Re: [PATCH v5 4/6] mm, proc: reduce cost of /proc/pid/smaps for unpopulated shmem mappings

[Michal Hocko]

On Wed 18-11-15 10:29:34, Vlastimil Babka wrote:
> Following the previous patch, further reduction of /proc/pid/smaps cost is
> possible for private writable shmem mappings with unpopulated areas where
> the page walk invokes the .pte_hole function. We can use radix tree iterator
> for each such area instead of calling find_get_entry() in a loop. This is
> possible at the extra maintenance cost of introducing another shmem function
> shmem_partial_swap_usage().
> 
> To demonstrate the diference, I have measured this on a process that creates a
> private writable 2GB mapping of a partially swapped out /dev/shm/file (which
> cannot employ the optimizations from the prvious patch) and doesn't populate it
> at all. I time how long does it take to cat /proc/pid/smaps of this process 100
> times.
> 
> Before this patch:
> 
> real    0m3.831s
> user    0m0.180s
> sys     0m3.212s
> 
> After this patch:
> 
> real    0m1.176s
> user    0m0.180s
> sys     0m0.684s
> 
> The time is similar to case where radix tree iterator is employed on the whole
> mapping.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Looks good as well.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
[...]
-- 
Michal Hocko
SUSE Labs
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