Re: [9fans] protection against resource exhaustion

[erik quanstrom <quanstro@quanstro.net>]

On Sat Jan 24 23:16:07 PST 2015, mischief@9.offblast.org wrote:

> You would need to implement something like ulimits to fix process
> exhaustion, or some kind of rate limit.  There are several forms of
> resource exhaustion in plan 9.  One other that comes to mind is cat
> /dev/random prevents other clients from making progress reading
> /dev/random, such as cpu.

this is the same route as oom killer, which can be depended on to kill
exactly the wrong thing at exactly the wrong time.  fixing things to be
oomkiller-proof tends to be an exercise in wack-a-mole.

about tail-calls mentioned elsewhere.  rc simple executes the file through
a system call and does not inspect its shebang line.  so there is no possibility
of optimization; rc doesn't know it's an rc script.  (and this problem
doesn't depend on the executible recursively executing itself being an rc script.)

i have eliminated the resrcwait() in the process allocation loop in my kernels.
this sounds much worse than it is.  the advantage is it breaks the loop and gets
the machine running again in a bounded amount of time.  the disadvantage
is one loses program state.  i've found it a win over all.  instead of getting paged
in the middle of the night, a few days later someone might mention that the
machine rebooted.  the logs are almost always intact, so debugging the issue
isn't too hard /during the middle of the day/.  :-)

- erik