From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: Relation between free() and remove_vm_struct() From: Arjan van de Ven In-Reply-To: References: Content-Type: text/plain Date: Thu, 17 Aug 2006 08:59:25 +0200 Message-Id: <1155797966.4494.29.camel@laptopd505.fenrus.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: "Abu M. Muttalib" Cc: kernelnewbies@nl.linux.org, linux-newbie@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm List-ID: On Thu, 2006-08-17 at 12:29 +0530, Abu M. Muttalib wrote: > Hi, > > In an application I am freeing some memory address, earlier reserved with > malloc. > > I have put prints in remove_vm_struct() function in ./mm/mmap.c. For few > calls to free(), there is no corresponding call to remove_vm_struct(). I am > not able to understand why the user space call to free() is not propagated > to kernel, where in the remove_vm_strcut() function should get called. Hi, there is 2 parts to this question first of all, glibc malloc doesn't always use mmap for it's allocations, it's a split between the brk() area and mmap() depending on the size of the allocation. (>= 128Kb uses mmap, smaller uses brk(). brk using allocations will not end up in remove_vm_struct at all) second of all, glibc delays freeing of some memory (in the brk() area) to optimize for cases of frequent malloc/free operations, so that it doesn't have to go to the kernel all the time (and a free would imply a cross cpu TLB invalidate which is *expensive*, so batching those up is a really good thing for performance) I hope this answer helps you... it's probably worth reading the malloc/malloc.c code in the glibc code tree, this behavior is documented there... Greetings, Arjan van de Ven -- if you want to mail me at work (you don't), use arjan (at) linux.intel.com -- 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