On Wed 18-11-15 10:29:33, Vlastimil Babka wrote:

The previous patch has improved swap accounting for shmem mapping, which
however made /proc/pid/smaps more expensive for shmem mappings, as we consult
the radix tree for each pte_none entry, so the overal complexity is
O(n*log(n)).

We can reduce this significantly for mappings that cannot contain COWed pages,
because then we can either use the statistics tha shmem object itself tracks
(if the mapping contains the whole object, or the swap usage of the whole
object is zero), or use the radix tree iterator, which is much more effective
than repeated find_get_entry() calls.

This patch therefore introduces a function shmem_swap_usage(vma) and makes
/proc/pid/smaps use it when possible. Only for writable private mappings of
shmem objects (i.e. tmpfs files) with the shmem object itself (partially)
swapped outwe have to resort to the find_get_entry() approach. Hopefully
such mappings are relatively uncommon.

To demonstrate the diference, I have measured this on a process that creates
a 2GB mapping and dirties single pages with a stride of 2MB, and time how long
does it take to cat /proc/pid/smaps of this process 100 times.

Private writable mapping of a /dev/shm/file (the most complex case):

real    0m3.831s
user    0m0.180s
sys     0m3.212s

Shared mapping of an almost full mapping of a partially swapped /dev/shm/file
(which needs to employ the radix tree iterator).

real    0m1.351s
user    0m0.096s
sys     0m0.768s

Same, but with /dev/shm/file not swapped (so no radix tree walk needed)

real    0m0.935s
user    0m0.128s
sys     0m0.344s

Private anonymous mapping:

real    0m0.949s
user    0m0.116s
sys     0m0.348s

The cost is now much closer to the private anonymous mapping case, unless the
shmem mapping is private and writable.

Signed-off-by: Vlastimil Babka <redacted>

Looks good to me
Acked-by: Michal Hocko <mhocko@suse.com>
[...]
-- 
Michal Hocko
SUSE Labs

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