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Message-ID: <20100107163923.GB6764@linux.vnet.ibm.com>
Date: Thu, 7 Jan 2010 08:39:23 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
akpm@...ux-foundation.org, josh@...htriplett.org,
tglx@...utronix.de, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
barrier
On Thu, Jan 07, 2010 at 09:32:16AM +0100, Peter Zijlstra wrote:
> On Wed, 2010-01-06 at 21:02 -0800, Paul E. McKenney wrote:
> >
> > Beats the heck out of user-mode signal handlers!!! And it is hard
> > to imagine groveling through runqueues ever being a win, even on very
> > large systems. The only reasonable optimization I can imagine is to
> > turn this into a no-op for a single-threaded process, but there are
> > other ways to do that optimization.
> >
> > Reviewed-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>
> Then imagine someone doing:
>
> while (1)
> sys_membarrier();
>
> on your multi node machine, see how happy you are then.
I guess in that situation, I would be feeling no pain. Or anything else
for that matter. :-/
So, good point!!! I stand un-Reviewed-By.
I could imagine throttling the requests, as well as batching them. If
any CPU does a sys_membarrier() after this CPU's sys_membarrier has
entered the kernel, then this CPU can simply return. A token-bucket
approach could throttle things nicely, but at some point it becomes
better to just do POSIX signals.
Thanx, Paul
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