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| -rw-r--r-- | ChangeLog | 5 | ||||
| -rw-r--r-- | manual/llio.texi | 54 |
2 files changed, 54 insertions, 5 deletions
@@ -1,3 +1,8 @@ +2017-11-20 Florian Weimer <fweimer@redhat.com> + + * manual/llio.texi (Memory-mapped I/O): Document MAP_HUGETLB, + MADV_HUGEPAGE, MADV_NOHUGEPAGE. + 2017-11-19 Florian Weimer <fweimer@redhat.com> manual: Document mprotect diff --git a/manual/llio.texi b/manual/llio.texi index 7dd4e06..41c3e06 100644 --- a/manual/llio.texi +++ b/manual/llio.texi @@ -1377,15 +1377,18 @@ available. Memory mapping only works on entire pages of memory. Thus, addresses for mapping must be page-aligned, and length values will be rounded up. -To determine the size of a page the machine uses one should use +To determine the default size of a page the machine uses one should use: @vindex _SC_PAGESIZE @smallexample size_t page_size = (size_t) sysconf (_SC_PAGESIZE); @end smallexample -@noindent -These functions are declared in @file{sys/mman.h}. +On some systems, mappings can use larger page sizes +for certain files, and applications can request larger page sizes for +anonymous mappings as well (see the @code{MAP_HUGETLB} flag below). + +The following functions are declared in @file{sys/mman.h}: @deftypefun {void *} mmap (void *@var{address}, size_t @var{length}, int @var{protect}, int @var{flags}, int @var{filedes}, off_t @var{offset}) @standards{POSIX, sys/mman.h} @@ -1452,6 +1455,29 @@ On some systems using private anonymous mmaps is more efficient than using @code{malloc} for large blocks. This is not an issue with @theglibc{}, as the included @code{malloc} automatically uses @code{mmap} where appropriate. +@item MAP_HUGETLB +@standards{Linux, sys/mman.h} +This requests that the system uses an alternative page size which is +larger than the default page size for the mapping. For some workloads, +increasing the page size for large mappings improves performance because +the system needs to handle far fewer pages. For other workloads which +require frequent transfer of pages between storage or different nodes, +the decreased page granularity may cause performance problems due to the +increased page size and larger transfers. + +In order to create the mapping, the system needs physically contiguous +memory of the size of the increased page size. As a result, +@code{MAP_HUGETLB} mappings are affected by memory fragmentation, and +their creation can fail even if plenty of memory is available in the +system. + +Not all file systems support mappings with an increased page size. + +The @code{MAP_HUGETLB} flag is specific to Linux. + +@c There is a mechanism to select different hugepage sizes; see +@c include/uapi/asm-generic/hugetlb_encode.h in the kernel sources. + @c Linux has some other MAP_ options, which I have not discussed here. @c MAP_DENYWRITE, MAP_EXECUTABLE and MAP_GROWSDOWN don't seem applicable to @c user programs (and I don't understand the last two). MAP_LOCKED does @@ -1468,8 +1494,11 @@ Possible errors include: @item EINVAL -Either @var{address} was unusable, or inconsistent @var{flags} were -given. +Either @var{address} was unusable (because it is not a multiple of the +applicable page size), or inconsistent @var{flags} were given. + +If @code{MAP_HUGETLB} was specified, the file or system does not support +large page sizes. @item EACCES @@ -1670,6 +1699,21 @@ The region is no longer needed. The kernel may free these pages, causing any changes to the pages to be lost, as well as swapped out pages to be discarded. +@item MADV_HUGEPAGE +@standards{Linux, sys/mman.h} +Indicate that it is beneficial to increase the page size for this +mapping. This can improve performance for larger mappings because the +system needs to handle far fewer pages. However, if parts of the +mapping are frequently transferred between storage or different nodes, +performance may suffer because individual transfers can become +substantially larger due to the increased page size. + +This flag is specific to Linux. + +@item MADV_NOHUGEPAGE +Undo the effect of a previous @code{MADV_HUGEPAGE} advice. This flag +is specific to Linux. + @end vtable The POSIX names are slightly different, but with the same meanings: |