On Wed, 2009-10-14 at 12:48 -0700, Jiaying Zhang wrote: > 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. > > > >> 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, > > Hmm, do you mean the filemap_write_and_wait_range() in > __blockdev_direct_IO? yes, that's the one to flush the page cache before direct read. > 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 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 -- 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
