[systemd-devel] question on special configuration case

[Greg KH]

On Wed, Jun 08, 2016 at 06:43:04AM +0000, Hebenstreit, Michael wrote:
> > Really?  No journal messages are getting created at all?  No users logging in/out?  What does strace show on those processes?
> 
> Yes, messages are created - but I'm not interested in them. Maybe a
> user logs in for a 6h job - that's already tracked by the cluster
> software. There are virtually no demons running, no changes to the
> hardware - so all those demons are doing are looking out for
> themselves. Not really productive

If messages are created, they have to go somewhere, to think that they
would be "free" is crazy :)

> > So you are hurting all 253 cores because you can't spare 1?
> 
> Situation is a bit more complex. I have 64 physical cores, with 4
> units each for integer operations and 2 floating point units. So
> essentially if I reserve one integer unit for the OS, due to cache
> hierarchies and other oddities, I essentially take down 4 cores. The
> applications typically scale best if they run on a power of 2 number
> of cores. 

You can still run the applications on the "non-reserved" core, it's just
that the kernel can't get access to any of the others.  So you only take
the hit of any potential wakeups and other kernel housekeeping on that
one core.

Again, try it, you might be pleasantly surprised as your workload is
_exactly_ what that feature was created for.  To ignore it without
testing seems bizarre to me.  If it doesn't work for you, then either
that kernel feature needs to be fixed, or maybe we can just rip it out,
so you need to tell the kernel developers about it.

> > Again, that's not the issue, you can't see the time the kernel is using to do its work, but it is there (interrupts, scheduling, housekeeping,
> etc.)
> 
> shouldn't that show up in the time for worker threads?

How do you account for interrupts, I/O, scheduler processing time, etc?
:)

> And I'm not arguing you are wrong. We should minimize that and if
> possible keep all OS on an extra core. That does not make my argument
> invalid those demons are doing nothing more than housekeeping
> themselves in a very complicated fashion and they are wasting
> resources. 

Again, I think you are wasting more resources than you realize just
because you can't see it :)

And as others have pointed out, turn off watchdogs and you should be
fine from a systemd point of view.

thanks,

greg k-h