linux-kernel - Re: [RFC PATCH] greatly reduce SLOB external fragmentation

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Message-ID: <Pine.LNX.4.64.0801101112350.20353@schroedinger.engr.sgi.com>
Date:	Thu, 10 Jan 2008 11:16:39 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Matt Mackall <mpm@...enic.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>, Hugh Dickins <hugh@...itas.com>,
	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] greatly reduce SLOB external fragmentation

On Thu, 10 Jan 2008, Matt Mackall wrote:

> Here I'm going to differ with you. The premises of the SLAB concept
> (from the original paper) are: 
> 
> a) fragmentation of conventional allocators gets worse over time

Even fragmentation of SLAB/SLUB gets worses over time. That is why we need 
a defrag solution.

> b) grouping objects of the same -type- (not size) together should mean
> they have similar lifetimes and thereby keep fragmentation low

I agree that is crap. The lifetimes argument is mostly only exploitable in 
benchmarks. That is why SLUB just groups them by size if possible.

> d) constructors and destructors are cache-friendly

I agree. Crap too. We removed the destructors. The constructors are needed 
so that objects in slab pages always have a definite state. That is f.e.
necessary for slab defragmentation because it has to be able to inspect an 
object at an arbitrary time and either remove it or move it to another 
slab page.

Constructors also make sense because the initialization of a cache object 
may be expensive. Initializing list heads and spinlocks can take some code 
and that code can be omitted if objects have a definite state when they 
are free. We saw that when measuring the buffer_head constructors effect 
on performance.
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