cvs commit: src/sys/sys time.h src/sys/kern kern_time.c

[Robert Watson]

rwatson     2005-11-27 00:55:18 UTC

  FreeBSD src repository

  Modified files:
    sys/sys              time.h 
    sys/kern             kern_time.c 
  Log:
  Add several aliases for existing clockid_t names to indicate that the
  application wishes to request high precision time stamps be returned:
  
  Alias                           Existing
  
  CLOCK_REALTIME_PRECISE          CLOCK_REALTIME
  CLOCK_MONOTONIC_PRECISE         CLOCK_MONOTONIC
  CLOCK_UPTIME_PRECISE            CLOCK_UPTIME
  
  Add experimental low-precision clockid_t names corresponding to these
  clocks, but implemented using cached timestamps in kernel rather than
  a full time counter query.  This offers a minimum update rate of 1/HZ,
  but in practice will often be more frequent due to the frequency of
  time stamping in the kernel:
  
  New clockid_t name              Approximates existing clockid_t
  
  CLOCK_REALTIME_FAST             CLOCK_REALTIME
  CLOCK_MONOTONIC_FAST            CLOCK_MONOTONIC
  CLOCK_UPTIME_FAST               CLOCK_UPTIME
  
  Add one additional new clockid_t, CLOCK_SECOND, which returns the
  current second without performing a full time counter query or cache
  lookup overhead to make sure the cached timestamp is stable.  This is
  intended to support very low granularity consumers, such as time(3).
  
  The names, visibility, and implementation of the above are subject
  to change, and will not be MFC'd any time soon.  The goal is to
  expose lower quality time measurement to applications willing to
  sacrifice accuracy in performance critical paths, such as when taking
  time stamps for the purpose of rescheduling select() and poll()
  timeouts.  Future changes might include retrofitting the time counter
  infrastructure to allow the "fast" time query mechanisms to use a
  different time counter, rather than a cached time counter (i.e.,
  TSC).
  
  NOTE: With different underlying time mechanisms exposed, using
  different time query mechanisms in the same application may result in
  relative non-monoticity or the appearance of clock stalling for a
  single clockid_t, as a cached time stamp queried after a precision
  time stamp lookup may be "before" the time returned by the earlier
  live time counter query.
  
  Revision  Changes    Path
  1.125     +26 -2     src/sys/kern/kern_time.c
  1.71      +8 -1      src/sys/sys/time.h