Date: Wed, 14 Jan 2009 16:22:07 +0100
From: Nick Piggin <npiggin@suse.de>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Lin Ming <ming.m.lin@intel.com>,
       Christoph Lameter <cl@linux-foundation.org>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch] SLQB slab allocator
Message-ID: <20090114152207.GD25401@wotan.suse.de>
References: <20090114090449.GE2942@wotan.suse.de> <84144f020901140253s72995188vb35a79501c38eaa3@mail.gmail.com> <20090114114707.GA24673@wotan.suse.de> <84144f020901140544v56b856a4w80756b90f5b59f26@mail.gmail.com> <20090114142200.GB25401@wotan.suse.de> <84144f020901140645o68328e01ne0e10ace47555e19@mail.gmail.com> <20090114150900.GC25401@wotan.suse.de>
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Hit send a bit too early here.

On Wed, Jan 14, 2009 at 04:09:00PM +0100, Nick Piggin wrote:
> On Wed, Jan 14, 2009 at 04:45:15PM +0200, Pekka Enberg wrote:
> > 
> > Don't get me wrong, though. I am happy you are able to work with the
> > Intel engineers to fix the long standing issue (I want it fixed too!)
> > but I would be happier if the end-result was few simple patches
> > against mm/slub.c :-).
> 
> Right, but that regression isn't my only problem with SLUB. I think
> higher order allocations could be much more damaging for more a wider
> class of users. It is less common to see higher order allocation failure

Sorry, *more* common.


> reports in places other than lkml, where people tend to have systems
> stay up longer and/or do a wider range of things with them.
> 
> The idea of removing queues doesn't seem so good to me. Queues are good.
> You amortize or avoid all sorts of things with queues. We have them
> everywhere in the kernel ;)
> 
>  
> > On Wed, Jan 14, 2009 at 4:22 PM, Nick Piggin <npiggin@suse.de> wrote:
> > > I'd love to be able to justify replacing SLAB and SLUB today, but actually
> > > it is simply never going to be trivial to discover performance regressions.
> > > So I don't think outright replacement is great either (consider if SLUB
> > > had replaced SLAB completely).
> > 
> > If you ask me, I wish we *had* removed SLAB so relevant people could
> > have made a huge stink out of it and the regression would have been
> > taken care quickly ;-).
> 
> Well, presumably the stink was made because we've been stuck with SLAB
> for 2 years for some reason. But it is not only that one but there were
> other regressions too. Point simply is that it would have been much
> harder for users to detect if there even is a regression, what with all
> the other changes happening.

It would have been harder to detect SLUB vs SLAB regressions if they
had been forced to bisect (which could lead eg. to CFS, or GSO), rather
than select between two allocators.

And... IIRC, the Intel guys did make a stink but it wasn't considered
so important or worthwhile to fix for some reason? Anyway, the fact is
that it hadn't been fixed in SLUB. Hmm, I guess it is a significant
failure of SLUB that it hasn't managed to replace SLAB by this point.

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