On Wed, Apr 01 2009, Chris Mason wrote: > On Tue, 2009-03-31 at 14:18 +0900, Hisashi Hifumi wrote: > > Hi Chris. > > > > I noticed performance of fsync() and write() with O_SYNC flag on Btrfs is > > very slow as compared to ext3/4. I used blktrace to try to investigate the > > cause of this. One of cause is that unplug is done by kblockd even if the > > I/O is > > issued through fsync() or write() with O_SYNC flag. kblockd's unplug timeout > > is 3msec, so unplug via blockd can decrease I/O response. To increase > > fsync/osync write performance, speeding up unplug should be done here. > > > > I realized today that all of the async thread handling btrfs does for > writes gives us plenty of time to queue up IO for the block device. If > that's true, we can just unplug the block device in async helper thread > and get pretty good coverage for the problem you're describing. > > Could you please try the patch below and see if it performs well? I did > some O_DIRECT testing on a 5 drive array, and tput jumped from 386MB/s > to 450MB/s for large writes. > > Thanks again for digging through this problem. > > -chris > > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index dd06e18..bf377ab 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -146,7 +146,7 @@ static noinline int run_scheduled_bios(struct > btrfs_device *device) > unsigned long num_run = 0; > unsigned long limit; > > - bdi = device->bdev->bd_inode->i_mapping->backing_dev_info; > + bdi = blk_get_backing_dev_info(device->bdev); > fs_info = device->dev_root->fs_info; > limit = btrfs_async_submit_limit(fs_info); > limit = limit * 2 / 3; > @@ -231,6 +231,19 @@ loop_lock: > if (device->pending_bios) > goto loop_lock; > spin_unlock(&device->io_lock); > + > + /* > + * IO has already been through a long path to get here. Checksumming, > + * async helper threads, perhaps compression. We've done a pretty > + * good job of collecting a batch of IO and should just unplug > + * the device right away. > + * > + * This will help anyone who is waiting on the IO, they might have > + * already unplugged, but managed to do so before the bio they > + * cared about found its way down here. > + */ > + if (bdi->unplug_io_fn) > + bdi->unplug_io_fn(bdi, NULL);
blk_run_backing_dev(bdi, NULL); :-) -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
