ca_pend_event()

[Mark Rivers <RIVERS@cars3.uchicago.edu>]

    Folks,

I just wanted to follow up on Jeff Hill's post regarding my observations on
the time required for ca_pend_event().

There is an error in the R3.12 Channel Access Reference Manual. In the
comments under the description of ca_pend_event() it says that:
    "A very short TIMEOUT such as .0001 seconds will result in a poll 
     (i.e. channel access will just check for file descriptor activity and 
     then return)."

This is incorrect. In fact the threshold is 1 microsecond.  Only TIMEOUT
values below 1 microsecond result in a poll.  TIMEOUTs of 2 microseconds will
often delay ca_pend_event by 1 tick of the system clock.  Measured values of
the time required for ca_pend_event(2.e-6) are 10 milliseconds on Sun4 and
SGI, 1 msec on Alpha, <.4 msec on Solaris.

Jeff has suggested 2 solutions to the problem (in addition to fixing the
documentation):

-  Setting the threshold to (1.0/CLOCKS_PER_SEC). Since CLOCKS_PER_SEC is ansi
   this would be portable.  Note that, for example, this would result in any 
   TIMEOUT less than 10 msec on a Sun4 result in a poll.

-  Making the TIMEOUT value which will result in a poll a symbolic constant.  
   Note that the value of SELECT_POLL, defined in iocinf.h cannot be used,
   since it is defined as 25 msec.

I think that both of the above suggestions are good ones and should be
implemented.

The performance impact of using the correct values for TIMEOUT can be very
large.  A factor of 5 improvement in the performance of 1000 calls to
ezcaGet() was observed when SHORT_TIME in ezca was redefined from 10
microseconds to 1 nanosecond.

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