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Date: Sun, 17 Mar 2002 17:50:09 +1100
From: Rusty Russell <rusty@rustcorp.com.au>
To: Martin Wirth <martin.wirth@dlr.de>
Cc: pwaechtler@loewe-komp.de, linux-kernel@vger.kernel.org, drepper@redhat.com
Subject: Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)
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On Sat, 16 Mar 2002 12:23:26 +0100
Martin Wirth <martin.wirth@dlr.de> wrote:
> Rusty Russell wrote:
> >The solution I was referring to before, using full semaphores, would
> >look like so:

[snip]

> In principle that works. But one of  things that's less nice with 
> pthread_cond_wait is
> that you sometimes have a (most of the time) unnecessary schedule 
> ping-pong, and with the
> approach above you always have this (due to ack).

Only vs. pthread_cond_broadcast.  And if you're using that you probably
have some other performance issues anyway?

> And secondly if 
> futex_up(&f, N) for N > 1
> relies on the chained wakeup in the kernels futex_up routine the 
> broadcast may take a while to
> complete (the lowest priority waiter penalizes all others queued behind 
> him). A semaphore simply is no full replacement for a waitqueue with 
> wake_all.

Yes, we could have a "wake N" variant, which would be more efficient here.

Hope that clarifies,
Rusty.
-- 
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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