From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 8 Jan 2001 09:43:28 -0800 (PST) From: Linus Torvalds Subject: Re: Subtle MM bug In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Marcelo Tosatti Cc: "David S. Miller" , Rik van Riel , linux-mm@kvack.org List-ID: On Mon, 8 Jan 2001, Marcelo Tosatti wrote: > > On Sun, 7 Jan 2001, Linus Torvalds wrote: > > > and just get rid of all the logic to try to "find the best mm". It's bogus > > anyway: we should get perfectly fair access patterns by just doing > > everything in round-robin, and each "swap_out_mm(mm)" would just try to > > walk some fixed percentage of the RSS size (say, something like > > > > count = (mm->rss >> 4) > > > > and be done with it. > > I have the impression that a fixed percentage of the RSS will be a problem > when you have a memory hog (or hogs) running. Nothing but testing can prove it, but I don't think that's really an issue. Remember: we're not actually swapping stuff out any more in VM scanning. We're just saying "we're low on memory, let's evict the page tables so that we _could_ swap stuff out if necessary". We're going to have to evict _something_, and walking the page tables really gives us a lot better knowledge of WHAT to evict. The cost of scanning the VM is (a) the cost of scanning itself and (b) the cost of soft-faults and CPU TLB invalidate cross-calls for the scanning. Both of which might be noticeable - but I have this fairly strong feeling that neither of them is big enough to offset the cost of paging out the wrong page. Which we definitely do now - I've got some simple test-programs that have a VM footprint that is not _that_ much more than the available memory, and they _really_ show problems. (The "lots of dirty pages" case is not the common case under most loads, so the fact that 2.4.0 has some performance problems with it was not a show-stopper for me - during my testing with low memory most loads were very nice indeed). Linus -- 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.eu.org/Linux-MM/