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Re: "lttng comparison with other tracers" page


* Frank Ch. Eigler (fche@redhat.com) wrote:
> Hi -
> 
> On Wed, Mar 18, 2009 at 11:14:55AM -0400, Mathieu Desnoyers wrote:
> 
> > [...]  Have you measured the performance impact of disabling
> > interrupts around the probe ?
> 
> Yes.  Among others, we received reports where oprofile was indicating
> more traffic at interrupt-enable time along with somewhat lower
> interrupt-handling-latency-related performance.
> 

Was oprofile triggered by NMIs or by standard interrupts ? If it is the
second option, then it's very normal to have lots of events at
interrupt-enable time and no events at all distributed within the
interrupt disabled section.

Are you sure that the performance regression you got was caused by added
interrupt latency ? How much time can be typically spent in a systemtap
probe (order of magnitude) ?

> > Basically, it sounds like you are shipping a car without brakes nor
> > air bags simply because it makes the car too heavy and therefore too
> > slow. I don't think trading stability and robustness for speed is a
> > good tradeoff.
> 
> We are doing neither - stability and robustness are not at risk by
> this option.  You noticed that the system responded with a clean
> script stop.
> 

I guess we have totally different definitions of "clean" then. Being
unable to report correctly events coming from reentrant execution
contexts is at the very least misleading to the user.

> We're starting to see this happen more now only because the kprobes
> machinery blocked reentrancy in other ways for us, so we never had to
> think hard about what is a good value for the MAXSKIPPED parameter.

I am still not convinced that irqoff is not a good solution. Skipping
events seems like the wrong course of action to me.

Mathieu

> 
> - FChE
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68


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