From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 1 Nov 2005 15:29:59 +0100 From: Ingo Molnar Subject: Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19 Message-ID: <20051101142959.GA9272@elte.hu> References: <20051030235440.6938a0e9.akpm@osdl.org> <27700000.1130769270@[10.10.2.4]> <4366A8D1.7020507@yahoo.com.au> <4366C559.5090504@yahoo.com.au> <4366D469.2010202@yahoo.com.au> <20051101135651.GA8502@elte.hu> <1130854224.14475.60.camel@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1130854224.14475.60.camel@localhost> Sender: owner-linux-mm@kvack.org Return-Path: To: Dave Hansen Cc: Mel Gorman , Nick Piggin , "Martin J. Bligh" , Andrew Morton , kravetz@us.ibm.com, linux-mm , Linux Kernel Mailing List , lhms List-ID: * Dave Hansen wrote: > > can you always, under any circumstance hot unplug RAM with these patches > > applied? If not, do you have any expectation to reach 100%? > > With these patches, no. There are currently some very nice, > pathological workloads which will still cause fragmentation. But, in > the interest of incremental feature introduction, I think they're a > fine first step. We can effectively reach toward a more comprehensive > solution on top of these patches. > > Reaching truly 100% will require some other changes such as being able > to virtually remap things like kernel text. then we need to see that 100% solution first - at least in terms of conceptual steps. Not being able to hot-unplug RAM in a 100% way wont satisfy customers. Whatever solution we choose, it must work 100%. Just to give a comparison: would you be content with your computer failing to start up apps 1 time out of 100, saying that 99% is good enough? Or would you call it what it is: buggy and unreliable? to stress it: hot unplug is a _feature_ that must work 100%, _not_ some optimization where 99% is good enough. This is a feature that people will be depending on if we promise it, and 1% failure rate is not acceptable. Your 'pathological workload' might be customer X's daily workload. Unless there is a clear definition of what is possible and what is not (which definition can be relied upon by users), having a 99% solution is much worse than the current 0% solution! worse than that, this is a known _hard_ problem to solve in a 100% way, and saying 'this patch is a good first step' just lures us (and customers) into believing that we are only 1% away from the desired 100% solution, while nothing could be further from the truth. They will demand the remaining 1%, but can we offer it? Unless you can provide a clear, accepted-upon path towards the 100% solution, we have nothing right now. I have no problems with using higher-order pages for performance purposes [*], as long as 'failed' allocation (and freeing) actions are user-invisible. But the moment you make it user-visible, it _must_ work in a deterministic way! Ingo [*] in which case any slowdown in the page allocator must be offset by the gains. -- 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