From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 16 Apr 2007 12:30:57 -0400 From: Jakub Jelinek Subject: Re: [PATCH] make MADV_FREE lazily free memory Message-ID: <20070416163057.GH355@devserv.devel.redhat.com> Reply-To: Jakub Jelinek References: <461C6452.1000706@redhat.com> <461D6413.6050605@cosmosbay.com> <461D67A9.5020509@redhat.com> <461DC75B.8040200@cosmosbay.com> <461DCCEB.70004@yahoo.com.au> <461DCDDA.2030502@yahoo.com.au> <461DDE44.2040409@redhat.com> <20070416161039.GA979@kryten> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070416161039.GA979@kryten> Sender: owner-linux-mm@kvack.org Return-Path: To: Anton Blanchard Cc: Rik van Riel , Nick Piggin , Eric Dumazet , linux-kernel , linux-mm , Ulrich Drepper List-ID: On Mon, Apr 16, 2007 at 11:10:39AM -0500, Anton Blanchard wrote: > > Making the pte clean also needs to clear the hardware writable > > bit on architectures where we do pte dirtying in software. > > > > If we don't, we would have corruption problems all over the VM, > > for example in the code around pte_clean_one :) > > > > >But as Linus recently said, even hardware handled faults still > > >take expensive microarchitectural traps. > > > > Nowhere near as expensive as a full page fault, though... > > Unfortunately it will be expensive on architectures that have software > referenced and changed. It would be great if we could just leave them > dirty in the pagetables and transition between a clean and dirty state > via madvise calls, but thats just wishful thinking on my part :) That would mean an additional syscall. Furthermore, if you allocate a big chunk of memory, dirty it, then free (with madvise (MADV_FREE)) it and soon allocate the same size of memory again, it is better to start that with non-dirty memory, it might be that this time you e.g. don't modify a big part of the chunk. If all that memory was kept dirty all the time and just marked/unmarked for lazy reuse with MADV_FREE/MADV_UNDO_FREE, all that memory would need to be saved to disk when paging out as it was marked dirty, while with current Rik's MADV_FREE that will happen only for pages that were actually dirtied after the last malloc. Jakub -- 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