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Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable
From: Robert Love <rml@tech9.net>
To: Rob Landley <landley@trommello.org>
Cc: yodaiken@fsmlabs.com, Alan Cox <alan@lxorguk.ukuu.org.uk>, nigel@nrg.org,
        Andrew Morton <akpm@zip.com.au>, linux-kernel@vger.kernel.org
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On Fri, 2002-01-11 at 15:22, Rob Landley wrote:

> So the best approach is a combination of the two patches.  SMP-on-UP for 
> everything outside of spinlocks, and then manually yielding locks that cause 
> problems.  Both Robert Love and Andrew Morton have come out in favor of each 
> other's patches on lkml just in the past few days.  The patches work together 
> quite well, and each wants to see the other's patch applied.

Right.  Here is what I want for 2.5 as a _general_ step towards a better
kernel that will yield better performance:

Merge the preemptible kernel patch.  A version is now out for
2.5.2-pre11 with support for Ingo's scheduler:

	ftp://ftp.kernel.org/pub/linux/kernel/people/rml/preempt-kernel

Next, make available a tool for profiling kernel latencies.  I have one
available now, preempt-stats, at the above url.  Andrew has some
excellent tools available at his website, too.  Something like this
could even be merged.  Daniel Phillips suggested a passive tool on IRC. 
Preempt-stats works like this.  It is off-by-default and, when enabled,
measures time between lock and unlock, reporting the top 20 worst-cases.

Begin working on the worst-case locks.  Solutions like Andrew's
low-latency and my lock-break are a start.  Better (at least in general)
solutions are to analyze the locks.  Localize them; make them finer
grained.  Analyze the algorithms.  Find the big problems.  Anyone look
at the tty layer lately?  Ugh.  Using the preemptive kernel as a base
and the analysis of the locks as a list of culprits, clean this cruft
up.  This would benefit SMP, too.  Perhaps a better locking construct is
useful.

The immediate result is good; the future is better.

	Robert Love

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