On Oct 13, 2010, at 4:52 PM, Michael E. Thomadakis wrote:
> Does MPI_Wtime of OMPI 1.4.3/1.5.0 rely on high resolution clocks (for
> Linux) or does still rely on gettimeofday() ? How would one request at OMPI
> built time to let it use high resolution clocks?
Check the man page for MPI_Wtime(3):
On POSIX platforms, this function may utilize a timer that is cheaper
to invoke than the gettimeofday() system call, but will fall back to
gettimeofday() if a cheap high-resolution timer is not available. The
ompi_info command can be consulted to see if Open MPI supports a native
high-resolution timer on your platform; see the value for "MPI_WTIME
support" (or "options:mpi-wtime" when viewing the parsable output). If
this value is "native", a method that is likely to be cheaper than get-
timeofday() will be used to obtain the time when MPI_Wtime is invoked.
IIRC, the problem on Linux is that the native x86 timers are a per-chip value
(e.g., on a multi-socket system, the value is different on each socket).
Hence, if you MPI_Wtime() while your MPI process is on socket A, but then the
OS moves it to socket B and you call MPI_Wtime() again, the two values are
completely unrelated.
That being said, I see clock_gettime() has CLOCK_REALTIME, which is supposed to
be system-wide. Is it cheaper and/or more accurate than gettimeofday? I see
that it has nanosecond precision in the man page, but what is it (typically)
actually implemented as?
Additionally, I suppose that if a process is bound to a single native clock
scope (I don't know offhand if it's per socket or per core -- I said "socket"
up above, but that was a guess), we could make the *assumption* that the
process will never move and might be able to use the native x86 timer (there's
some complications here, but we might be able to figure it out). Bad Things
could happen if the process ever moved, though (e.g., if the application ever
manually changed the binding).
--
Jeff Squyres
jsquy...@cisco.com
For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/
