From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id C67B06B002D for ; Wed, 19 Oct 2011 21:01:53 -0400 (EDT) References: In-Reply-To: Mime-Version: 1.0 (iPhone Mail 8L1) Content-Type: text/plain; charset=us-ascii Message-Id: Content-Transfer-Encoding: quoted-printable From: Andreas Dilger Subject: Re: Latency writing to an mlocked ext4 mapping Date: Wed, 19 Oct 2011 19:02:46 -0600 Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-ext4@vger.kernel.org" What kernel are you using? A change to keep pages consistent during writeou= t was landed not too long ago (maybe Linux 3.0) in order to allow checksummi= ng of the data.=20 We discussed doing copy-on-write, but there are relatively few mmap users an= d it wasn't clear whether the complexity was worth it.=20 Cheers, Andreas On 2011-10-19, at 6:39 PM, Andy Lutomirski wrote: > I have a real-time program that has everything mlocked (i.e. > mlockall(MCL_CURRENT | MCL_FUTURE)). It has some log files opened for > writing. Those files are opened and memset to zero in another thread > to fault everything in. The system is under light I/O load with very > little memory pressure. >=20 > Latencytop shows frequent latency in the real-time threads. The main > offenders are: >=20 > schedule sleep_on_page wait_on_page_bit ext4_page_mkwrite do_wp_page > handle_pte_fault handle_mm_fault do_page_fault page_fault >=20 > schedule do_get_write_access jbd2_journal_get_write_access > __ext4_journal_get_write_access ext4_reserve_inode_write > ext4_mark_inode_dirty ext4_dirty_inode __mark_inode_dirty > file_update_time do_wp_page handle_pte_fault handle_mm_fault >=20 >=20 > I imagine the problem is that the system is periodically writing out > my dirty pages and marking them clean (and hence write protected). > When I try to write to them, the kernel makes them writable again, > which causes latency either due to updating the inode mtime or because > the file is being written to disk when I try to write to it. >=20 > Is there any way to prevent this? One possibility would be a way to > ask the kernel not to write the file out to disk. Another would be a > way to ask the kernel to make a copy of the file when it writes it > disk and leave the original mapping writable. >=20 > Obviously I can fix this by mapping anonymous memory, but then I need > another thread to periodically write my logs out to disk, and if that > crashes, I lose data. >=20 > --=20 > Andy Lutomirski > AMA Capital Management, LLC > Office: (310) 553-5322 > Mobile: (650) 906-0647 > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org