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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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