lkml.org 
[lkml]   [2012]   [Nov]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: Recent kernel "mount" slow
    On 2012-11-27 11:06, Jeff Chua wrote:
    > On Tue, Nov 27, 2012 at 3:38 PM, Jens Axboe <axboe@kernel.dk> wrote:
    >> On 2012-11-27 06:57, Jeff Chua wrote:
    >>> On Sun, Nov 25, 2012 at 7:23 AM, Jeff Chua <jeff.chua.linux@gmail.com> wrote:
    >>>> On Sun, Nov 25, 2012 at 5:09 AM, Mikulas Patocka <mpatocka@redhat.com> 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



    \
     
     \ /
      Last update: 2012-11-27 14:01    [W:2.554 / U:0.192 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site