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    SubjectRe: Slow swapon for big (12GB) swap
    On Mon, 10 Apr 2006, Andrew Morton wrote:
    > Grzegorz Kulewski <kangur@polcom.net> wrote:
    >> On Mon, 10 Apr 2006, Andrew Morton wrote:
    >>> Grzegorz Kulewski <kangur@polcom.net> wrote:
    >>>> I am using big swap here (as a backing for potentially huge tmpfs). And I
    >>>> wonder why swapon on such big (like 12GB) swap takes about 7 minutes
    >>>> (continuous disk IO).
    >>>
    >>> It's a bit quicker here:
    >>>
    >>> vmm:/usr/src/25# mkswap /dev/hda6
    >>> Setting up swapspace version 1, size = 54031826 kB
    >>> vmm:/usr/src/25# time swapon /dev/hda6
    >>> swapon /dev/hda6 0.00s user 0.04s system 74% cpu 0.054 total
    >>>
    >>>
    >>>> Is this expected?
    >>>
    >>> Nope.
    >>>
    >>>> Why it is like that?
    >>>
    >>> Are you using a swapfile or a swap partition?
    >>
    >> Swap file.
    >>
    >>
    >>> If it's a swapfile then perhaps the filesystem is being inefficient in its
    >>> bmap() function. Which filesystem is it?
    >>
    >> Ext3.
    >>
    >
    > OK. A 12G file is around 3000 indirect blocks. At 10 milliseconds each
    > I'd expect it to take 30-odd seconds. A 14G swapfile here takes 20 second
    > for swapon.

    That is what I would expect.


    > Maybe your disks are slow, or the fs is already quite full+fragmented.

    Well, the disk is slow but not that slow... For example dd creating this
    file goes at about 20-25MB/s and takes about 10 minutes IIRC, so it is not
    *that* bad...

    The disk is not full or even close to:
    /dev/hda4 /mnt/data 52552,1 27343,6 24140,4 54% ext3

    and file count is not that high too:
    /dev/hda4 6836224 42333 6793891 1% /mnt/data

    and it should not be fragmented in any way. It is rather new (3 months
    old) filesystem with several bigger files and small amount of smaller.
    Used mainly as a data store and backup. Very infrequently written to (new
    blocks/files).

    I think I will test new kernel soon. Maybe with and without -ck. Maybe
    disk scheduler (probably CFQ) does something stupid... Or swap prefetch...
    Or anything...


    Thanks,

    Grzegorz Kulewski

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