tmpfs swapoff oddity

tmpfs swapoff oddity

[Mike Galbraith]

Hi Christoph,

While testing Jens' loop-4 patch (and not being able to find
any way to lock it up), I stumbled onto a strange behavior.

I set up an interleaved swap with one swap partition, and one
swapfile in a loopback mounted reiserfs - populated tmpfs with
a kernel tree and did hefty make -j kernel builds to generate
much I/O.  Afterward (bored), I figured I'd bounce the data in
tmpfs back and forth between swap containers with swapoff.  It
took much longer than I expected.

This is likely only something a bored tester would do, but it
seems odd enough to report.. repeated without loop.

[root]:# pwd
/var/tmp/linux-2.4.2-pre1
[root]:# df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/hda6              4157873   3734124    213701  95% /
shm                     499092         0    499092   0% /dev/shm
/dev/hda7               256592    155715     87625  64% /var
tmpfs                   622228    123136    499092  20% /var/tmp
/dev/hda8             10445318   6450096   3669907  64% /usr/local
/dev/hda1              4088532   2286968   1801564  56% /dos_c
[root]:# mount
/dev/hda6 on / type ext2 (rw)
proc on /proc type proc (rw)
shm on /dev/shm type shm (rw)
/dev/hda7 on /var type ext2 (rw)
tmpfs on /var/tmp type tmpfs (rw)
/dev/hda8 on /usr/local type ext2 (rw)
/dev/hda1 on /dos_c type vfat (rw,noexec)
devpts on /dev/pts type devpts (rw,gid=5,mode=0620)
[root]:# swapon -s
Filename			Type		Size	Used	Priority
/dev/hda2                       partition	265064	127356	2
/dev/hda5                       partition	265032	0	2
[root]:# time swapoff /dev/hda2

real	7m17.555s
user	0m0.010s
sys	7m3.040s
[root]:#

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Re: tmpfs swapoff oddity

[Christoph Rohland]

Hi Mike,

On Thu, 8 Feb 2001, Mike Galbraith wrote:
> Hi Christoph,
> 
> While testing Jens' loop-4 patch (and not being able to find
> any way to lock it up), I stumbled onto a strange behavior.
> 
> I set up an interleaved swap with one swap partition, and one
> swapfile in a loopback mounted reiserfs - populated tmpfs with
> a kernel tree and did hefty make -j kernel builds to generate
> much I/O.  Afterward (bored), I figured I'd bounce the data in
> tmpfs back and forth between swap containers with swapoff.  It
> took much longer than I expected.

Oh, the swapoff handling in Linux is much less then suboptimal. So I
would expect that.

To explain: For every single page on swap we scan all processes vmas
and all shm/tmpfs objects swap tables :-( And we have to hold some
locks for that...

Greetings
		Christoph


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