From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Subject: Re: max CPU load recommended
Date: Thu, 28 Jun 2012 22:20:31 +0300
Date: 2012-06-28T22:20:31+03:00 [thread overview]
Message-ID: <a53p3vFdaqU1@mid.individual.net> (raw)
In-Reply-To: <e1f284a7-6b1a-41c1-85e0-016cab6bc5c5@h9g2000yqi.googlegroups.com>
On 12-06-25 12:53 , Gerd wrote:
> Sorry, this not an Ada related question, but maybe you could answer it
> best:
>
> What is a recommended value for maximum CPU load for safety systems?
> Are there any recommendations within 61508 or 26262?
I don't know what those standards recommend, but the idea of a "safe"
CPU load is wrong, IMO. The actual CPU load varies with time; when the
CPU is processing a task, its load is 100%; when it is waiting for some
task to become ready, it is 0%. Any measured "CPU load" figure is an
average over some time period and cannot be used to prove that the
system meets its deadlines within that time period.
For example, if the system is expected to respond in 1 ms to an
interrupt that occurs about twice per second, the average CPU load over
one second can be quite small even if the response takes 10 ms instead
of 1 ms.
You should rather compute the margins for all deadlines and put a limit
there, for example 50% margin, or some absolute time margin.
Some rigorous schedulability analysis methods, like RMA, give predicted
CPU loads and ensure that if this predicted load is below some limit
then all deadlines will be met. Other (more flexible) analysis methods
focus on response times rather than CPU loads.
--
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
. @ .
prev parent reply other threads:[~2012-06-28 19:20 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-25 9:53 max CPU load recommended Gerd
2012-06-26 22:14 ` Vinzent Hoefler
2012-06-26 23:00 ` Jeffrey Carter
2012-06-28 19:20 ` Niklas Holsti [this message]
replies disabled
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox