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All threads involved in an application
write trace points data in the same buffer.
At the same time,
an external process (daemon)
regularly accesses this buffer
to read it and copy data to disk (in a binary file).
To manage the access to the buffer,
the following variables are shared between the different threads
and the daemon:
reserved: indicate space reserved for writting.written: indicate the end of the written area.-
beg_read: index where the daemon starts to read.
-
PAUSE: to synchronise threads and the daemon
when the buffer is emptied.
The following algorithm is used:
trace function called by trace points:
Get the size of the trace point to write
if there is not enough place in the array to write trace data
wait for place
endif
if (PAUSE == TRUE)
wait for the daemon to authorize to continue (PAUSE == FALSE)
endif
index where the writting will begin = reserved
reserve space in the array for the trace point:
reserved = reserved + size of (all data to be writen)
write each data in the space reserved
increment the variable written: the writing is finished
written = written + size of (all written data)
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daemon:
open the binary file
write header information
beg_read = 0
launch the program to traced
while its launched program (child process) is not dead
if "reserved reaches the middle of the array"
PAUSE = 1: block threads to stop new writing
if there are still data to write
wait for the end of the writing (written == reserved)
endif
end_read = written: set the index where the reading will stop
PAUSE = 0: unblock threads. writting can continue
copy data between beg_read and end_read in the binary file
the reading is finished:
beg_read = end_read: set the beginning of the next one
endif
endwhile
if there are still data to write
wait for the end of the writting (written == reserved))
endif
end_read = writen: set the index where the reading will stop
copy data between beg_read and end_read in the binary file
close the binary file
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Data written for a trace point are:
- the trace point ID (one word)
- the timestamp (two words)
- the process pid (one word)
- the thread pid (one word)
- the trace point arguments
- a checksum
EXAMPLE:
This example helps to understand the trace mechanism.
It explains how the collect mechanism runs.
Let's consider this user program
with three trace points.
Each call to the trace mechanism is done by a different thread.
user_function(...) {
...
TRACE (TR_BARRIER_1, &th1, &th1, &bar1, lo1)
...
TRACE (TR_THREAD_2, &th2)
...
TRACE (TR_BARRIER_9, &th3, ret)
...
}
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