From: Jiaying Zhang Subject: Re: ext4 DIO read performance issue on SSD Date: Wed, 14 Oct 2009 12:48:03 -0700 Message-ID: <5df78e1d0910141248h1f537863n97991585e6147ca7@mail.gmail.com> References: <5df78e1d0910091634q22e6a372g3738b0d9e9d0e6c9@mail.gmail.com> <1255546117.4377.62.camel@mingming-laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: ext4 development , Andrew Morton , Michael Rubin , Manuel Benitez To: Mingming Return-path: Received: from smtp-out.google.com ([216.239.33.17]:61733 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759249AbZJNTtQ convert rfc822-to-8bit (ORCPT ); Wed, 14 Oct 2009 15:49:16 -0400 Received: from wpaz1.hot.corp.google.com (wpaz1.hot.corp.google.com [172.24.198.65]) by smtp-out.google.com with ESMTP id n9EJm6KV020658 for ; Wed, 14 Oct 2009 20:48:07 +0100 Received: from ywh34 (ywh34.prod.google.com [10.192.8.34]) by wpaz1.hot.corp.google.com with ESMTP id n9EJm4Vf016369 for ; Wed, 14 Oct 2009 12:48:04 -0700 Received: by ywh34 with SMTP id 34so142214ywh.1 for ; Wed, 14 Oct 2009 12:48:04 -0700 (PDT) In-Reply-To: <1255546117.4377.62.camel@mingming-laptop> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Oct 14, 2009 at 11:48 AM, Mingming wrote: > On Fri, 2009-10-09 at 16:34 -0700, Jiaying Zhang wrote: >> Hello, >> >> Recently, we are evaluating the ext4 performance on a high speed SSD= =2E >> One problem we found is that ext4 performance doesn't scale well wit= h >> multiple threads or multiple AIOs reading a single file with O_DIREC= T. >> E.g., with 4k block size, multiple-thread DIO AIO random read on ext= 4 >> can lose up to 50% throughput compared to the results we get via RAW= IO. >> >> After some initial analysis, we think the ext4 performance problem i= s caused >> by the use of i_mutex lock during DIO read. I.e., during DIO read, w= e grab >> the i_mutex lock in __blockdev_direct_IO because ext4 uses the defau= lt >> DIO_LOCKING from the generic fs code. I did a quick test by calling >> blockdev_direct_IO_no_locking() in ext4_direct_IO() and I saw ext4 D= IO read >> got 99% performance as raw IO. >> > > This is very interesting...and impressive number. > > I tried to change ext4 to call blockdev_direct_IO_no_locking() direct= ly, > but then realize that we can't do this all the time, as ext4 support > ext3 non-extent based files, and uninitialized extent is not support = on > ext3 format file. > >> As we understand, the reason why we want to take i_mutex lock during= DIO >> read is to prevent it from accessing stale data that may be exposed = by a >> simultaneous write. We saw that Mingming Cao has implemented a patch= set >> with which when a get_block request comes from direct write, ext4 on= ly >> allocates or splits an uninitialized extent. That uninitialized exte= nt >> will be marked as initialized at the end_io callback. > > Though I need to clarify that with all the patches in mainline, we on= ly > treat new allocated blocks form direct io write to holes, not to writ= es > to the end of file. I actually have proposed to treat the write to th= e > end of file also as unintialized extents, but there is some concerns > that this getting tricky with updating inode size when it is async IO > direct IO. So it didn't getting done yet. > >> =A0We are wondering >> whether we can extend this idea to buffer write as well. I.e., we al= ways >> allocate an uninitialized extent first during any write and convert = it >> as initialized at the time of end_io callback. This will eliminate t= he need >> to hold i_mutex lock during direct read because a DIO read should ne= ver get >> a block marked initialized before the block has been written with ne= w data. >> > > Oh I don't think so. For buffered IO, the data is being copied to > buffer, direct IO read would first flush what's in page cache to disk= , Hmm, do you mean the filemap_write_and_wait_range() in __blockdev_direct_IO? Or do we flush page cache after calling get_block in dio read? Jiaying > then read from disk. So if there is concurrent buffered write and dir= ect > read, removing the i_mutex locks from the direct IO path should still > gurantee the right order, without having to treat buffered allocation > with uninitialized extent/end_io. > > The i_mutex lock, from my understanding, is there to protect direct I= O > write to hole and concurrent direct IO read, we should able to remove > this lock for extent based ext4 file. > >> We haven't implemented anything yet because we want to ask here firs= t to >> see whether this proposal makes sense to you. >> > > It does make sense to me. > > Mingming >> Regards, >> >> Jiaying >> -- >> 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 =A0http://vger.kernel.org/majordomo-info.html > > > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html