have to work with are the pages that are
> > free *right now*.
> >
> Well, buddy always uses the smallest available page first. Even with
> deferred coalescing, it will merge up to order-5 at least. Lets say they
> could have merged up to order-10 in ordinary circumstances, they are
> still avoided for as long as possible. Granted, it might mean that an
> order-5 is split that could have been merged but it's hard to tell how
> much of a difference that makes.

But the kinds of pages *you* are interested in are order-10, right?

> > Your anti-frag tests probably don't stress this long term fragmentation
> > problem.
> >
> Probably not, but we have little data on long-term fragmentation other than
> anecdotal evidence that it's ok these days.

Well, I think before anti-frag there was lots of anecdotal evidence
that it's "ok", except for loads heavily using large higher order
allocations. I don't know if we'd have many systems running with
hundreds of days of uptime on such workloads post-anti-frag?

Google might? But I don't know how long their uptimes are. I expect
we'd have a better idea in a couple more years after the next
enterprise distro release cycles with anti-frag.

> > Still, it's significant enough that I think it should be made
> > optional (and arguably default to on) even if it does harm higher
> > order allocations a bit.
> >
> I could make PAGE_ORDER_MERGE_ORDER a proc tunable? If it's placed as a
> read-mostly variable beside the gfp_zone table, it might even fit in the
> same cache line.

Hmm, possibly. OTOH I don't like tunables. If you don't think it will
be a problem for hugepage allocations, then I would prefer just to
leave it on and 5 by default (or even less? COSTLY_ORDER?)

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