Re: [patch 4/7 -mm] oom: badness heuristic rewrite

[David Rientjes]

On Thu, 11 Feb 2010, Andrew Morton wrote:

> > Changing any value that may have a tendency to be hardcoded elsewhere is 
> > always controversial, but I think the nature of /proc/pid/oom_adj allows 
> > us to do so for two specific reasons:
> > 
> >  - hardcoded values tend not the fall within a range, they tend to either
> >    always prefer a certain task for oom kill first or disable oom killing
> >    entirely.  The current implementation uses this as a bitshift on a
> >    seemingly unpredictable and unscientific heuristic that is very 
> >    difficult to predict at runtime.  This means that fewer and fewer
> >    applications would hardcode a value of '8', for example, because its 
> >    semantics depends entirely on RAM capacity of the system to begin with
> >    since badness() scores are only useful when used in comparison with
> >    other tasks.
> 
> You'd be amazed what dumb things applications do.  Get thee to
> http://google.com/codesearch?hl=en&lr=&q=[^a-z]oom_adj[^a-z]&sbtn=Search
> and start reading.  All 641 matches ;)
> 
> Here's one which which writes -16:
> http://google.com/codesearch/p?hl=en#eN5TNOm7KtI/trunk/wlan/vendor/asus/eeepc/init.rc&q=[^a-z]oom_adj[^a-z]&sa=N&cd=70&ct=rc
> 
> Let's not change the ABI please.
> 

Sigh, this is going to require the amount of system memory to be 
partitioned into OOM_ADJUST_MAX, 15, chunks and that's going to be the 
granularity at which we'll be able to either bias or discount memory usage 
of individual tasks by: instead of being able to do this with 0.1% 
granularity we'll now be limited to 100 / 15, or ~7%.  That's ~9GB on my 
128GB system just because this was originally a bitshift.  The upside is 
that it's now linear and not exponential.
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