linux-kernel - Re: [PATCH 0/2] convert mmap_sem to a scalable rw

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Message-ID: <20070512154217.GA20228@elte.hu>
Date:	Sat, 12 May 2007 17:42:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Esben Nielsen <nielsen.esben@...glemail.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex


* Esben Nielsen <nielsen.esben@...glemail.com> wrote:

> Yeah, after sending that mail I realized I accepted this fact way 
> back... But I disagree in that it is easy to avoid not write-lcling 
> the mm semaphore: A simple malloc() might lead to a mmap() call 
> creating trouble. Am I right?

yeah - that's why "hard RT" apps generally either preallocate all memory 
in advance, or use special, deterministic allocators. And for "soft RT" 
it's all a matter of degree.

> > But mainline should not be bothered with this.
> 
> I disagree. You lay a large burdon on the users of PI futexes to avoid 
> write locking the mm semaphore. PI boosting those writers would be a 
> good idea even in the mainline.

only if it can be done without slowing down all the much more important 
uses of the MM semaphore.

> 1) How much slower would the pi_rw_mutex I suggested really be? As far 
> as I see there is only an overhead when there is congestion. I can not 
> see that that overhead is much larger than a non-PI boosting 
> implementation.

it could be measured, but it's certainly not going to be zero.

> 2) I know that execution time isn't bounded in the main-line - that is 
> why -rt is needed. But it is _that_ bad? How low can you get your 
> latencies with preemption on on a really busy machine?

on mainline? It can get arbitrarily large (read: seconds) in essence.

	Ingo
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