On 06/06/2012 08:39 PM, Robin Dong wrote:
> 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_add_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 will be moved from lru_add_pvecs to active_list
^^^^
inactive list
> (by __lru_cache_add() ) in first write_begin(), now page 14th *is* PageLRU(page) and after
> second write_end() it 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
> kswaped works, shrinks the inactive_list, the file only have 14th, 28th...pages in memory,
^^^^^
kswapd
> the readahead request size will be broken to only 52k (13*4k), system's performance falls
> dramatically.
Good catch!
>
> 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>
Nitpick:
Please comment in function as well as description.
It will prevent some guy restore original code for the consistency with other pagevec_add call sites.
Thanks.
--
Kind regards,
Minchan Kim
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