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SubjectRe: Slow swapon for big (12GB) swap
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.

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

I guess the only thing I can suggest is that you force the swapfile to put
its indirect blocks close together. You can do that by writing one byte
at each 4MB interval:

creat(swapfile);
for (i = 0; i < swapfile_size / 4MB; i++)
pwrite(fd, "\0", 1, i * 4MB);

then fill in all the holes:

dd if=/dev/zero of=swapfile bs=4MB count=swapfile_size/4MB conv=notrunc

rather kludgy, and hopefully ext3 reservations won't defeat it.
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