On Tue, 10 Aug 2010 14:37:22 +0200
casper....@sun.com wrote:
> But if you call time() in a loop, then even the best performance
> won't change the runtime: it will just call time() more times.
>
> Casper
Well, quite often you need to call time function in a loop. For
example, you have a loop that accepts network packets, or connections
and every so often you need to have current system time. The resolution
of the timer can be specific to application, i.e. it could be seconds,
milliseconds, or microseconds.a
uint64_t current_time;
while(1)
{
get_network_data();
current_time = update_current_time();
...
use_current_time();
}
So in a tight loop, if update_current_time() calls clock_gettime() on
every loop iteration, what would kill performance.
POSIX.4 provides interval timers, but I think they can be problematic:
1. There is a limit on the total number of timers a process can have,
that is TIMER_MAX in <limits.h> and this is usually 32. What if you
need more than 32 timers?
2. Timer notification is sent via Unix signals, which sucks if your
process is multi-threaded. Also, signal delivery is not particularly
efficient.
My solution was to develop a small Interval Time Sampling package,
below are the prototypes:
#ifndef _its_h
#define _its_h
#include <time.h>
#include <stdint.h>
#include <inttypes.h>
typedef struct its *its_t;
int32_t its_init(its_t *its, clockid_t clock_id, uint32_t interval);
void its_destroy(its_t *its);
uint64_t its_sample(its_t its);
#endif /* _its_h */
When calling its_init(), interval specifies at what intervals to call
clock_gettime(). No matter how often you call its_sample(), it will
call clock_gettime() at the specified intervals. It does it by checking
internal 64-bit counters, if the counter reaches a threshold, it calls
clock_gettime(), otherwise it increments the counter. If the counter
gets too high, it decrements it. This way it automatically adjusts to
the frequency of loop iterations.
Each thread has its own its_t object, so even if it's spinning in a
tight loop, most of the time the cost of its_sample() call is a 64-bit
increment instruction on a local variable.
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