On Thu, Oct 15, 2009 at 10:31 AM, Mingming <cmm@xxxxxxxxxx> wrote: > On Wed, 2009-10-14 at 22:14 -0700, Jiaying Zhang wrote: >> Mingming, >> > > Hi Jiaying, > >> On Wed, Oct 14, 2009 at 11:48 AM, Mingming <cmm@xxxxxxxxxx> 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. >> >> One problem we found is that ext4 performance doesn't scale well with >> >> multiple threads or multiple AIOs reading a single file with O_DIRECT. >> >> E.g., with 4k block size, multiple-thread DIO AIO random read on ext4 >> >> 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 is caused >> >> by the use of i_mutex lock during DIO read. I.e., during DIO read, we grab >> >> the i_mutex lock in __blockdev_direct_IO because ext4 uses the default >> >> 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 DIO 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() directly, >> > 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 only >> >> allocates or splits an uninitialized extent. That uninitialized extent >> >> will be marked as initialized at the end_io callback. >> > >> > Though I need to clarify that with all the patches in mainline, we only >> > treat new allocated blocks form direct io write to holes, not to writes >> > to the end of file. I actually have proposed to treat the write to the >> > 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. >> >> I read you previous email thread again. As I understand, the main >> concern for allocating uninitialized blocks in i_size extending write >> is that we may end up having uninitialized blocks beyond i_size >> if the system crashes during write. Can we protect this case by >> adding the inode into the orphan list in ext4_ext_direct_IO, >> i.e., same as we have done in ext4_ind_direct_IO? >> > > Sure we could do that, though initially I hoped we could get rid of > that:) > > The tricky part is async direct write to the end of file. By the time > the IO is completed, the inode may be truncated or extended larger. > Updating the most "safe" size is the part I haven't thought through yet. > Ok. I think I understand the problem better now :). Looking at the __blockdev_direct_IO(), I saw it actually treats size-extending aio dio write as synchronous and expects the dio to complete before return (fs/direct-io.c line 1204 and line 1056-1061). Can we follow the same rule and check whether it is a size-extending aio write in ext4_end_io_dio()? In such cases, we can call ext4_end_aio_dio_nolock() synchronously instead of queuing the work. I think this will protect us from truncate because we are still holding i_mutex and i_alloc_sem. Jiaying > > >> Jiaying >> >> > >> >> We are wondering >> >> whether we can extend this idea to buffer write as well. I.e., we always >> >> allocate an uninitialized extent first during any write and convert it >> >> as initialized at the time of end_io callback. This will eliminate the need >> >> to hold i_mutex lock during direct read because a DIO read should never get >> >> a block marked initialized before the block has been written with new 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, >> > then read from disk. So if there is concurrent buffered write and direct >> > 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 IO >> > 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 first 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@xxxxxxxxxxxxxxx >> >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > >> > >> > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
