From mboxrd@z Thu Jan 1 00:00:00 1970 From: Greg Harmon Subject: Re: [PATCH] ext4: Do not normalize request from fallocate Date: Fri, 22 Mar 2013 10:10:46 -0700 Message-ID: References: <1363881045-21673-1-git-send-email-lczerner@redhat.com> <877gl0yc2k.fsf@openvz.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: Lukas Czerner , "linux-ext4@vger.kernel.org" , "Theodore Ts'o" , Curt Wohlgemuth To: Dmitry Monakhov Return-path: Received: from mail-ve0-f172.google.com ([209.85.128.172]:48042 "EHLO mail-ve0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161033Ab3CVRL2 (ORCPT ); Fri, 22 Mar 2013 13:11:28 -0400 Received: by mail-ve0-f172.google.com with SMTP id cz11so3467466veb.31 for ; Fri, 22 Mar 2013 10:11:27 -0700 (PDT) In-Reply-To: <877gl0yc2k.fsf@openvz.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: [Resending without html.] I haven't looked at ext4 in awhile now. CC'ing Ted and Curt in case they want to chime in. See comment below as well. On Thu, Mar 21, 2013 at 9:03 AM, Dmitry Monakhov wrote: > On Thu, 21 Mar 2013 16:50:45 +0100, Lukas Czerner wrote: >> Block requests from fallocate has been normalized originally. Then it was >> changed by 556b27abf73833923d5cd4be80006292e1b31662 not to normalize it. >> And then it was changed by 3c6fe77017bc6ce489f231c35fed3220b6691836 >> again to normalize the request. >> >> The fact is that we _never_ want to normalize the request from >> fallocate. We know exactly how much space we're going to use and we do >> not want anyone to mess with the request and there is no point in doing >> so. > Looks reasonable. > Reviewed-by:Dmitry Monakhov >> >> Commit 3c6fe77017bc6ce489f231c35fed3220b6691836 mentioned that Is this a publicly available commit? I did not find it with a quick google search. Anyway, I don't see a problem with this commit off-hand, but I'll defer to Ted or Curt. -Greg Harmon >> large fallocate requests were not physically contiguous. However it is >> important to see why that is the case. Because the request is so big the >> allocator will try to find free group to allocate from skipping block >> groups which are used, which is fine. However it will only allocate >> extents of 2^15-1 block (limitation of uninitialized extent size) >> which will leave one block in each block group free which will make the >> extent tree physically non-contiguous, however _only_ by one block which >> is perfectly fine. >> >> This will never happen when we normalize the request because for some >> reason (maybe bug) it will be normalized to much smaller request (2048 >> blocks) and those extents will then be merged together not leaving any >> free block in between - hence physically contiguous. However the fact >> that we're splitting huge requests into ton of smaller ones and then >> merging extents together is very _very_ bad for fallocate performance. >> >> The situation is even worst since with commit >> ec22ba8edb507395c95fbc617eea26a6b2d98797 we no longer merge >> uninitialized extents so we end up with absolutely _huge_ extent tree >> for bigger fallocate requests which is also bad for performance but not >> only when fallocate itself, but even when working with the file >> later on. >> >> Fix this by disabling normalization for fallocate. From my simple testing >> with this commit fallocate is much faster on non fragmented file >> system. On my system fallocate 15T is almost 3x faster with this patch >> and removing this file is almost 2x faster - tested on real hardware. >> >> Signed-off-by: Lukas Czerner >> --- >> fs/ext4/extents.c | 18 ++++++++++-------- >> 1 files changed, 10 insertions(+), 8 deletions(-) >> >> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c >> index e2bb929..a40a602 100644 >> --- a/fs/ext4/extents.c >> +++ b/fs/ext4/extents.c >> @@ -4422,16 +4422,18 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len) >> trace_ext4_fallocate_exit(inode, offset, max_blocks, ret); >> return ret; >> } >> - flags = EXT4_GET_BLOCKS_CREATE_UNINIT_EXT; >> - if (mode & FALLOC_FL_KEEP_SIZE) >> - flags |= EXT4_GET_BLOCKS_KEEP_SIZE; >> + >> /* >> - * Don't normalize the request if it can fit in one extent so >> - * that it doesn't get unnecessarily split into multiple >> - * extents. >> + * We do NOT want the requests from fallocate to be normalized >> + * ever!. We know exactly how much we want to allocate and >> + * we do not need to do any mumbo-jumbo with it. Requests bigger >> + * than uninit extent size, will be divided automatically into >> + * biggest possible extents. >> */ >> - if (len <= EXT_UNINIT_MAX_LEN << blkbits) >> - flags |= EXT4_GET_BLOCKS_NO_NORMALIZE; >> + flags = EXT4_GET_BLOCKS_CREATE_UNINIT_EXT | >> + EXT4_GET_BLOCKS_NO_NORMALIZE; >> + if (mode & FALLOC_FL_KEEP_SIZE) >> + flags |= EXT4_GET_BLOCKS_KEEP_SIZE; >> >> retry: >> while (ret >= 0 && ret < max_blocks) { >> -- >> 1.7.7.6 >> >> -- >> 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