From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 21 Mar 2008 14:33:23 -0700 (PDT) From: Christoph Lameter Subject: Re: [13/14] vcompound: Use vcompound for swap_map In-Reply-To: <8763vfixb8.fsf@basil.nowhere.org> Message-ID: References: <20080321061703.921169367@sgi.com> <20080321061727.269764652@sgi.com> <8763vfixb8.fsf@basil.nowhere.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Andi Kleen Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Fri, 21 Mar 2008, Andi Kleen wrote: > > is larger then there is no way around the use of vmalloc. > > Have you considered the potential memory wastage from rounding up > to the next page order now? (similar in all the other patches > to change vmalloc). e.g. if the old size was 64k + 1 byte it will > suddenly get 128k now. That is actually not a uncommon situation > in my experience; there are often power of two buffers with > some small headers. Yes the larger the order the more significant the problem becomes. > A long time ago (in 2.4-aa) I did something similar for module loading > as an experiment to avoid too many TLB misses. The module loader > would first try to get a continuous range in the direct mapping and > only then fall back to vmalloc. > > But I used a simple trick to avoid the waste problem: it allocated a > continuous range rounded up to the next page-size order and then freed > the excess pages back into the page allocator. That was called > alloc_exact(). If you replace vmalloc with alloc_pages you should > use something like that too I think. That trick is still in use for alloc_large_system_hash.... But cutting off the tail of compound pages would make treating them as order N pages difficult. The vmalloc fallback situation is easy to deal with. Maybe we can think about making compound pages being N consecutive pages of PAGE_SIZE rather than an order O page? The api would be a bit different then and it would require changes to the page allocator. More fragmentation if pages like that are freed. -- 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