[lttng-dev] hi,I have some question about lttng

[Francis Giraldeau]

Le 2012-08-22 09:56, 寇晓晖 a écrit :
> 1.I already use this macro TRACEPOINT_EVENT(I have finished some
> program)in my code,but except these  usage your mentioned before,but
> When i need a new tracepoint() formation,i must define a corresponding
> macro TRACEPOINT_EVENT in header file.I need to change Argument Listing:
> TP_ARGS(...),and Fields Listing TP_FIELDS(...).Compare to Variadic
> Function like printf(...),it's too complicated and inconvenience.Is
> there any reason lttng chose this method?

Yes. First, it's typesafe. The compiler will tell you if arguments do
not match the signature. Also, the macro is used as a way to generates
metadata in the trace that describe the format and length of each
fields. It's less convenient than having varargs, but it's the most
performant and reliable.

> 2.Now we have a clusters environment,So company chose lttng as default
> log system.Is this sentence (while LTTng uses per-CPU buffers and has a
> lockless data structure and will scale perfectly.) your mentioned
> primary cause?If it is ,would you please explain me the detail about
> these.And if not,how lttng improve the performance.I want to know
> why,not comparison results.Thanks again for resolving my questions.

The details are very complicated, but to make a summary, mention that
you have a shared resource (like a buffer or a file) to write into, then
you need to protect it by locks. To avoid that, LTTng uses per-CPU
buffers. Let's say on a two cores system there are at most 2 threads
running concurrently at any time. Then, if they got each their own
buffer, they will not fight because they have their own buffer. The
other major aspect is about RCU data structure used, that is itself
lockless, is capable to allow concurrent read and write access to a data
structure and produces much less cache-line replication and misses than
normal lock. Please look at the LWN article on this for more details.

http://lwn.net/Articles/263130/

> And how did lttng consumerd work?

In a sentence, it's using splice system call to write blocks of memory
to disk.

> Why we need a macro
> TRACEPOINT_CREATE_PROBES in a new .c file.

It's just a way to compile the tracepoint code. It's boilerplate code
and the macro is taking care of generating this code.

Cheers,

Francis

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