[PATCH] Document how to setup/use huge pages

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/HOWTO b/HOWTO
index f5e2449c22d64956015d983c04c1f93aa8dc9840..996da23a1ce998665d1f9af7d65faaf064eced64 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -367,7 +367,22 @@ mem=str            Fio can use various types of memory as the io unit buffer.
                                mem=mmaphuge:/hugetlbfs/file
 
                The area allocated is a function of the maximum allowed
                                mem=mmaphuge:/hugetlbfs/file
 
                The area allocated is a function of the maximum allowed

-               bs size for the job, multiplied by the io depth given.
+               bs size for the job, multiplied by the io depth given. Note
+               that for shmhuge and mmaphuge to work, the system must have
+               free huge pages allocated. This can normally be checked
+               and set by reading/writing /proc/sys/vm/nr_hugepages on a
+               Linux system. Fio assumes a huge page is 4MiB in size. So
+               to calculate the number of huge pages you need for a given
+               job file, add up the io depth of all jobs (normally one unless
+               iodepth= is used) and multiply by the maximum bs set. Then
+               divide that number by the huge page size. You can see the
+               size of the huge pages in /proc/meminfo. If no huge pages
+               are allocated by having a non-zero number in nr_hugepages,
+               using mmaphuge or shmhuge will fail.
+
+               mmaphuge also needs to have hugetlbfs mounted and the file
+               location should point there. So if it's mounted in /huge,
+               you would use mem=mmaphuge:/huge/somefile.

 
 exitall                When one job finishes, terminate the rest. The default is
                to wait for each job to finish, sometimes that is not the
 
 exitall                When one job finishes, terminate the rest. The default is
                to wait for each job to finish, sometimes that is not the