From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 27 Dec 2002 16:45:01 -0800 From: "Martin J. Bligh" Subject: Re: shared pagetable benchmarking Message-ID: <311610000.1041036301@flay> In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline Sender: owner-linux-mm@kvack.org Return-Path: To: Linus Torvalds , Dave McCracken Cc: Daniel Phillips , Andrew Morton , linux-mm@kvack.org List-ID: > 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. 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. M. -- 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/