path: root/mm/filemap.c
diff options
authorWu Fengguang <>2009-06-16 15:31:38 -0700
committerLinus Torvalds <>2009-06-16 19:47:30 -0700
commit7ffc59b4d0bdfa00e882339f85b8a969bb7021e2 (patch)
tree6b6d96208f08bc394c8e64efed6767b9a95e7a6d /mm/filemap.c
parent61b7cbdba2f3c588a0cf3db574c562805454b09b (diff)
readahead: enforce full sync mmap readahead size
Now that we do readahead for sequential mmap reads, here is a simple evaluation of the impacts, and one further optimization. It's an NFS-root debian desktop system, readahead size = 60 pages. The numbers are grabbed after a fresh boot into console. approach pgmajfault RA miss ratio mmap IO count avg IO size(pages) A 383 31.6% 383 11 B 225 32.4% 390 11 C 224 32.6% 307 13 case A: mmap sync/async readahead disabled case B: mmap sync/async readahead enabled, with enforced full async readahead size case C: mmap sync/async readahead enabled, with enforced full sync/async readahead size or: A = vanilla 2.6.30-rc1 B = A plus mmap readahead C = B plus this patch The numbers show that - there are good possibilities for random mmap reads to trigger readahead - 'pgmajfault' is reduced by 1/3, due to the _async_ nature of readahead - case C can further reduce IO count by 1/4 - readahead miss ratios are not quite affected The theory is - readahead is _good_ for clustered random reads, and can perform _better_ than readaround because they could be _async_. - async readahead size is guaranteed to be larger than readaround size, and they are _async_, hence will mostly behave better However for B - sync readahead size could be smaller than readaround size, hence may make things worse by produce more smaller IOs which will be fixed by this patch. Final conclusion: - mmap readahead reduced major faults by 1/3 and no obvious overheads; - mmap io can be further reduced by 1/4 with this patch. Signed-off-by: Wu Fengguang <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'mm/filemap.c')
1 files changed, 2 insertions, 1 deletions
diff --git a/mm/filemap.c b/mm/filemap.c
index 2e9bcc2e797..6846a902f5c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1471,7 +1471,8 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
if (VM_SequentialReadHint(vma) ||
offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
- page_cache_sync_readahead(mapping, ra, file, offset, 1);
+ page_cache_sync_readahead(mapping, ra, file, offset,
+ ra->ra_pages);