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

Andrew Morton
2010-02-11T22:31:51Z

On Thu, 11 Feb 2010 13:51:36 -0800 (PST)
David Rientjes <rientjes@google.com> wrote:

> 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.

Can you add newly-named knobs (rather than modifying the existing
ones), deprecate the old ones and then massage writes to the old ones
so that they talk into the new framework?