From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from digeo-nav01.digeo.com (digeo-nav01.digeo.com [192.168.1.233]) by packet.digeo.com (8.9.3+Sun/8.9.3) with SMTP id SAA11560 for ; Fri, 27 Dec 2002 18:34:02 -0800 (PST) Message-ID: <3E0D0D99.5EB318E5@digeo.com> Date: Fri, 27 Dec 2002 18:34:01 -0800 From: Andrew Morton MIME-Version: 1.0 Subject: Re: shared pagetable benchmarking References: <311610000.1041036301@flay> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: "Martin J. Bligh" Cc: Linus Torvalds , Dave McCracken , Daniel Phillips , linux-mm@kvack.org List-ID: "Martin J. Bligh" wrote: > > > I don't consider it important enough to qualify unless there are some real > > loads where it really matters. I can well imagine that such loads exist > > (where low-memory usage by page tables is a real problem), but I'd like to > > have that confirmed as a bug-report and that the sharing really does fix > > it. > > We had over 10Gb of PTEs running Oracle Apps (on 2.4 without RMAP) - > RMAP would add another 5Gb or so to that (2Gb shared memory segment > across many processes). But you can stick PTEs in highmem, whereas > it's not easy to do that with pte_chains ... sticking 5Gb of overhead > into ZONE_NORMAL is tricky ;-) The really nice thing about shared > pagetables as a solution is that it's totally transparent, and requires > no app modifications. Obviously degrading fork for small tasks is > unacceptable, but Dave seems to have fixed that issue now. To what extent is that a "real" workload? What other applications are affected, and to what extent? Why are hugepages not a sufficient solution? Is this problem sufficiently common to warrant the inclusion of pagetable sharing in the main kernel, as opposed to a specialised Oracle/DB2 derivative? > I think the long-term fix for the rmap performance hit is object-based > RMAP (doing the reverse mappings shared on a per-area basis) which we've > talked about, but not for 2.6 ... it may not turn out to be that hard > though ... K42 did it before. I think we can do a few things still in the 2.6 context. The fact that my "apply seventy patches with patch-scripts" test takes 350,000 pagefaults in 13 seconds makes one go "hmm". -- 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/