2014-09-30

Timer

When I write asynchronous script, sometimes I want to a timer so that I can invoke some procedure periodically or so. So I've looked at POSIX's timer_create and Windows' CreateWaitableTimer. Then found out both needs some special treatment. For example, POSIX timer_create requires signal handling which is lacking on Sagittarius. (Honestly, I've never properly understood how signal masking works...)

So I've wrote sort of mimic code with thread.
(import (rnrs) (srfi :18))

;; simple timer
(define-record-type ( make-timer timer?)
  (fields (immutable thread timer-thread))
  (protocol (lambda (p)
              (lambda (interval thunk)
                (p (make-thread 
                    (lambda ()
                      (let loop ()
                        (thread-sleep! interval)
                        (thunk)
                        (loop)))))))))

(define (timer-start! timer) (thread-start! (timer-thread timer)) timer)
(define (timer-cancel! timer) (thread-terminate! (timer-thread timer)))

;; use it
(define t (timer-start! (make-timer 2 (lambda () (print "It's time!!")))))

(define (heavy-to-do)
  (thread-sleep! 5)
  (print "It was heavy!"))
(heavy-to-do)
Above prints It's time!! twice then finish heavy-to-do. Now I'm wondering if this is enough or not. Without deep consideration, I've got couple of pros and cons with this implementation.

[Pros]
  • Easy to implement and could be portable.
  • Asynchronous.
[Cons]
  • Could be expensive. (Making thread is not cheap on Sagittarius)
  • Timer can't change parameters which is thread local.
I think above points are more like how we want it to be but it seems better that timer runs the same thread for me. Now, look at both Windows and POSIX timer APIs. Seems both can take callback function. However on POSIX, if I use SIGEV_THREAD then it would create a new thread (it only says "as if" so may not). And not sure if Sagittarius can call a procedure using parent thread's VM without breaking something. So, it's most likely not an option...

Then Windows. SetWaitableTimer can also take a callback function. And according to MSDN, the callback function will be executed on the same thread.
The completion routine will be executed by the same thread that called SetWaitableTimer. This thread must be in an alertable state to execute the completion routine. It accomplishes this by calling the SleepEx function, which is an alertable function.
Using Waitable Timers with an Asynchronous Procedure Call
Now, I'm not sure what's alertable state exactly means. Seems the target thread should be sleeping and if so, sucks...

Hmmmm, it may not an easy thing to do.

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