[mpich-commits] [mpich] MPICH primary repository branch, master, updated. v3.2b3-196-ga849468

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http://git.mpich.org/mpich.git/commitdiff/a849468b5efbf1e0ea4bd82e1fad86c0d9326a43

commit a849468b5efbf1e0ea4bd82e1fad86c0d9326a43
Author: Halim Amer <aamer at anl.gov>
Date:   Wed Jul 8 18:47:52 2015 -0500

    Advise against using sleeping routines to implement the yield functionality
    
    Details are given in the added comment.
    
    Signed-off-by: Xin Zhao <xinzhao3 at illinois.edu>

diff --git a/src/util/wrappers/mpiu_process_wrappers.h b/src/util/wrappers/mpiu_process_wrappers.h
index 7675c1b..8ec43ee 100644
--- a/src/util/wrappers/mpiu_process_wrappers.h
+++ b/src/util/wrappers/mpiu_process_wrappers.h
@@ -9,6 +9,16 @@
 #include "mpichconf.h"
 
 /* MPIU_PW_SCHED_YIELD() - Yield the processor to OS scheduler */
+/* On a typical Linux system (verified with kernels 3.2 and 3.5),
+ * usleep has a resolution of more than 1000 cycles. This makes
+ * it impractical if the desired sleeping period is shorter. On
+ * the other hand, sleep(0) returns immediately without going to
+ * the kernel. This means that there is no actual yielding, which
+ * is equivalent to doing nothing. Thus, usleep and sleep are not
+ * recommended as ways to yield the CPU, and sched_yield would be
+ * preferred if available.
+ * Note that nanosleep has the same shortcomings as usleep.*/
+
 #if defined(USE_SWITCHTOTHREAD_FOR_YIELD)
     #include <winsock2.h>
     #include <windows.h>

-----------------------------------------------------------------------

Summary of changes:
 src/util/wrappers/mpiu_process_wrappers.h |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)


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