linux-kernel - Re: [15/17] SLUB: Support virtual fallback via SLAB

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Date:	Tue, 2 Oct 2007 18:43:14 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Christoph Hellwig <hch@....de>, Mel Gorman <mel@...net.ie>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Chinner <dgc@....com>, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [15/17] SLUB: Support virtual fallback via SLAB_VFALLBACK

On Tuesday 02 October 2007 06:50, Christoph Lameter wrote:
> On Fri, 28 Sep 2007, Nick Piggin wrote:
> > I thought it was slower. Have you fixed the performance regression?
> > (OK, I read further down that you are still working on it but not
> > confirmed yet...)
>
> The problem is with the weird way of Intel testing and communication.
> Every 3-6 month or so they will tell you the system is X% up or down on
> arch Y (and they wont give you details because its somehow secret). And
> then there are conflicting statements by the two or so performance test
> departments. One of them repeatedly assured me that they do not see any
> regressions.

Just so long as there aren't known regressions that would require higher
order allocations to fix them.

> > OK, so long as it isn't going to depend on using higher order pages,
> > that's fine. (if they help even further as an optional thing, that's fine
> > too. You can turn them on your huge systems and not even bother about
> > adding this vmap fallback -- you won't have me to nag you about these
> > purely theoretical issues).
>
> Well the vmap fallback is generally useful AFAICT. Higher order
> allocations are common on some of our platforms. Order 1 failures even
> affect essential things like stacks that have nothing to do with SLUB and
> the LBS patchset.

I don't know if it is worth the trouble, though. The best thing to do is to
ensure that contiguous memory is not wasted on frivolous things... a few
order-1 or 2 allocations aren't too much of a problem.

The only high order allocation failure I've seen from fragmentation for a
long time IIRC are the order-3 failures coming from e1000. And obviously
they cannot use vmap.
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