>> >> > Can I pass UTF16 strings? Do they need to be null-terminated? >> >> >> >> You should convert them to UTF8. >> >> >> >> A ctf_string() needs to be null-terminated. A ctf_sequence_text() is >> >> not required to be null-terminated. See >> >> http://lttng.org/docs/#doc-liblttng-ust-tp-fields >> > >> > This sounds like tracing would then incur a huge runtime overhead, when I >> > need to convert all my UTF-16 strings to UTF-8. How does one deal with >> > that? >> >> Are you concerned about the runtime overhead when tracing is disabled, or >> enabled ? It would be good to gather some metrics on the overhead of this >> conversion. > > I'm mostly concerned about the overhead when tracing is disabled. Ideally, I > would like to see these tracepoints unconditionally compiled into Qt. But if > the overhead is noticeable when they are not in use, I may have a hard time > achieving this goal. Instead, one will then probably need to recompile Qt with > the tracepoints enabled.
As Mathieu wrote, the overhead of disabled LTTng-UST tracepoints is pretty much unnoticeable. You can use the tracepoint_enabled() and do_tracepoint() macros to perform some computation, conversions, and prepare stuff specifically for the payload of the event. > > When in use, the overhead of the tracepoints should also be minimal. > Allocating memory for a temporary UTF-16 to UTF-8 conversion alone has a large > overhead, and then converting the data to UTF-8 also adds on top of that. > Thus, if at all possible, I would like to prevent that. There's no way to support UTF-16 as of the current versions of LTTng and CTF. Even if you find a way to store the string as is, for example using a sequence of bytes, the existing CTF viewers and analyzers won't recognize the field as a string. However we could think about a way to support UTF-16 in the future, and possibly other Unicode encodings too. > > Similarly, I am looking for a way to put URLs into tracepoints, and converting > a QUrl to string data is far from cheap. Yes, I see that this code is executed: <https://github.com/qt/qtbase/blob/dev/src/corelib/io/qurl.cpp#L3279>. You could allocate one tracepoint field for each individual attribute of the QUrl object, for example the scheme, the path, the query string, the fragment, etc. But I guess you'd still have the UTF-16 issue. > > At this point, I don't see a way around only enabling tracepoints optionally. > Independent of the mechanism in use for the actual tracepoints (i.e. lttng-ust > or sdt). The conditional to check whether tracing is enabled may not be too > bad. But then in the conditional it looks like $some code will be required to > massage the data into a form that the tracepoints accept them. This increase > in code size negatively influences code caches and I don't see any way around > that. I leave this part for Mathieu ;-). Phil > >> > Looking at the API documentation, using ctf_array instead with ushort as C >> > type sounds like a better approach. In a custom babeltrace consumer >> > script, I could then convert it to UTF8 offline, if needed. >> > >> > The advantage here is that I have "zero" overhead while tracing, which is >> > crucial for me to get the tracepoints accepted upstream in Qt, without >> > hiding them behind a compiler switch. >> >> As a short term solution this appears to be fine. >> >> In the long run, we're currently designing CTF 2.0, and we're been wondering >> whether we keep it limited to UTF8, or extend it somehow. I'm CCing >> Philippe Proulx who is driving this effort. > > Thanks again! > > Bye > > -- > Milian Wolff | [email protected] | Software Engineer > KDAB (Deutschland) GmbH&Co KG, a KDAB Group company > Tel: +49-30-521325470 > KDAB - The Qt Experts > > _______________________________________________ > lttng-dev mailing list > [email protected] > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > _______________________________________________ lttng-dev mailing list [email protected] https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
