Add gtod_reduce option

This cuts down even more on gtod() calls. While the three disable_X
options halved the gettimeofday() call count, this can reduce it to
less than 1 percent of what it otherwise would have been.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 1d1aa61..f4efd28 100644
--- a/HOWTO
+++ b/HOWTO
@@ -848,6 +848,13 @@
 disable_bw=bool	Disable measurements of throughput/bandwidth numbers. See
 		disable_clat.
 
+gtod_reduce=bool Enable all of the gettimeofday() reducing options
+		(disable_clat, disable_slat, disable_bw) plus reduce
+		precision of the timeout somewhat to really shrink
+		the gettimeofday() call count. With this option enabled,
+		we only do about 0.4% of the gtod() calls we would have
+		done if all time keeping was enabled.
+
 
 6.0 Interpreting the output
 ---------------------------