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:[email protected]]
Sent: Wednesday, December 11, 2013 4:48 PM
To: Máté Ferenczy
Cc: [email protected]
Subject: Re: efficient use of LTTng in legacy codebase
________________________________
From: "Máté Ferenczy"
<[email protected]<mailto:[email protected]>>
To: "mathieu desnoyers"
<[email protected]<mailto:[email protected]>>
Cc: [email protected]<mailto:[email protected]>
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/[email protected]/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: '[email protected]'
Cc: '[email protected]'
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
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