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readahead: trigger mmap sequential readahead on PG_readahead 

Previously the mmap sequential readahead is triggered by updating
ra->prev_pos on each page fault and compare it with current page offset.

It costs dirtying the cache line on each _minor_ page fault.  So remove
the ra->prev_pos recording, and instead tag PG_readahead to trigger the
possible sequential readahead.  It's not only more simple, but also will
work more reliably and reduce cache line bouncing on concurrent page
faults on shared struct file.

In the mosbench exim benchmark which does multi-threaded page faults on
shared struct file, the ra->mmap_miss and ra->prev_pos updates are found
to cause excessive cache line bouncing on tmpfs, which actually disabled
readahead totally (shmem_backing_dev_info.ra_pages == 0).

So remove the ra->prev_pos recording, and instead tag PG_readahead to
trigger the possible sequential readahead.  It's not only more simple, but
also will work more reliably on concurrent reads on shared struct file.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Tested-by: Tim Chen <tim.c.chen@intel.com>
Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Wu Fengguang 2011-05-24 17:12:30 -07:00 committed by Linus Torvalds
parent 207d04baa3
commit 2cbea1d3ab
1 changed files with 2 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
mm/filemap.c
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@ -1559,8 +1559,7 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
if (!ra->ra_pages)
return;
if (VM_SequentialReadHint(vma) ||
offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
if (VM_SequentialReadHint(vma)) {
page_cache_sync_readahead(mapping, ra, file, offset,
ra->ra_pages);
return;
@ -1583,7 +1582,7 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
ra_pages = max_sane_readahead(ra->ra_pages);
ra->start = max_t(long, 0, offset - ra_pages / 2);
ra->size = ra_pages;
ra->async_size = 0;
ra->async_size = ra_pages / 4;
ra_submit(ra, mapping, file);
}
@ -1689,7 +1688,6 @@ retry_find:
return VM_FAULT_SIGBUS;
}
ra->prev_pos = (loff_t)offset << PAGE_CACHE_SHIFT;
vmf->page = page;
return ret | VM_FAULT_LOCKED;