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Forum Topic - cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue): (11 Items)

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Claudio Fontana(deleted)

05/17/2018 2:46 AM

post118821

cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

Hi all,

is it possible at all to synchronize events captured with TraceEvent across cpus?

The issue here is that in the trace buffers the events coming from different cpus
have a timestamp that is apparently generated with ClockCycles(), whose counters are not synchronized.

How can I properly calculate time intervals in this situation?

Thank you,

Claudio

Elad Lahav

05/17/2018 1:12 PM

post118823

Re: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

ClockCycles() is assumed to be synchronized across cores (in theory it
doesn't have to be, but there are various things that would break if
that assumption doesn't hold).
What board and BSP are you using?

--Elad

On Thu, 2018-05-17 at 02:46 -0400, Claudio Fontana wrote:
> Hi all,
> 
> is it possible at all to synchronize events captured with TraceEvent
> across cpus?
> 
> The issue here is that in the trace buffers the events coming from
> different cpus
> have a timestamp that is apparently generated with ClockCycles(),
> whose counters are not synchronized.
> 
> How can I properly calculate time intervals in this situation?
> 
> Thank you,
> 
> Claudio
> 
> 
> 
> _______________________________________________
> 
> OSTech
> http://community.qnx.com/sf/go/post118821
> To cancel your subscription to this discussion, please e-mail ostech-
> core_os-unsubscribe@community.qnx.com

Claudio Fontana(deleted)

05/18/2018 2:28 AM

post118826

Re: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

> ClockCycles() is assumed to be synchronized across cores (in theory it
> doesn't have to be, but there are various things that would break if
> that assumption doesn't hold).
> What board and BSP are you using?
> 
> --Elad

Hello Elad,

thank you for your answer,
I assumed ClockCycles counters were not synchronized between cores due to the documentation at:

http://www.qnx.com/developers/docs/7.0.0/#com.qnx.doc.neutrino.lib_ref/topic/c/clockcycles.html

But if they are always synchronized that solves my issue..

I am using an AArch64-based board (Renesas Car-M3 Starter Kit Pro) with the QNX bsp package from the Software Center.

Elad Lahav

05/18/2018 9:51 AM

post118827

Re: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

As odd as it may sound, both the documentation and what I said are
correct ;-)
In *principle* it is true that the ClockCycle() callout (implemented
for each BSP separately) does not have to return a synchronized value.
In *practice* all aarch64 BSPs I know of will use a single clock
source. x86_64 BSPs use the rdtsc call which, again, is not guaranteed
to be synchronized in principle but seems to be in practice (at least
on the boards we tested, and that's quite a few).
There are certain features in 7.0 (specifically HRTs) that will not
function properly if ClockCycles() does not return a synchronized
value.

The bottom line is that you should not observe a problem with combining
traces from different cores in any of the official BSPs. If you do,
please let me know.

--Elad

On Fri, 2018-05-18 at 02:28 -0400, Claudio Fontana wrote:
> > 
> > ClockCycles() is assumed to be synchronized across cores (in theory
> > it
> > doesn't have to be, but there are various things that would break
> > if
> > that assumption doesn't hold).
> > What board and BSP are you using?
> > 
> > --Elad
> Hello Elad,
> 
> thank you for your answer,
> I assumed ClockCycles counters were not synchronized between cores
> due to the documentation at:
> 
> http://www.qnx.com/developers/docs/7.0.0/#com.qnx.doc.neutrino.lib_re
> f/topic/c/clockcycles.html
> 
> But if they are always synchronized that solves my issue..
> 
> I am using an AArch64-based board (Renesas Car-M3 Starter Kit Pro)
> with the QNX bsp package from the Software Center.
> 
> 
> 
> 
> 
> 
> 
> _______________________________________________
> 
> OSTech
> http://community.qnx.com/sf/go/post118826
> To cancel your subscription to this discussion, please e-mail ostech-
> core_os-unsubscribe@community.qnx.com

Claudio Fontana(deleted)

Re: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

Claudio Fontana(deleted)

05/18/2018 9:59 AM

post118828

Re: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

thank you very much for your clarification, super-helpful!

Ciao,

Claudio

John Kearney

RE: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

John Kearney

05/18/2018 10:01 AM

post118829

RE: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

If you want to be on the safe side you can verify this in the syspage.
https://www.qnx.com/developers/docs/7.0.0/com.qnx.doc.neutrino.building/topic/syspage/qtime.html

qtime->flags&QTIME_FLAG_GLOBAL_CLOCKCYCLES

cheers
________________________________________
From: Elad Lahav [community-noreply@qnx.com]
Sent: Friday, May 18, 2018 3:33 PM
To: ostech-core_os@community.qnx.com
Subject: Re: cross-cpu synchronization of events captured with TraceEvent (ClockCycles issue)

As odd as it may sound, both the documentation and what I said are
correct ;-)
In *principle* it is true that the ClockCycle() callout (implemented
for each BSP separately) does not have to return a synchronized value.
In *practice* all aarch64 BSPs I know of will use a single clock
source. x86_64 BSPs use the rdtsc call which, again, is not guaranteed
to be synchronized in principle but seems to be in practice (at least
on the boards we tested, and that's quite a few).
There are certain features in 7.0 (specifically HRTs) that will not
function properly if ClockCycles() does not return a synchronized
value.

The bottom line is that you should not observe a problem with combining
traces from different cores in any of the official BSPs. If you do,
please let me know.

--Elad

On Fri, 2018-05-18 at 02:28 -0400, Claudio Fontana wrote:
> >
> > ClockCycles() is assumed to be synchronized across cores (in theory
> > it
> > doesn't have to be, but there are various things that would break
> > if
> > that assumption doesn't hold).
> > What board and BSP are you using?
> >
> > --Elad
> Hello Elad,
>
> thank you for your answer,
> I assumed ClockCycles counters were not synchronized between cores
> due to the documentation at:
>
> http://www.qnx.com/developers/docs/7.0.0/#com.qnx.doc.neutrino.lib_re
> f/topic/c/clockcycles.html
>
> But if they are always synchronized that solves my issue..
>
> I am using an AArch64-based board (Renesas Car-M3 Starter Kit Pro)
> with the QNX bsp package from the Software Center.
>
>
>
>
>
>
>
> _______________________________________________
>
> OSTech
> http://community.qnx.com/sf/go/post118826
> To cancel your subscription to this discussion, please e-mail ostech-
> core_os-unsubscribe@community.qnx.com

_______________________________________________

OSTech
http://community.qnx.com/sf/go/post118827
To cancel your subscription to this discussion, please e-mail ostech-core_os-unsubscribe@community.qnx.com

Claudio Fontana(deleted)

05/25/2018 2:10 AM

post118844