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Subject: Re: ps performance sucks (was Re: dcache_rcu [performance results])
From: Robert Love <rml@tech9.net>
To: William Lee Irwin III <wli@holomorphy.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>, Alexander Viro <viro@math.psu.edu>,
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On Thu, 2002-11-07 at 22:57, William Lee Irwin III wrote:

> One way to at least "postpone" having to do things like making a fair
> tasklist_lock is to make readers well-behaved. /proc/ is the worst
> remaining offender left with its quadratic (!) get_pid_list(). After
> "kernel, you're being bad and spinning in near-infinite loops with the
> tasklist_lock readlocked" is (completely?) solved, then we can wait for
> boxen with higher cpu counts to catch fire anyway when the arrival rate
> of readers * hold time of readers > 1, which will happen because arrival
> rates are O(cpus), and cpus will grow without bound as machines advance.
> 
> I'm not sure RCU would help this any; I'd be very much afraid of the
> writes being postponed indefinitely or just too long in the presence
> of what's essentially perpetually in-progress read access. Does RCU
> have a guarantee of forward progress for writers?

I am not sure I like the idea of RCU for the tasklist_lock.

I do agree 100% with your first point, though - the problem is
ill-behaved readers.  I think the writing vs. reading is such that the
rw-lock we have now is fine, we just need to make e.g. /proc play way
way more fair.

	Robert Love

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