Re: [PATCH 1/4] oom: move oom_adj to signal_struct

[KAMEZAWA Hiroyuki]

On Wed, 5 Aug 2009 15:37:01 +0900
Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> Hmm. I can't understand why it is troublesome. 
> I think it's related to moving oom_adj to singal_struct. 
> Unfortunately, I can't understand why we have to put oom_adj 
> in singal_struct?
> 
> That's why I have a question to Kosaki a while ago. 
> I can't understand it still. :-(
> 
> Could you elaborate it ?
> 

Current code is as following
==
  do_each_thread(g,p) {
	......
	p = badness();

	record p of highest badness.
  }
  p = higest badness thread.

  Scan all threads which shares mm_struct of p. and check oom_adj
  
==	
Assume a process which has 20000 threads. And 1 of thread has OOM_DISABLE.

Then, at worst, this scan will needs
	(1+2+3+....+20000) * (20000-1) scan. (when ignoring other processes)
even with your patch.

This means the kernel wastes enough long time that Cluster-Management-Software can
detetct this as livelock, and do reboot/cluster-fail-over.

Fixing livelock is not the last goal. I (we) would like to reduct stall time
to reasonable level. If we move oom_adj to signal_struct or mm_struct, scan-cost
will be only 20000. No retry at all.

And, if we can use for_each_process() rather than do_each_thread(),
scan-cost will be 1.

(BTW, "signal" struct is bad name I think, it should be "process" struct ;)


Thanks,
-Kame

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