On May 22, 2018, at 5:15 AM, Jonathan Taylor <jonathan.taylor@...> wrote:

Hi all,


I’ve been trying to figure out what happened on an overnight run of the code that drives our scientific experiment, and I’m seeing logs that imply that a timer has been firing much more rapidly than intended. I’m hoping somebody can help me figure out why, and what I should be changing.

The calls that set up the timer can be condensed down to:

timerSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, DISPATCH_TIMER_STRICT, dispatch_get_main_queue());
dispatch_source_set_event_handler(timerSource, myHandler);
dispatch_source_set_timer(timerSource, dispatch_time(DISPATCH_TIME_NOW, repeatIntervalNs), repeatIntervalNs, flexibilityNs);

where repeatIntervalNs is 4*NSEC_PER_SEC and flexibilityNs is 2*NSEC_PER_SEC. The CPU is under fairly high load from multithreaded code all night, but normally the timer fires on average every 4 seconds. Note that I have specified DISPATCH_TIMER_STRICT in a perhaps-misguided attempt to protect against the fact that occasionally, under unusually high load I presume, the timer was not firing for 30 seconds or more at a time. However, the problem that I am seeing in practice is that (in this overnight run, at least) the timer was firing at a rate of up to 200 times per second! All I am trying to achieve with this periodic timer is to check in with the secondary process and provide it with a status update.

The perhaps-unusual thing here is that the timer callback makes a function call via an NSConnection to a secondary process. Sometimes that secondary process is busy (for seconds at a time) and therefore the timer callback blocks. My theory is that what is happening is that the OS is building up a “backlog” of timer callbacks that it wants to make - many more, indeed, than than it would make if it really was firing every ~4secs.

None of this is a huge problem (except that it’s led to an enormous log file!), but it makes me worry that I am going about all of this the wrong way. What do people think? Is this a terrible way to achieve what I want to achieve, for some reason? Does it seem plausible that the number of timer callbacks would be disproportionately high like this? Any suggestions most welcome!

Cheers
Jonny