Re: User space out of memory approach

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 2005-01-11 at 04:03 +0200, Edjard Souza Mota wrote:
> > I have no objections against the userspace provided candidate list
> > option, but as long as the main sources of trouble
> > 
> >         - invocation
> >         - reentrancy
> >         - timed, counted, blah ugly protection
> >         - selection problem
> > 
> > are not fixed properly, we don't need to discuss the inclusion of a
> > userspace provided candidate list.
> 
> Any solution that doesn't  offer a proper approach to the above issues
> should not be discussed anyway. By allowing the ranking goes up to the
> user space is not meant only for user testing ranking, but to keep the
> OOM Killer kernel code simpler and clean. As a matter of fact, even
> protected.
>
> Consider the invocation for example. It comes in two phases with this proposal:

I consider the invocation of out_of_memory in the first place. This is
the real root of the problems. The ranking is a different playground.
Your solution does not solve
- invocation madness
- reentrancy protection
- the ugly mess of timers, counters... in out_of_memory, which aren't
neccecary at all

This must be solved first in a proper way, before we talk about ranking.

You are definitely curing the symptom instead of the cause.

> 1) ranking for the most likely culprits only starts when memory consumption
>     gets close to the red zone (for example 98% or something like that).
> 2) killing just gets the first candidate from the list and kills it.
> No need to calculate
>     at kernel level.

What is the default behaviour when no userspace settings are available -
Nothing ? Are you really expecting that we change every root fs in order
to be able to upgrade the kernel for solving this _kernel_ problem ?

Who is setting up those userspace constraints ? Joe User, who is barely
able to find the power on button on his box ? The sysadmin, who will
have to adjust the list for each box depending on the apps it runs or
the user who is logged into the box ?

Memory management _is_ a kernel task and so the shortage of memory has
to be handled by the kernel on its own in the first place. Adding user
space support for certain tasks is a good thing, but not a solution to
the problem itself.

> The selection problem is very dependent on the ranking algorithm. For PCs it
> may not be a trouble, but for emdedded devices? yes it is. The ranking at the
> kernel level uses only int type of integer. If you get the log file
> for the ranking
> in any embedded device you will notice that many processes end up with
> the same ranking point. Thus, there will never be the best choice in this way.

I know the constrains of embedded boxes well enough to know that there
is a bit of a difference to a desktop machine.

> By moving just the ranking to the user space fix this problem 'cause you may
> use float to order PIDs with different indexes. The good side effect is that we 
> allow better ways of choosing the culprit by means of diffrent calculations to 
> meet different patterns of memory consumtion.

I'm running Andrea's and my combined fixes on a couple of embedded and
desktop boxes and it has proven to be a proper in kernel solution for
the in kernel problem. 

I don't argue againts the ability to provide a culprit list to the
kernel, but as I said before it only can be a optional addon to a proper
in kernel solution.

tglx