trace2: add stopwatch timers

Add stopwatch timer mechanism to Trace2.

Timers are an alternative to Trace2 Regions.  Regions are useful for
measuring the time spent in various computation phases, such as the
time to read the index, time to scan for unstaged files, time to scan
for untracked files, and etc.

However, regions are not appropriate in all places.  For example,
during a checkout, it would be very inefficient to use regions to
measure the total time spent inflating objects from the ODB from
across the entire lifetime of the process; a per-unzip() region would
flood the output and significantly slow the command; and some form of
post-processing would be requried to compute the time spent in unzip().

Timers can be used to measure a series of timer intervals and emit
a single summary event (at thread and/or process exit).

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 90 insertions, 0 deletions

diff --git a/Documentation/technical/api-trace2.txt b/Documentation/technical/api-trace2.txt
index 9d43909d06..75ce6f4560 100644
--- a/Documentation/technical/api-trace2.txt
+++ b/Documentation/technical/api-trace2.txt
@@ -769,6 +769,42 @@ The "value" field may be an integer or a string.
 }
 ------------
 
+`"th_timer"`::
+	This event logs the amount of time that a stopwatch timer was
+	running in the thread.  This event is generated when a thread
+	exits for timers that requested per-thread events.
++
+------------
+{
+	"event":"th_timer",
+	...
+	"category":"my_category",
+	"name":"my_timer",
+	"intervals":5,         # number of time it was started/stopped
+	"t_total":0.052741,    # total time in seconds it was running
+	"t_min":0.010061,      # shortest interval
+	"t_max":0.011648       # longest interval
+}
+------------
+
+`"timer"`::
+	This event logs the amount of time that a stopwatch timer was
+	running aggregated across all threads.  This event is generated
+	when the process exits.
++
+------------
+{
+	"event":"timer",
+	...
+	"category":"my_category",
+	"name":"my_timer",
+	"intervals":5,         # number of time it was started/stopped
+	"t_total":0.052741,    # total time in seconds it was running
+	"t_min":0.010061,      # shortest interval
+	"t_max":0.011648       # longest interval
+}
+------------
+
 == Example Trace2 API Usage
 
 Here is a hypothetical usage of the Trace2 API showing the intended
@@ -1200,6 +1236,60 @@ d0 | main                     | data         | r0  |  0.002126 |  0.002126 | fsy
 d0 | main                     | exit         |     |  0.000470 |           |              | code:0
 d0 | main                     | atexit       |     |  0.000477 |           |              | code:0
 ----------------
+
+Stopwatch Timer Events::
+
+	Measure the time spent in a function call or span of code
+	that might be called from many places within the code
+	throughout the life of the process.
++
+----------------
+static void expensive_function(void)
+{
+	trace2_timer_start(TRACE2_TIMER_ID_TEST1);
+	...
+	sleep_millisec(1000); // Do something expensive
+	...
+	trace2_timer_stop(TRACE2_TIMER_ID_TEST1);
+}
+
+static int ut_100timer(int argc, const char **argv)
+{
+	...
+
+	expensive_function();
+
+	// Do something else 1...
+
+	expensive_function();
+
+	// Do something else 2...
+
+	expensive_function();
+
+	return 0;
+}
+----------------
++
+In this example, we measure the total time spent in
+`expensive_function()` regardless of when it is called
+in the overall flow of the program.
++
+----------------
+$ export GIT_TRACE2_PERF_BRIEF=1
+$ export GIT_TRACE2_PERF=~/log.perf
+$ t/helper/test-tool trace2 100timer 3 1000
+...
+$ cat ~/log.perf
+d0 | main                     | version      |     |           |           |              | ...
+d0 | main                     | start        |     |  0.001453 |           |              | t/helper/test-tool trace2 100timer 3 1000
+d0 | main                     | cmd_name     |     |           |           |              | trace2 (trace2)
+d0 | main                     | exit         |     |  3.003667 |           |              | code:0
+d0 | main                     | timer        |     |           |           | test         | name:test1 intervals:3 total:3.001686 min:1.000254 max:1.000929
+d0 | main                     | atexit       |     |  3.003796 |           |              | code:0
+----------------
+
+
 == Future Work
 
 === Relationship to the Existing Trace Api (api-trace.txt)