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SubjectRe: [PATCH -v2 -mm] add extra free kbytes tunable
On Thu, 1 Sep 2011, Andrew Morton wrote:

> > Add a userspace visible knob
>
> argh. Fear and hostility at new knobs which need to be maintained for
> ever, even if the underlying implementation changes.
>

Do we really need to maintain tunables that lose their purpose either
because the implementation changes or is patched to fix the issue that the
tunable was intended to address without requiring it?

Are there really userspace tools written to not be able to handle -ENOENT
when one of these gets removed?

> > It may also be useful to reduce the memory use of virtual
> > machines (temporarily?), in a way that does not cause memory
> > fragmentation like ballooning does.
>
> Maybe. You need to alter the setting, then somehow persuade all the
> targeted kswapd's to start running, then somehow determine that they've
> done their thing, then unalter the /proc setting. Not the best API
> we've ever designed ;)
>

And, unfortunately, this could have negative effects if using cpusets
and/or mempolicies since this is global across all zones such that jobs
that do not require such "extra" memory would be unfairly penalized with
work incurred by additional reclaim they don't need. And if the job is
constrained to a memory cgroup, there's no guarantee it will reclaim back
to these altered watermarks.


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