Re: get-internal-real-time and internal-time-units-per-second

["Mark A. Tapia" <markt@dgp.toronto.edu>]

On Monday 13 DEC, Wong Joo Fung asks
   Right now, I'm a little puzzled about the timing accuracy that I can get from
   MCL.  That is, MCL 2.0 tells me that internal-time-units-per-second has the
   value of 1000 -- and this seems to imply that (get-internal-real-time) has an
   accuracy of up to 1 millisecond.  However, the Mac's time manager is only
   updated 60 times a second (or so) -- meaning that all times are only accurate
   to about 16 milliseconds.

Here's a simple example that shows that the resolution is greater than
1/60 ms (a clock tick).

? (defun time-test (n)
  (loop for i fixnum from 1 to n
              with j = 0
              do (incf j)
              finally (return j)))

? (loop for i from 1 to 6
      initially (gc)
      with j = 10
      do (time (time-test j))
      do (setq j (* j 10)))
? 
Here are the statistics on Mac IIcx with 8mb running Lisp in
a 6mb partition. As you can see, the resolution is creatly improved.
This is probably a reflection of the faster clock speeds
J       Total Time    Multi
                     tasking 
10             52ms   12ms
100             1ms       
1000            6ms       
10000          53ms    1ms
100000        542ms    5ms
1000000      6213ms  781ms 

Here are the relevant disassembled functions for reporting the internal
time.

mark

? (disassemble #'GET-INTERNAL-RUN-TIME)
0 (JSR_SUBPRIM $SP-NOARGS)
4 (MOVE.L (FIXNUM -846) D1)
10 (MOVE.L (FIXNUM -842) D0)
16 (SET_NARGS 2)
18 (ADDQ.W 4 SP)
20 (JMP #'CCL::%TMTASK-TIME)
? 
? (disassemble #'CCL::%TMTASK-TIME)
0 (JSR_SUBPRIM $SP-TWO-ARGS-VPUSH)
4 (ASR.L 3 D1)
6 (MOVE.L (A5 D1.L 0) ATEMP0)
10 (ASR.L 3 D0)
12 (PEA (A5 D0.L 4))
16 (ATRAP #xA059)
18 (MOVE.L (ATEMP0 10) D1)
22 (ATRAP #xA058)
24 (MOVE.L (FIXNUM 125) D0)
30 (ATRAP #xA05A)
32 (MOVE.L (FIXNUM 125000) D0)
38 (ADD.L D1 D0)
40 (SPOP ATEMP0)
42 (MOVE.L SP ATEMP1)
44 (PEA (@W 0))
48 (ADD.L D0 @ATEMP0)
50 (ADDX.L -@ATEMP1 -@ATEMP0)
52 (ADDQ.L 4 SP)
54 (JSR #'CCL::%%DOUBLE-US->MS)
60 (SPOP VSP)
62 (RTS)

? (disassemble #'CCL::%%DOUBLE-US->MS)
0 (MOVEM.L ATEMP0@+ #(D1 D0))
4 (MOVE.L (FIXNUM 125) D3)
10 (DIVU.L D3 (D0 D1))
14 (BVS.S LABEL-24)
16 (MOVE.L D1 D0)
18 (JMP_SUBPRIM $SP-MKULONG)
22 (BRA.S 'CCL::%%DOUBLE-US->MS)
LABEL-24 (MOVE.L D0 D3)
26 (CLR.W D0)
28 (VPUSH D0)
30 (SWAP D3)
32 (CLR.W D3)
34 (VPUSH D3)
36 (MOVE.L D1 D0)
38 (JSR_SUBPRIM $SP-MKULONG)
42 (MOVE.L D0 D1)
44 (VPOP D3)
46 (SWAP D3)
48 (VPOP D0)
50 (MOVE.W D3 D0)
52 (VPUSH D1)
54 (JSR_SUBPRIM $SP-MKULONG)
58 (MOVE.L D0 D1)
60 (MOVE.L (FIXNUM 32) D0)
66 (SET_NARGS 2)
68 (JSR #'ASH)
74 (VPOP D1)
76 (JSR_SUBPRIM $SP-ADD2ACC)
80 (MOVE.L D0 D1)
82 (MOVE.L (FIXNUM 1000) D0)
88 (SET_NARGS 2)