Re: Some questions about linux kernel.

From: David Whysong (
Date: Tue Mar 21 2000 - 01:23:39 EST

On Mon, 20 Mar 2000, Jesse Pollard wrote:
>On Mon, 20 Mar 2000, David Whysong wrote:
>>When OOM the kernel has to kill something. The trick is for the kernel to
>>kill tasks that are considered "less important". Don't kill init, X
>>server, syslogd, whatever. Some of this policy could be set by a userspace
>By the time the userspace daemon runs it is too late.

Not necessarrily. Here is the (rough) idea:

        1. Modify Rik van Riel's patch to understand multiple "nice" levels.
        2. Have a daemon that finds new processes and renices them,
           based on a config file. This puts policy in user-space.

In theory, the daemon could be woken up by the kernel when new processes
are started, and renice them before they actually run.

The beauty of this is that the daemon is not required for OOM handling;
Rik van Riel's kernel patch takes care of that. The daemon simply allows
policy to be placed in user-space.

>The kernel must do the bookkeeping for the resource allocation. It must
>be able to decrement from the users quota what is being allocated by the
>various allocation methods - fork, exec, sbrk, stack page fault, COW.

Ugh, no thanks. Overhead in common functions...

>The kernel can enforce the limit, but it cannot determine what the limit
>should be. This is the place for the userspace daemon.

Ideally you want policy set in userspace, but executed by the kernel (with
a reasonable default case). That's what I'm trying to do, though not with

>On some systems, the quotas are staticly set by administration, and loaded
>into the kernel on login (or cron/batch). If the quotas to be set are
>not available (determined by the userspace daemon) then the user should
>not be logged in (or cron/batch job not started).

(I may be misunderstanding you here...)

Refusing user login is yet another failure mode that will cause great
moaning and mayhem, and result in an out of work sysadmin. No thanks.

>It may be possible for the userspace daemon to be able to carry out
>adjustments - If the policy is that from 8-5 all logins/cron/batch may
>have the quota set by administration until N logins are active. No more

I'm still thinking from a quota-free point of view.

>>This is completely independent of overcommit or rlimits (which can cause
>>the OOM behavior to happen more often, though in more predictable and
>>perhaps less drastic fashion).
>It is a way to prevent OOM from occuring at the system level.

Maybe, but I'm not convinced. You still have to deal with kernel
allocations. And these memory quotas come at huge expense, if they are to
be useful at preventing system-wide OOM.

>>I'm a scientific app programmer, not a system programmer... but I'll try
>>to code up a simple daemon in the next couple days that works in
>>conjunction with a modified version of Rik van Riel's patch.
>Watch out for the kernel level accounting that is needed.

What is needed is sane handling of system wide OOM situations. Killing the
offending process when the system runs OOM is a lot more sane than
crippling everybody's access to memory resources. IMNSHO.


David Whysong
Astrophysics graduate student University of California, Santa Barbara
My public PGP keys are on my web page -
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