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Date: Mon, 10 Apr 2006 00:40:30 -0700
From: Andrew Morton <akpm@osdl.org>
To: Grzegorz Kulewski <kangur@polcom.net>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Slow swapon for big (12GB) swap
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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?

If it's a swapfile then perhaps the filesystem is being inefficient in its
bmap() function.  Which filesystem is it?