Re: [PATCH] Memory usage limit notification addition to memcg

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 13 Apr 2009 16:45:17 -0700
Dan Malek <dan@embeddedalley.com> wrote:

> On Apr 13, 2009, at 4:08 PM, Andrew Morton wrote:
> 
> > We've run into problems in the past where a percentage number is too
> > coarse on large-memory systems.
> >
> > Proabably that won't be an issue here, but I invite you to convince us
> > of this ;)
> 
> The challenge here is that the absolute limit of the memcg can
> be dynamically changed, so I wanted to avoid a couple of problems.
> One is just a system configuration error where someone forgets
> to modify both.  For example, if you start with the memcg limit of 100M,
> and the notification limit to 80M, then come back and change the memcg
> limit to 90M (or worse, < 80M) you now have a clearly incorrect
> configuration.   Another problem is the operation isn't atomic, at some
> point during the changes, even if you remember to do it correctly, you
> will have the two values not representing what you really want.  It
> could trigger an erroneous notification, or simply OOM kill before you
> get the configuration correct.
> 
> If an integer number turns out to not be sufficient, we could change  
> this
> to some fixed point representation and adjust the arithmetic in the  
> tests.
> I believe the integer number will be fine, even in large memory systems.
> This is just a notification model, if we want something more fine  
> grained
> I believe it would need different semantics.

I agree.  But it would be a mighty mess if we were to turn around in
two years time and add a second centi-percent interface.  So we should
give this careful thought now and really convince ourselves that we
will never ever ever want sub-1% resolution.