On Wed, 2009-10-14 at 14:42 -0700, Jiaying Zhang wrote: > On Wed, Oct 14, 2009 at 1:57 PM, Mingming <cmm@xxxxxxxxxx> wrote: > > 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. > > > I don't think that function is called with DIO_NO_LOCKING. Oh, I mean the filemap_write_and_wait_range() in generic_file_aio_read() > Also, if we no longer hold i_mutex lock during dio read, I think > there is a time window that a buffer write can allocate an > initialize block after dio read flushes page cache but > before it calls get_block. Then that dio read can get that > initialized block with stale data. > ah, I think it over, the key is prevent get_block() expose initialized extent to direct read. concurrent buffered write to hole could result in get_block() allocate blocks before direct IO read. That could be addressed in a similar way we did for async direct IO write to hole... > Jiaying > > >> 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 -- 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
