From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762190AbXF0QAU (ORCPT ); Wed, 27 Jun 2007 12:00:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757043AbXF0QAF (ORCPT ); Wed, 27 Jun 2007 12:00:05 -0400 Received: from x35.xmailserver.org ([64.71.152.41]:3381 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752894AbXF0QAB (ORCPT ); Wed, 27 Jun 2007 12:00:01 -0400 X-AuthUser: davidel@xmailserver.org Date: Wed, 27 Jun 2007 08:59:52 -0700 (PDT) From: Davide Libenzi X-X-Sender: davide@alien.or.mcafeemobile.com To: Hugh Dickins cc: Ulrich Drepper , Linux Kernel Mailing List Subject: Re: [patch 2/3] MAP_NOZERO - implement sys_brk2() In-Reply-To: Message-ID: References: X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 27 Jun 2007, Hugh Dickins wrote: > On Tue, 26 Jun 2007, Ulrich Drepper wrote: > > On 6/26/07, Davide Libenzi wrote: > > > > > OTOH glibc could implement __morecore using mmap(MAP_NOZERO), and hence > > > brk2() would not be needed, no? > > > > No. mmap calls create individual VMAs which gets expensive. There > > are also some hardware drivers which get more expensive the more VMAs > > there are. I want to go away as much as possible from mmap for > > malloc. > > Not so: if an mmap can be done by extending either adjacent vma (prot > and flags and file and offset all match up), that's what's done and no > separate vma is created. (And adjacent vmas get merged when mprotect > removes the difference in protection.) > > I don't think there's any such reason to prefer brk to mmap. But you > may have encountered something which we in the kernel are thinking of > as an insignificant corner case, which is actually breaking things up > badly in practice (I recall the kernel's internal VM_ACCOUNT flag, > relating to non-overcommit accounting, which might get turned on at > any time, sometimes preventing a vma merge you'd otherwise expect). > Please let me know if you've a test case which shows more vmas than > expected. The only way I can see vma fragmentation happen in that case, is if userspace uses a mixture of mmaps and mallocs, and flags+prots of the two does not match. The glibc allocator seems to support it just fine. There's a macro where you specify if the heaps are contiguous or not. - Davide