Re: 1us latency?

[Clark Williams <williams@redhat.com>]

On Thu, 6 Aug 2015 15:12:59 +0200
Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> * Clark Williams | 2015-08-03 13:53:26 [-0500]:
> 
> >On Mon, 3 Aug 2015 21:36:26 +0300
> >
> >Interesting. Betting that's page faults and cache filling. 
> >
> >I don't think we want to arbitrarily pick some number of cycles for a
> >"settle time" (i.e. a grace period for the application to reach steady
> >state). Possibly we should add an option for that? Specify some number
> >of cycles or some amount of time that where the measurement threads run
> >before actual measurements start?
> >
> >  $ cyclictest --numa -p95 -m --settle=10ms
> >
> >That would say "run the measurement threads for ten milliseconds before
> >actually starting the measurement period". That would allow them to
> >fault in and fill cache lines before starting real work.
> >
> >Anyone else have an opinion?
> 
> Wouldn't you have everything in-memory after once cycle of each thread?

I had to go through the timerthread() routine a couple of times to
convince myself, but I think you're right. 

So if we wanted to discount the paging-in overhead, we could have each
thread do a "dummy" pass through the timer loop (i.e. do everything but
just not record the results) and then start recording measurements. I
may hack together an option to try that and see what sort of results
we get. 

Clark