Re: [patch 4/4] oom: serialize for cpusets

[Christoph Lameter <clameter@sgi.com>]

On Thu, 28 Jun 2007, David Rientjes wrote:

> If you attach all your system tasks to a single small node and then 
> attempt to allocate large amounts of memory in that node, tasks get killed 
> unnecessarily.  This is a good way to approximate a cpuset's memory 
> pressure in real-world examples.  The actual rogue task can avoid getting 
> killed by simply not allocating the last N kB in that node while other 
> tasks, such as sshd or sendmail, require memory on a spurious basis.  So 
> we've often seen tasks such as those get OOM killed even though they don't 
> alleviate the condition much at all: sshd and sendmail are not normally 
> memory hogs.

Yeah but to get there seems to require intention on the part of the 
rogue tasks.

> The much better policy in terms of sharing memory among a cpuset's task is 
> to kill the actual rogue task which we can estimate pretty well with 
> select_bad_process() since it takes into consideration, most importantly, 
> the total VM size.

Sorry that is too expensive. I did not see that initially. Thanks Paul for 
reminding me. I am at the OLS and my mindshare for this is pretty limited 
right now.

> So my belief is that it is better to kill one large memory-hogging task in 
> a cpuset instead of killing multiple smaller ones based on their 
> scheduling and unfortunate luck of being the one to enter the OOM killer.  
> Even worse is when the OOM killer, which is not at all serialized for 
> cpuset-constrained allocations at present, kills multiple smaller tasks 
> before killing the rogue task.  Then those previous kills were unnecessary 
> and certainly would qualify as a strong example for why current git's 
> behavior is broken.

The current behavior will usually kill the memory hogging task and it can 
do so with minimal effort. If there is a whole array of memory hogging 
tasks then the existing approach will be much easier on the system.

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