linux-kernel - Re: [patch] SLQB slab allocator

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Date:	Wed, 21 Jan 2009 19:19:08 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Lin Ming <ming.m.lin@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] SLQB slab allocator

On Mon, 19 Jan 2009, Nick Piggin wrote:

> The thing IMO you forget with all these doomsday scenarios about SGI's peta
> scale systems is that no matter what you do, you can't avoid the fact that
> computing is about locality. Even if you totally take the TLB out of the
> equation, you still have the small detail of other caches. Code that jumps
> all over that 1024 TB of memory with no locality is going to suck regardless
> of what the kernel ever does, due to physical limitations of hardware.

Typically we traverse lists of objects that are in the same slab cache.

> > Sorry not at all. SLAB and SLQB queue objects from different pages in the
> > same queue.
>
> The last sentence is what I was replying to. Ie. "simplification of
> numa handling" does not follow from the SLUB implementation of per-page
> freelists.

If all objects are from the same page then you need not check
the NUMA locality of any object on that queue.

> > As I sad it pins a single page in the per cpu page and uses that in a way
> > that you call a queue and I call a freelist.
>
> And you found you have to increase the size of your pages because you
> need bigger queues. (must we argue semantics? it is a list of free
> objects)

Right. That may be the case and its a similar tuning to what SLAB does.

> > SLAB and SLUB can have large quantities of objects in their queues that
> > each can keep a single page out of circulation if its the last
> > object in that page. This is per queue thing and you have at least two
>
> And if that were a problem, SLQB can easily be runtime tuned to keep no
> objects in its object lists. But as I said, queueing is good, so why
> would anybody want to get rid of it?

Queing is sometimes good....

> Again, this doesn't really go anywhere while we disagree on the
> fundamental goodliness of queueing. This is just describing the
> implementation.

I am not sure that you understand the fine points of queuing in slub. I am
not a fundamentalist: Queues are good if used the right way and as you say
SLUB has "queues" designed in a particular fashion that solves issus that
we had with SLAB queues.


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