Message-ID: <20071209085030.GA14264@elte.hu>
Date:	Sun, 9 Dec 2007 09:50:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: tipc_init(), WARNING: at arch/x86/mm/highmem_32.c:52,
	[2.6.24-rc4-git5: Reported regressions from 2.6.23]


* Pekka Enberg <penberg@...helsinki.fi> wrote:

> Hi Ingo,
> 
> On Dec 8, 2007 10:29 PM, Ingo Molnar <mingo@...e.hu> wrote:
> > so it has a "free list", which is clearly per cpu. Hang on! Isnt that
> > actually a per CPU queue? Which SLUB has not, we are told? The "U" in
> > SLUB. How on earth can an allocator in 2007 claim to have no queuing
> > (which is in essence caching)? Am i on crack with this? Did i miss
> > something really obvious?
> 
> I think you did. The difference is explained in Christoph's 
> announcement:
> 
> "A particular concern was the complex management of the numerous 
> object queues in SLAB. SLUB has no such queues. Instead we dedicate a 
> slab for each allocating CPU and use objects from a slab directly 
> instead of queueing them up."
> 
> Which, I think, is where SLUB gets its name from (the "unqueued" 
> part).

yes, i understand the initial announcement (and the Kconfig entry still 
says the same), but that is not matched up by the reality i see in the 
actual code - SLUB clearly uses a queue/list of objects (as cited in my 
previous mail), for obvious performance reasons.

unless i'm missing something obvious (and i easily might), i see SLUB as 
SLAB reimplemented with a different queueing model. Not "without 
queueing".

> Now, while SLAB code is "pleasant and straightforward code" (thanks, 
> btw) for UMA, it's really hairy for NUMA plus the "alien caches" eat 
> tons of memory (which is why Christoph wrote SLUB in the first place, 
> the current code in SLAB is mostly unfixable due to its *queuing* 
> nature).

i'm curious about the real facts behind this "alien cache problem". I 
heard about it and asked around and was told that there's some sort of 
bad quadratic behavior of memory consumption on NUMA - but i cannot 
actually see that in the code.

The alien caches feature of SLAB i see as a spread out clustered 
index/cache of objects on other nodes. It's not increasing the average 
per object memory consumption per se! The number of alien caches 
increases with increasing number of nodes, but _of course_, as memory 
size increases too so there's more stuff and a larger expected spreadout 
of memory to keep track of. ("Fixing" that would be like reintroducing a 
single runqueue for the scheduler, based on the argument that it's an 
O(1) number of runqueues against O(N) number of runqueues - which would 
be complete nonsense.)

so i see SLAB alien caches as an automatic self-partitioning mechanism 
... which has complexities but which also has _obvious_ performance 
benefits. Yes, it has some disadvantages like all caching schemes do - 
there's more cached memory tied up in the allocator at any given moment 
- but arguing against that would be like arguing against a 2MB L2 cache 
purely on the basis that a 1MB L2 cache is smaller and hence more 
space-efficient. Caches are there to cache stuff, and more caches ... 
use more memory. It's all a question of proportion and tuning, but the 
_design_ should be based on having as thorough caching as possible.

	Ingo
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