From 1dd68d48a8039b4582822d989f97b81e17bcc124 Mon Sep 17 00:00:00 2001 From: Jeff Squyres Date: Wed, 3 Dec 2014 17:07:42 -0800 Subject: [PATCH] MPI_Wtime.3: give further explanation about high-res timers --- ompi/mpi/man/man3/MPI_Wtime.3in | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/ompi/mpi/man/man3/MPI_Wtime.3in b/ompi/mpi/man/man3/MPI_Wtime.3in index 011afbfdf1..b41a6bad04 100644 --- a/ompi/mpi/man/man3/MPI_Wtime.3in +++ b/ompi/mpi/man/man3/MPI_Wtime.3in @@ -1,6 +1,7 @@ .\" -*- nroff -*- .\" Copyright 2006-2008 Sun Microsystems, Inc. .\" Copyright (c) 1996 Thinking Machines Corporation +.\" Copyright (c) 2014 Cisco Systems, Inc. All rights reserved. .\" $COPYRIGHT$ .TH MPI_Wtime 3 "#OMPI_DATE#" "#PACKAGE_VERSION#" "#PACKAGE_NAME#" .SH NAME @@ -65,6 +66,18 @@ output). If this value is "native", a method that is likely to be cheaper than gettimeofday() will be used to obtain the time when MPI_Wtime is invoked. .PP +For example, on platforms that support it, the +.I clock_gettime() +function will be used to obtain a monotonic clock value with whatever +precision is supported on that platform (e.g., nanoseconds). +.PP +Note, too, that the MCA parameter opal_timer_require_monotonic can +influcence this behavior. It defaults to true, but if set to false, +Open MPI may use a finer-grained timing mechanism (e.g., the +RDTSC/RDTSCP clock ticks on x86_64 platforms), but is not guaranteed +to be monotonic in some cases (e.g., if the MPI process is not bound +to a single processor core). +.PP This function does not return an error value. Consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined. .SH SEE ALSO