Quick question.

Wed Aug 4 06:39:13 UTC 2004

On Tue, 3 Aug 2004, Alexei Roudnev wrote:

> It is not mad idea - 2 CPU servers are not sugnificantly more 
> expansive as 1CPU (and notice, we count P-IV MMultiThread as 2 CPU)

Well, you have to compare like for like, so system with multiple CPUs 
versus exact system without. No diffference in cost, other than for 
the CPUs.

And if you want reliability, you're not going to be buying your 
machines from the nearest Lidl (unless your application is engineered 
to take advantage of dozens of cheap throwaway PCs).

> but increases system redundancy to the run-away processes. Of 
> course, it is not hardware redundancy, but it REALLY works.

Not really.. this is a resource exhaustion problem, and you can not 
cure this, given buggy apps, by throwing more CPUs at it.

Let's say you have some multi-process or multi-threaded application 
which regularly spawns/forks new processes/threads, but it is buggy 
and prone to having individual processes/threads spin.

So one spins, but you still have plenty of CPU time left cause you 
have two CPUs. Another spins, and the machine starts to crawl. So you 
solve this problem by upgrading to a quad-SMP machine. And guess what 
happens? :)

Sure, there are some application bugs you can mask a wee bit with 
SMP, but it's not much cop, its not a solution, and you need 
an infinite-SMP machine to guarantee that a bad application 
can never hog all CPU time.

What you really want is a good OS with:

- a good scheduler (to prevent spinning tasks from starving other 
tasks)

- ability to set resource limits, ie per-task and/or per-user (if 
your apps run under dedicated user accounts) limits on cpu time, 
resident memory, etc..

Both of these will allow you to constrain the impact bad tasks can 
have on the system, whether your machine is 1, 2, ... or n CPUs.

The real solution though is to fix the buggy application.

regards,
-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Fortune:
The life which is unexamined is not worth living.
 		-- Plato