linux-kernel - Re: [RFC PATCH 00/19] Cleanup and optimise the page allocator V2

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Date:	Tue, 3 Mar 2009 11:31:46 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	Lin Ming <ming.m.lin@...el.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH 00/19] Cleanup and optimise the page allocator V2

On Mon, 2 Mar 2009, Mel Gorman wrote:

> Going by the vanilla kernel, a *large* amount of time is spent doing
> high-order allocations. Over 25% of the cost of buffered_rmqueue() is in
> the branch dealing with high-order allocations. Does UDP-U-4K mean that 8K
> pages are required for the packets? That means high-order allocations and
> high contention on the zone-list. That is bad obviously and has implications
> for the SLUB-passthru patch because whether 8K allocations are handled by
> SL*B or the page allocator has a big impact on locking.
>
> Next, a little over 50% of the cost get_page_from_freelist() is being spent
> acquiring the zone spinlock. The implication is that the SL*B allocators
> passing in order-1 allocations to the page allocator are currently going to
> hit scalability problems in a big way. The solution may be to extend the
> per-cpu allocator to handle magazines up to PAGE_ALLOC_COSTLY_ORDER. I'll
> check it out.

Then we are increasing the number of queues dramatically in the page
allocator. More of a memory sink. Less cache hotness.

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