From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Sat, 12 May 2007 17:42:17 +0200 From: Ingo Molnar Subject: Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex Message-ID: <20070512154217.GA20228@elte.hu> References: <20070511131541.992688403@chello.nl> <1178964103.6810.55.camel@twins> <20070512143328.GA24803@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org Return-Path: To: Esben Nielsen Cc: Peter Zijlstra , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Oleg Nesterov , Andrew Morton , Thomas Gleixner , Nick Piggin List-ID: * Esben Nielsen 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org