Mingming, 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? 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
