From: Robin Dong <sanbai@xxxxxxxxxx>
When writing a new file with 2048 bytes buffer, such as write(fd, buffer, 2048), it will
call generic_perform_write() twice for every page:
write_begin
mark_page_accessed(page)
write_end
write_begin
mark_page_accessed(page)
write_end
The page 1~13th will be added to lru-pvecs in write_begin() and will *NOT* be added to
active_list even they have be accessed twice because they are not PageLRU(page).
But when page 14th comes, all pages in lru-pvecs will be moved to inactive_list
(by __lru_cache_add() ) in first write_begin(), now page 14th *is* PageLRU(page).
And after second write_end() only page 14th will be in active_list.
In Hadoop environment, we do comes to this situation: after writing a file, we find
out that only 14th, 28th, 42th... page are in active_list and others in inactive_list. Now
kswapd works, shrinks the inactive_list, the file only have 14th, 28th...pages in memory,
the readahead request size will be broken to only 52k (13*4k), system's performance falls
dramatically.
This problem can also replay by below steps (the machine has 8G memory):
1. dd if=/dev/zero of=/test/file.out bs=1024 count=1048576
2. cat another 7.5G file to /dev/null
3. vmtouch -m 1G -v /test/file.out, it will show:
/test/file.out
[oooooooooooooooooooOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO] 187847/262144
the 'o' means same pages are in memory but same are not.
The solution for this problem is simple: the 14th page should be added to lru_add_pvecs
before mark_page_accessed() just as other pages.
Signed-off-by: Robin Dong <sanbai@xxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>
Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
mm/swap.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/mm/swap.c b/mm/swap.c
index 4e7e2ec..bf03903 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -394,13 +394,22 @@ void mark_page_accessed(struct page *page)
}
EXPORT_SYMBOL(mark_page_accessed);
+/*
+ * Order of operations is important: flush the pagevec when it's already
+ * full, not when adding the last page, to make sure that last page is
+ * not added to the LRU directly when passed to this function. Because
+ * mark_page_accessed() (called after this when writing) only activates
+ * pages that are on the LRU, linear writes in subpage chunks would see
+ * every PAGEVEC_SIZE page activated, which is unexpected.
+ */
void __lru_cache_add(struct page *page, enum lru_list lru)
{
struct pagevec *pvec = &get_cpu_var(lru_add_pvecs)[lru];
page_cache_get(page);
- if (!pagevec_add(pvec, page))
+ if (!pagevec_space(pvec))
__pagevec_lru_add(pvec, lru);
+ pagevec_add(pvec, page);
put_cpu_var(lru_add_pvecs);
}
EXPORT_SYMBOL(__lru_cache_add);
--
1.7.9.5
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