Re: Break 2.4 VM in five easy steps

Re: Break 2.4 VM in five easy steps

[Bulent Abali]

>> O.k.  I think I'm ready to nominate the dead swap pages for the big
>> 2.4.x VM bug award.  So we are burning cpu cycles in sys_swapoff
>> instead of being IO bound?  Just wanting to understand this the cheap
way :)
>
>There's no IO being done whatsoever (that I can see with only a blinky).
>I can fire up ktrace and find out exactly what's going on if that would
>be helpful.  Eating the dead swap pages from the active page list prior
>to swapoff cures all but a short freeze.  Eating the rest (few of those)
>might cure the rest, but I doubt it.
>
>    -Mike

1)  I second Mike's observation.  swapoff either from command line or
during
shutdown, just hangs there.  No disk I/O is being done as I could see
from the blinkers.  This is not a I/O boundness issue.  It is more like
a deadlock.

I happened to saw this one with debugger attached serial port.
The system was alive.  I think I was watching the free page count and
it was decreasing very slowly may be couple pages per second.  Bigger
the swap usage longer it takes to do swapoff.  For example, if I had
1GB in the swap space then it would take may be an half hour to shutdown...


2)  Now why I would have 1 GB in the swap space, that is another problem.
Here is what I observe and it doesn't make much sense to me.
Let's say I have 1GB of memory and plenty of swap.  And let's
say there is process with little less than 1GB size.  Suppose the system
starts swapping because it is short few megabytes of memory.
Within *seconds* of swapping, I see that the swap disk usage balloons to
nearly 1GB. Nearly entire memory moves in to the page cache.  If you
run xosview you will know what I mean.  Memory usage suddenly turns from
green to red :-).   And I know for a fact that my disk cannot do 1GB per
second :-). The SHARE column of the big process in "top" goes up by
hundreds
of megabytes.
So it appears to me that MM is marking the whole process memory to be
swapped out and probably reserving nearly 1 GB in the swap space and
furthermore moves entire process pages to apparently to the page cache.
You would think that if you are short by few MB of memory MM would put
few MB worth of pages in the swap. But it wants to move entire processes
in to swap.

When the 1GB process exits, the swap usage doesn't change (dead swap
pages?).
And shutdown or swapoff will take forever due to #1 above.

Bulent




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Re: Break 2.4 VM in five easy steps

[Mike Galbraith]

On Thu, 7 Jun 2001, Bulent Abali wrote:

> I happened to saw this one with debugger attached serial port.
> The system was alive.  I think I was watching the free page count and
> it was decreasing very slowly may be couple pages per second.  Bigger
> the swap usage longer it takes to do swapoff.  For example, if I had
> 1GB in the swap space then it would take may be an half hour to shutdown...

I took a ~300ms ktrace snapshot of the no IO spot with 2.4.4.ikd..

  % TOTAL    TOTAL USECS    AVG/CALL   NCALLS
  0.0693%         208.54        0.40      517 c012d4b9 __free_pages
  0.0755%         227.34        1.01      224 c012cb67 __free_pages_ok
  ...
 34.7195%      104515.15        0.95   110049 c012de73 unuse_vma
 53.3435%      160578.37      303.55      529 c012dd38 __swap_free
Total entries: 131051  Total usecs:    301026.93 Idle: 0.00%

Andrew Morton could be right about that loop not being wonderful.

	-Mike

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Re: Break 2.4 VM in five easy steps

[Eric W. Biederman]

Derek Glidden <dglidden@illusionary.com> writes:


> The problem I reported is not that 2.4 uses huge amounts of swap but
> that trying to recover that swap off of disk under 2.4 can leave the
> machine in an entirely unresponsive state, while 2.2 handles identical
> situations gracefully.  
> 

The interesting thing from other reports is that it appears to be kswapd
using up CPU resources.  Not the swapout code at all.  So it appears
to be a fundamental VM issue.  And calling swapoff is just a good way
to trigger it. 

If you could confirm this by calling swapoff sometime other than at
reboot time.  That might help.  Say by running top on the console.

Eric



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Re: Break 2.4 VM in five easy steps

[Mike Galbraith]

On 6 Jun 2001, Eric W. Biederman wrote:

> Derek Glidden <dglidden@illusionary.com> writes:
>
>
> > The problem I reported is not that 2.4 uses huge amounts of swap but
> > that trying to recover that swap off of disk under 2.4 can leave the
> > machine in an entirely unresponsive state, while 2.2 handles identical
> > situations gracefully.
> >
>
> The interesting thing from other reports is that it appears to be kswapd
> using up CPU resources.  Not the swapout code at all.  So it appears
> to be a fundamental VM issue.  And calling swapoff is just a good way
> to trigger it.
>
> If you could confirm this by calling swapoff sometime other than at
> reboot time.  That might help.  Say by running top on the console.

The thing goes comatose here too. SCHED_RR vmstat doesn't run, console
switch is nogo...

After running his memory hog, swapoff took 18 seconds.  I hacked a
bleeder valve for dead swap pages, and it dropped to 4 seconds.. still
utterly comatose for those 4 seconds though.

	-Mike

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Re: Break 2.4 VM in five easy steps

[Eric W. Biederman]

Mike Galbraith <mikeg@wen-online.de> writes:

> On 6 Jun 2001, Eric W. Biederman wrote:
> 
> > Derek Glidden <dglidden@illusionary.com> writes:
> >
> >
> > > The problem I reported is not that 2.4 uses huge amounts of swap but
> > > that trying to recover that swap off of disk under 2.4 can leave the
> > > machine in an entirely unresponsive state, while 2.2 handles identical
> > > situations gracefully.
> > >
> >
> > The interesting thing from other reports is that it appears to be kswapd
> > using up CPU resources.  Not the swapout code at all.  So it appears
> > to be a fundamental VM issue.  And calling swapoff is just a good way
> > to trigger it.
> >
> > If you could confirm this by calling swapoff sometime other than at
> > reboot time.  That might help.  Say by running top on the console.
> 
> The thing goes comatose here too. SCHED_RR vmstat doesn't run, console
> switch is nogo...
> 
> After running his memory hog, swapoff took 18 seconds.  I hacked a
> bleeder valve for dead swap pages, and it dropped to 4 seconds.. still
> utterly comatose for those 4 seconds though.

At the top of the while(1) loop in try_to_unuse what happens if you put in.
if (need_resched) schedule(); 
It should be outside all of the locks.  It might just be a matter of everything
serializing on the SMP locks, and the kernel refusing to preempt itself.

Eric

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Re: Break 2.4 VM in five easy steps

[Mike Galbraith]

On 6 Jun 2001, Eric W. Biederman wrote:

> Mike Galbraith <mikeg@wen-online.de> writes:
>
> > > If you could confirm this by calling swapoff sometime other than at
> > > reboot time.  That might help.  Say by running top on the console.
> >
> > The thing goes comatose here too. SCHED_RR vmstat doesn't run, console
> > switch is nogo...
> >
> > After running his memory hog, swapoff took 18 seconds.  I hacked a
> > bleeder valve for dead swap pages, and it dropped to 4 seconds.. still
> > utterly comatose for those 4 seconds though.
>
> At the top of the while(1) loop in try_to_unuse what happens if you put in.
> if (need_resched) schedule();
> It should be outside all of the locks.  It might just be a matter of everything
> serializing on the SMP locks, and the kernel refusing to preempt itself.

That did it.

	-Mike

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Re: Break 2.4 VM in five easy steps

[Eric W. Biederman]

Mike Galbraith <mikeg@wen-online.de> writes:

> On 6 Jun 2001, Eric W. Biederman wrote:
> 
> > Mike Galbraith <mikeg@wen-online.de> writes:
> >
> > > > If you could confirm this by calling swapoff sometime other than at
> > > > reboot time.  That might help.  Say by running top on the console.
> > >
> > > The thing goes comatose here too. SCHED_RR vmstat doesn't run, console
> > > switch is nogo...
> > >
> > > After running his memory hog, swapoff took 18 seconds.  I hacked a
> > > bleeder valve for dead swap pages, and it dropped to 4 seconds.. still
> > > utterly comatose for those 4 seconds though.
> >
> > At the top of the while(1) loop in try_to_unuse what happens if you put in.
> > if (need_resched) schedule();
> > It should be outside all of the locks.  It might just be a matter of
> everything
> 
> > serializing on the SMP locks, and the kernel refusing to preempt itself.
> 
> That did it.

Does this improve the swapoff speed or just allow other programs to
run at the same time?  If it is still slow under that kind of load it
would be interesting to know what is taking up all time.

If it is no longer slow a patch should be made and sent to Linus.

Eric
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Re: Break 2.4 VM in five easy steps

[Mike Galbraith]

On 7 Jun 2001, Eric W. Biederman wrote:

> Mike Galbraith <mikeg@wen-online.de> writes:
>
> > On 6 Jun 2001, Eric W. Biederman wrote:
> >
> > > Mike Galbraith <mikeg@wen-online.de> writes:
> > >
> > > > > If you could confirm this by calling swapoff sometime other than at
> > > > > reboot time.  That might help.  Say by running top on the console.
> > > >
> > > > The thing goes comatose here too. SCHED_RR vmstat doesn't run, console
> > > > switch is nogo...
> > > >
> > > > After running his memory hog, swapoff took 18 seconds.  I hacked a
> > > > bleeder valve for dead swap pages, and it dropped to 4 seconds.. still
> > > > utterly comatose for those 4 seconds though.
> > >
> > > At the top of the while(1) loop in try_to_unuse what happens if you put in.
> > > if (need_resched) schedule();
> > > It should be outside all of the locks.  It might just be a matter of
> > everything
> >
> > > serializing on the SMP locks, and the kernel refusing to preempt itself.
> >
> > That did it.
>
> Does this improve the swapoff speed or just allow other programs to
> run at the same time?  If it is still slow under that kind of load it
> would be interesting to know what is taking up all time.
>
> If it is no longer slow a patch should be made and sent to Linus.

No, it only cures the freeze.  The other appears to be the slow code
pointed out by Andrew Morton being tickled by dead swap pages.

	-Mike

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Re: Break 2.4 VM in five easy steps

[Eric W. Biederman]

Mike Galbraith <mikeg@wen-online.de> writes:

> On 7 Jun 2001, Eric W. Biederman wrote:
> 
> > Does this improve the swapoff speed or just allow other programs to
> > run at the same time?  If it is still slow under that kind of load it
> > would be interesting to know what is taking up all time.
> >
> > If it is no longer slow a patch should be made and sent to Linus.
> 
> No, it only cures the freeze.  The other appears to be the slow code
> pointed out by Andrew Morton being tickled by dead swap pages.

O.k.  I think I'm ready to nominate the dead swap pages for the big
2.4.x VM bug award.  So we are burning cpu cycles in sys_swapoff
instead of being IO bound?  Just wanting to understand this the cheap way :)

Eric
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Re: Break 2.4 VM in five easy steps

[Mike Galbraith]

On 7 Jun 2001, Eric W. Biederman wrote:

> Mike Galbraith <mikeg@wen-online.de> writes:
>
> > On 7 Jun 2001, Eric W. Biederman wrote:
> >
> > > Does this improve the swapoff speed or just allow other programs to
> > > run at the same time?  If it is still slow under that kind of load it
> > > would be interesting to know what is taking up all time.
> > >
> > > If it is no longer slow a patch should be made and sent to Linus.
> >
> > No, it only cures the freeze.  The other appears to be the slow code
> > pointed out by Andrew Morton being tickled by dead swap pages.
>
> O.k.  I think I'm ready to nominate the dead swap pages for the big
> 2.4.x VM bug award.  So we are burning cpu cycles in sys_swapoff
> instead of being IO bound?  Just wanting to understand this the cheap way :)

There's no IO being done whatsoever (that I can see with only a blinky).
I can fire up ktrace and find out exactly what's going on if that would
be helpful.  Eating the dead swap pages from the active page list prior
to swapoff cures all but a short freeze.  Eating the rest (few of those)
might cure the rest, but I doubt it.

	-Mike

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Re: Break 2.4 VM in five easy steps

[Marcelo Tosatti]

On Thu, 7 Jun 2001, Mike Galbraith wrote:

> On 6 Jun 2001, Eric W. Biederman wrote:
> 
> > Mike Galbraith <mikeg@wen-online.de> writes:
> >
> > > > If you could confirm this by calling swapoff sometime other than at
> > > > reboot time.  That might help.  Say by running top on the console.
> > >
> > > The thing goes comatose here too. SCHED_RR vmstat doesn't run, console
> > > switch is nogo...
> > >
> > > After running his memory hog, swapoff took 18 seconds.  I hacked a
> > > bleeder valve for dead swap pages, and it dropped to 4 seconds.. still
> > > utterly comatose for those 4 seconds though.
> >
> > At the top of the while(1) loop in try_to_unuse what happens if you put in.
> > if (need_resched) schedule();
> > It should be outside all of the locks.  It might just be a matter of everything
> > serializing on the SMP locks, and the kernel refusing to preempt itself.
> 
> That did it.

What about including this workaround in the kernel ? 

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Re: Break 2.4 VM in five easy steps

[Derek Glidden]

"Eric W. Biederman" wrote:
> 
> Derek Glidden <dglidden@illusionary.com> writes:
> 
> > The problem I reported is not that 2.4 uses huge amounts of swap but
> > that trying to recover that swap off of disk under 2.4 can leave the
> > machine in an entirely unresponsive state, while 2.2 handles identical
> > situations gracefully.
> >
> 
> The interesting thing from other reports is that it appears to be kswapd
> using up CPU resources.  Not the swapout code at all.  So it appears
> to be a fundamental VM issue.  And calling swapoff is just a good way
> to trigger it.
> 
> If you could confirm this by calling swapoff sometime other than at
> reboot time.  That might help.  Say by running top on the console.

That's exactly what my original test was doing.  I think it was Jeffrey
Baker complaining about "swapoff" at reboot.  See my original post that
started this thread and follow the "five easy steps."  :)  I'm sucking
down a lot of swap, although not all that's available which is something
I am specifically trying to avoid - I wanted to stress the VM/swap
recovery procedure, not "out of RAM and swap" memory pressure - and then
running 'swapoff' from an xterm or a console.

The problem with being able to see what's eating up CPU resources is
that the whole machine stops responding for me to tell.  consoles stop
updating, the X display freezes, keyboard input is locked out, etc.  As
far as anyone can tell, for several minutes, the whole machine is locked
up. (except, strangely enough, the machine will still respond to ping) 
I've tried running 'top' to see what task is taking up all the CPU time,
but the system hangs before it shows anything meaningful.  I have been
able to tell that it hits 100% "system" utilization very quickly though.

I did notice that the first thing sys_swapoff() does is call
lock_kernel() ... so if sys_swapoff() takes a long time, I imagine
things will get very unresponsive quickly.  (But I'm not intimately
familiar with the various kernel locks, so I don't know what
granularity/atomicity/whatever lock_kernel() enforces.)

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$_='while(read+STDIN,$_,2048){$a=29;$b=73;$c=142;$t=255;@t=map
{$_%16or$t^=$c^=($m=(11,10,116,100,11,122,20,100)[$_/16%8])&110;
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unxV,xb25,$_;$e=256|(ord$b[4])<<9|ord$b[3];$d=$d>>8^($f=$t&($d
>>12^$d>>4^$d^$d/8))<<17,$e=$e>>8^($t&($g=($q=$e>>14&7^$e)^$q*
8^$q<<6))<<9,$_=$t[$_]^(($h>>=8)+=$f+(~$g&$t))for@a[128..$#a]}
print+x"C*",@a}';s/x/pack+/g;eval 

usage: qrpff 153 2 8 105 225 < /mnt/dvd/VOB_FILENAME \
    | extract_mpeg2 | mpeg2dec - 

http://www.eff.org/                    http://www.opendvd.org/ 
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Re: Break 2.4 VM in five easy steps

[Rik van Riel]

On 6 Jun 2001, Eric W. Biederman wrote:
> Derek Glidden <dglidden@illusionary.com> writes:
> 
> > The problem I reported is not that 2.4 uses huge amounts of swap but
> > that trying to recover that swap off of disk under 2.4 can leave the
> > machine in an entirely unresponsive state, while 2.2 handles identical
> > situations gracefully.  
> 
> The interesting thing from other reports is that it appears to be
> kswapd using up CPU resources.

This part is being worked on, expect a solution for this thing
soon...


Rik
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