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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Daniel Phillips <phillips@phunq.net>
Subject: Re: [Tux3] Tux3 report: Tux3 Git tree available
Date: Sun, 15 Mar 2009 15:14:27 +1100
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Cc: Matthew Wilcox <matthew@wil.cx>, linux-fsdevel@vger.kernel.org,
       tux3@tux3.org, Andrew Morton <akpm@linux-foundation.org>,
       linux-kernel@vger.kernel.org
References: <200903110925.37614.phillips@phunq.net> <200903151450.51726.nickpiggin@yahoo.com.au> <200903142108.53155.phillips@phunq.net>
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On Sunday 15 March 2009 15:08:52 Daniel Phillips wrote:
> On Saturday 14 March 2009, Nick Piggin wrote:
> > On Sunday 15 March 2009 14:24:29 Daniel Phillips wrote:
> > > I expect implementing VM extents to be a brutally complex project, as
> > > filesystem extents always turn out to be, even though one tends to
> > > enter such projects thinking, how hard could this be?  Answer: harder
> > > than you think.  But VM extents would be good for a modest speedup, so
> > > somebody is sure to get brave enough to try it sometime.
> >
> > I don't think there is enough evidence to be able to make such an
> > assertion.
> >
> > When you actually implement extent splitting and merging in a deadlock
> > free manner and synchronize everything properly I wouldn't be surprised
> > if it is slower most of the time. If it was significantly faster, then
> > memory fragmentation means that it is going to get significantly slower
> > over the uptime of the kernel, so you would have to virtually map the
> > kernel and implement memory defragmentation, at which point you get even
> > slower and more complex.
>
> You can make exactly the same argument about filesystem extents, and
> we know that extents are faster there.  So what is the fundamental
> difference?

Uh, aside from all the obvious fundamental differences there are, you
can only make such an assertion if performance characteristics and
usage patterns are very similar, nevermind fundamentally different...


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