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SubjectRe: [PATCH] add some drop_caches documentation and info messsge
On Wed, 24 Oct 2012 08:29:45 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> > >
> > > + printk(KERN_NOTICE "%s (%d): dropped kernel caches: %d\n",
> > > + current->comm, task_pid_nr(current), sysctl_drop_caches);
> >
> > urgh. Are we really sure we want to do this? The system operators who
> > are actually using this thing will hate us :(
>
> I have no problems with lowering the priority (how do you see
> KERN_INFO?) but shouldn't this message kick them that they are doing
> something wrong? Or if somebody uses that for "benchmarking" to have a
> clean table before start is this really that invasive?

hmpf. This patch worries me. If there are people out there who are
regularly using drop_caches because the VM sucks, it seems pretty
obnoxious of us to go dumping stuff into their syslog. What are they
supposed to do? Stop using drop_caches? But that would unfix the
problem which they fixed with drop_caches in the first case.

And they might not even have control over the code - they need to go
back to their supplier and say "please send me a new version", along
with all the additional costs and risks involed in an update.

> > More friendly alternatives might be:
> >
> > - Taint the kernel. But that will only become apparent with an oops
> > trace or similar.
> >
> > - Add a drop_caches counter and make that available in /proc/vmstat,
> > show_mem() output and perhaps other places.
>
> We would loose timing and originating process name in both cases which
> can be really helpful while debugging. It is fair to say that we could
> deduce the timing if we are collecting /proc/meminfo or /proc/vmstat
> already and we do collect them often but this is not the case all of the
> time and sometimes it is important to know _who_ is doing all this.

But how important is all that? The main piece of information the
kernel developer wants is "this guy is using drop_caches a lot". All
the other info is peripheral and can be gathered by other means if so
desired.


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