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Date: Thu, 7 Nov 2002 20:29:25 -0800
From: William Lee Irwin III <wli@holomorphy.com>
To: Robert Love <rml@tech9.net>
Cc: Rusty Russell <rusty@rustcorp.com.au>, Alexander Viro <viro@math.psu.edu>,
       mbligh@aracnet.com, ahu@ds9a.nl, peter@chubb.wattle.id.au,
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Subject: Re: ps performance sucks (was Re: dcache_rcu [performance results])
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On Thu, 2002-11-07 at 22:57, William Lee Irwin III wrote:
>> 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?

On Thu, Nov 07, 2002 at 11:17:21PM -0500, Robert Love wrote:
> 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.

This is only feasible for small numbers of cpus. Any compensation
provided by algorithmic improvements on the read-side is outweighed by
NR_CPUS. Making readers well-behaved only solves half of the issue.
Whether the other half is addressible in a 2.6.x time scale is an open
question, but a question I'd like to see answered in favor of fixing
the livelocks sooner rather than later (esp. as 2.7+ issues are unlikely
to be resolved in line with hardware release schedules). I have a
strong bias toward code which works everywhere, all the time.

Bill
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