Long line style fix + comment

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 4298f3245daed2b850cc0bcf3077fad48f8404e5..731684cb9d62c7766155ba050ee66178ea19c428 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -583,11 +583,16 @@ cpumask=int       Set the CPU affinity of this job. The parameter given is a
                the allowed CPUs to be 1 and 5, you would pass the decimal
                value of (1 << 1 | 1 << 5), or 34. See man
                sched_setaffinity(2). This may not work on all supported
-               operating systems or kernel versions.
+               operating systems or kernel versions. This option doesn't
+               work well for a higher CPU count than what you can store in
+               an integer mask, so it can only control cpus 1-32. For
+               boxes with larger CPU counts, use cpus_allowed.
 
 cpus_allowed=str Controls the same options as cpumask, but it allows a text
                setting of the permitted CPUs instead. So to use CPUs 1 and
-               5, you would specify cpus_allowed=1,5.
+               5, you would specify cpus_allowed=1,5. This options also
+               allows a range of CPUs. Say you wanted a binding to CPUs
+               1, 5, and 8-15, you would set cpus_allowed=1,5,8-15.
 
 startdelay=time        Start this job the specified number of seconds after fio
                has started. Only useful if the job file contains several
@@ -864,6 +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
 ---------------------------