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+#ifndef TR2_TMR_H
+#define TR2_TMR_H
+
+#include "trace2.h"
+#include "trace2/tr2_tgt.h"
+
+/*
+ * Define a mechanism to allow "stopwatch" timers.
+ *
+ * Timers can be used to measure "interesting" activity that does not
+ * fit the "region" model, such as code called from many different
+ * regions (like zlib) and/or where data for individual calls are not
+ * interesting or are too numerous to be efficiently logged.
+ *
+ * Timer values are accumulated during program execution and emitted
+ * to the Trace2 logs at program exit.
+ *
+ * To make this model efficient, we define a compile-time fixed set of
+ * timers and timer ids using a "timer block" array in thread-local
+ * storage. This gives us constant time access to each timer within
+ * each thread, since we want start/stop operations to be as fast as
+ * possible. This lets us avoid the complexities of dynamically
+ * allocating a timer on the first use by a thread and/or possibly
+ * sharing that timer definition with other concurrent threads.
+ * However, this does require that we define time the set of timers at
+ * compile time.
+ *
+ * Each thread uses the timer block in its thread-local storage to
+ * compute partial sums for each timer (without locking). When a
+ * thread exits, those partial sums are (under lock) added to the
+ * global final sum.
+ *
+ * Using this "timer block" model costs ~48 bytes per timer per thread
+ * (we have about six uint64 fields per timer). This does increase
+ * the size of the thread-local storage block, but it is allocated (at
+ * thread create time) and not on the thread stack, so I'm not worried
+ * about the size.
+ *
+ * Partial sums for each timer are optionally emitted when a thread
+ * exits.
+ *
+ * Final sums for each timer are emitted between the "exit" and
+ * "atexit" events.
+ *
+ * A parallel "timer metadata" table contains the "category" and "name"
+ * fields for each timer. This eliminates the need to include those
+ * args in the various timer APIs.
+ */
+
+/*
+ * The definition of an individual timer and used by an individual
+ * thread.
+ */
+struct tr2_timer {
+ /*
+ * Total elapsed time for this timer in this thread in nanoseconds.
+ */
+ uint64_t total_ns;
+
+ /*
+ * The maximum and minimum interval values observed for this
+ * timer in this thread.
+ */
+ uint64_t min_ns;
+ uint64_t max_ns;
+
+ /*
+ * The value of the clock when this timer was started in this
+ * thread. (Undefined when the timer is not active in this
+ * thread.)
+ */
+ uint64_t start_ns;
+
+ /*
+ * Number of times that this timer has been started and stopped
+ * in this thread. (Recursive starts are ignored.)
+ */
+ uint64_t interval_count;
+
+ /*
+ * Number of nested starts on the stack in this thread. (We
+ * ignore recursive starts and use this to track the recursive
+ * calls.)
+ */
+ unsigned int recursion_count;
+};
+
+/*
+ * Metadata for a timer.
+ */
+struct tr2_timer_metadata {
+ const char *category;
+ const char *name;
+
+ /*
+ * True if we should emit per-thread events for this timer
+ * when individual threads exit.
+ */
+ unsigned int want_per_thread_events:1;
+};
+
+/*
+ * A compile-time fixed-size block of timers to insert into
+ * thread-local storage. This wrapper is used to avoid quirks
+ * of C and the usual need to pass an array size argument.
+ */
+struct tr2_timer_block {
+ struct tr2_timer timer[TRACE2_NUMBER_OF_TIMERS];
+};
+
+/*
+ * Private routines used by trace2.c to actually start/stop an
+ * individual timer in the current thread.
+ */
+void tr2_start_timer(enum trace2_timer_id tid);
+void tr2_stop_timer(enum trace2_timer_id tid);
+
+/*
+ * Add the current thread's timer data to the global totals.
+ * This is called during thread-exit.
+ *
+ * Caller must be holding the tr2tls_mutex.
+ */
+void tr2_update_final_timers(void);
+
+/*
+ * Emit per-thread timer data for the current thread.
+ * This is called during thread-exit.
+ */
+void tr2_emit_per_thread_timers(tr2_tgt_evt_timer_t *fn_apply);
+
+/*
+ * Emit global total timer values.
+ * This is called during atexit handling.
+ *
+ * Caller must be holding the tr2tls_mutex.
+ */
+void tr2_emit_final_timers(tr2_tgt_evt_timer_t *fn_apply);
+
+#endif /* TR2_TMR_H */