[lttng-dev] efficient use of LTTng in legacy codebase

[Mathieu Desnoyers]

Just to speed things up, please have a look at the proposed UST patch. (it's not in mainline yet) 

This is specifically for a demo program example so far. 

Comments are welcome, 

Thanks, 

Mathieu 

----- Original Message -----

> From: "Máté Ferenczy" <mate.ferenczy at ericsson.com>
> To: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> Cc: lttng-dev at lists.lttng.org
> Sent: Wednesday, December 11, 2013 8:52:30 PM
> Subject: RE: efficient use of LTTng in legacy codebase

> It sounds great, I like the idea. Just to make sure I understand your
> suggestion, are we talking about converting the variable argument list to
> string in include/lttng/ust-tracepoint-event-write.h?

> For example:

> #define ctf_format_string(_item, …) \

> do { \

> char _buf[BUF_MAXLEN]; \

> snprintf(buf, sizeof(_buf), __VA_ARGS__); \

> _ctf_string(_item, _buf, 0); \

> } while(0)

> Or do you have some better place to put this logic?

> Thanks,

> Mate

> From: Mathieu Desnoyers [mailto:mathieu.desnoyers at efficios.com]
> Sent: Wednesday, December 11, 2013 4:48 PM
> To: Máté Ferenczy
> Cc: lttng-dev at lists.lttng.org
> Subject: Re: efficient use of LTTng in legacy codebase

> ----- Original Message -----

> > From: "Máté Ferenczy" < mate.ferenczy at ericsson.com >
> 
> > To: "mathieu desnoyers" < mathieu.desnoyers at efficios.com >
> 
> > Cc: lttng-dev at lists.lttng.org
> 
> > Sent: Wednesday, December 11, 2013 6:52:22 PM
> 
> > Subject: RE: efficient use of LTTng in legacy codebase
> 

> > I found this email thread:
> > https://www.mail-archive.com/lttng-dev@lists.lttng.org/msg02278.html
> 

> > According to this I could just check
> > caa_unlikely(__tracepoint_##provider##___##name.state). However, Dave
> > claims
> > that this state will stay true even if the tracepoint is disabled. Is there
> > anything else I should check for?
> 
> I would recommend doing this in the UST probe provider instead. Within the
> TRACEPOINT_EVENT "TP_FIELD" section, when you declare your string field, you
> could parse the format string there in the last argument of a ctf_string()
> (a statement expression). The format string would be received as parameter
> by the tracepoint, along with a variable arg list (...). In the probe, it
> would be turned into a va_list (see stdarg(3)), and the parameters could be
> decoded into a string there. This has the benefit of moving the
> serialization call out-of-line into the probe provider rather than
> clobbering the tracepoint call site.

> Thoughts ?

> Thanks,

> Mathieu

> > Thanks,
> 

> > Mate
> 

> > From: Máté Ferenczy
> 
> > Sent: Wednesday, December 11, 2013 12:17 PM
> 
> > To: 'mathieu.desnoyers at efficios.com'
> 
> > Cc: 'lttng-dev at lists.lttng.org'
> 
> > Subject: efficient use of LTTng in legacy codebase
> 

> > Hello Mathieu,
> 

> > Our team was asked to support LTTng UST solution in a legacy codebase. The
> > existing codebase is fairly large, and the requirement is that we provide
> > all the existing logs and debugs in the system as LTTng traces.
> 

> > The planned solution so far was just to pass everything we have as text
> > (strings) to LTTng. For that we obviously have to convert the printf-like
> > format string and parameter list to a string by calling snprintf before
> > giving the string to the LTTng API. That would however mean, that these
> > snprintfs are getting called every time even if there is no listener/active
> > trace session for the given trace. In this case, all those unnecessary
> > snprintf calls may end up significantly impacting the performance of our
> > applications.
> 

> > In order to work around this, we were thinking that if there was a way to
> > query the LTTng library from our application code whether there is an
> > active
> > tracing session for the given trace level, we could avoid calling those
> > snprintfs in case they are not needed. Do you think it would be feasible to
> > easily get this data from LTTng lib? If yes, can you provide us some
> > pointers where to look at in order to do those changes ourselves?
> 

> > Any suggestions are welcome.
> 

> > Thank you,
> 

> > Mate Ferenczy
> 
> --

> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers 
EfficiOS Inc. 
http://www.efficios.com 
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