Add gtod_cpu option for pinning gettimeofday() to a single CPU

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 7bd71a9a961c64c69ba8fec686fa1cb262c45a8c..731684cb9d62c7766155ba050ee66178ea19c428 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -869,6 +869,18 @@ gtod_reduce=bool Enable all of the gettimeofday() reducing options
                we only do about 0.4% of the gtod() calls we would have
                done if all time keeping was enabled.
 
+gtod_cpu=int   Sometimes it's cheaper to dedicate a single thread of
+               execution to just getting the current time. Fio (and
+               databases, for instance) are very intensive on gettimeofday()
+               calls. With this option, you can set one CPU aside for
+               doing nothing but logging current time to a shared memory
+               location. Then the other threads/processes that run IO
+               workloads need only copy that segment, instead of entering
+               the kernel with a gettimeofday() call. The CPU set aside
+               for doing these time calls will be excluded from other
+               uses. Fio will manually clear it from the CPU mask of other
+               jobs.
+
 
 6.0 Interpreting the output
 ---------------------------