[PATCH] Warning on hugepage-size setting in HOWTO

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 6ac744cd3d436124f8474ff135c7a842fb92136b..6a7f58ed92d8fcc9e7a041e22b8a393c5786c425 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -362,8 +362,34 @@ mem=str            Fio can use various types of memory as the io unit buffer.
                        mmap    Use anonymous memory maps as the buffers.
                                Allocated through mmap(2).
 
                        mmap    Use anonymous memory maps as the buffers.
                                Allocated through mmap(2).
 

+                       mmaphuge Use a memory mapped huge file as the buffer
+                               backing. Append filename after mmaphuge, ala
+                               mem=mmaphuge:/hugetlbfs/file
+

                The area allocated is a function of the maximum allowed
                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. Also see hugepage-size.
+
+               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.
+
+hugepage-size=siint
+               Defines the size of a huge page. Must at least be equal
+               to the system setting, see /proc/meminfo. Defaults to 4MiB.
+               Should probably always be a multiple of megabytes, so using
+               hugepage-size=Xm is the preferred way to set this to avoid
+               setting a non-pow-2 bad value.

 
 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