On 2012-11-27 11:06, Jeff Chua wrote: > On Tue, Nov 27, 2012 at 3:38 PM, Jens Axboe <axboe@xxxxxxxxx> wrote: >> On 2012-11-27 06:57, Jeff Chua wrote: >>> On Sun, Nov 25, 2012 at 7:23 AM, Jeff Chua <jeff.chua.linux@xxxxxxxxx> wrote: >>>> On Sun, Nov 25, 2012 at 5:09 AM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >>>>> So it's better to slow down mount. >>>> >>>> I am quite proud of the linux boot time pitting against other OS. Even >>>> with 10 partitions. Linux can boot up in just a few seconds, but now >>>> you're saying that we need to do this semaphore check at boot up. By >>>> doing so, it's inducing additional 4 seconds during boot up. >>> >>> By the way, I'm using a pretty fast SSD (Samsung PM830) and fast CPU >>> (2.8GHz). I wonder if those on slower hard disk or slower CPU, what >>> kind of degradation would this cause or just the same? >> >> It'd likely be the same slow down time wise, but as a percentage it >> would appear smaller on a slower disk. >> >> Could you please test Mikulas' suggestion of changing >> synchronize_sched() in include/linux/percpu-rwsem.h to >> synchronize_sched_expedited()? > > Tested. It seems as fast as before, but may be a "tick" slower. Just > perception. I was getting pretty much 0.012s with everything reverted. > With synchronize_sched_expedited(), it seems to be 0.012s ~ 0.013s. > So, it's good. Excellent >> linux-next also has a re-write of the per-cpu rw sems, out of Andrews >> tree. It would be a good data point it you could test that, too. > > Tested. It's slower. 0.350s. But still faster than 0.500s without the patch. Makes sense, it's 2 synchronize_sched() instead of 3. So it doesn't fix the real issue, which is having to do synchronize_sched() in the first place. > # time mount /dev/sda1 /mnt; sync; sync; umount /mnt > > > So, here's the comparison ... > > 0.500s 3.7.0-rc7 > 0.168s 3.7.0-rc2 > 0.012s 3.6.0 > 0.013s 3.7.0-rc7 + synchronize_sched_expedited() > 0.350s 3.7.0-rc7 + Oleg's patch. I wonder how many of them are due to changing to the same block size. Does the below patch make a difference? diff --git a/fs/block_dev.c b/fs/block_dev.c index 1a1e5e3..f041c56 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -126,29 +126,28 @@ int set_blocksize(struct block_device *bdev, int size) if (size < bdev_logical_block_size(bdev)) return -EINVAL; - /* Prevent starting I/O or mapping the device */ - percpu_down_write(&bdev->bd_block_size_semaphore); - /* Check that the block device is not memory mapped */ mapping = bdev->bd_inode->i_mapping; mutex_lock(&mapping->i_mmap_mutex); if (mapping_mapped(mapping)) { mutex_unlock(&mapping->i_mmap_mutex); - percpu_up_write(&bdev->bd_block_size_semaphore); return -EBUSY; } mutex_unlock(&mapping->i_mmap_mutex); /* Don't change the size if it is same as current */ if (bdev->bd_block_size != size) { - sync_blockdev(bdev); - bdev->bd_block_size = size; - bdev->bd_inode->i_blkbits = blksize_bits(size); - kill_bdev(bdev); + /* Prevent starting I/O */ + percpu_down_write(&bdev->bd_block_size_semaphore); + if (bdev->bd_block_size != size) { + sync_blockdev(bdev); + bdev->bd_block_size = size; + bdev->bd_inode->i_blkbits = blksize_bits(size); + kill_bdev(bdev); + } + percpu_up_write(&bdev->bd_block_size_semaphore); } - percpu_up_write(&bdev->bd_block_size_semaphore); - return 0; } @@ -1649,14 +1648,12 @@ EXPORT_SYMBOL_GPL(blkdev_aio_write); static int blkdev_mmap(struct file *file, struct vm_area_struct *vma) { + struct address_space *mapping = file->f_mapping; int ret; - struct block_device *bdev = I_BDEV(file->f_mapping->host); - - percpu_down_read(&bdev->bd_block_size_semaphore); + mutex_lock(&mapping->i_mmap_mutex); ret = generic_file_mmap(file, vma); - - percpu_up_read(&bdev->bd_block_size_semaphore); + mutex_unlock(&mapping->i_mmap_mutex); return ret; } -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html