Re: mmap vs fs cache

Jan Kara
2013-03-11T12:04:32Z

On Fri 08-03-13 12:04:46, Howard Chu wrote:
> Johannes Weiner wrote:
> >On Fri, Mar 08, 2013 at 07:00:55AM -0800, Howard Chu wrote:
> >>Chris Friesen wrote:
> >>>On 03/08/2013 03:40 AM, Howard Chu wrote:
> >>>
> >>>>There is no way that a process that is accessing only 30GB of a mmap
> >>>>should be able to fill up 32GB of RAM. There's nothing else running on
> >>>>the machine, I've killed or suspended everything else in userland
> >>>>besides a couple shells running top and vmstat. When I manually
> >>>>drop_caches repeatedly, then eventually slapd RSS/SHR grows to 30GB and
> >>>>the physical I/O stops.
> >>>
> >>>Is it possible that the kernel is doing some sort of automatic
> >>>readahead, but it ends up reading pages corresponding to data that isn't
> >>>ever queried and so doesn't get mapped by the application?
> >>
> >>Yes, that's what I was thinking. I added a
> >>posix_madvise(..POSIX_MADV_RANDOM) but that had no effect on the
> >>test.
> >>
> >>First obvious conclusion - kswapd is being too aggressive. When free
> >>memory hits the low watermark, the reclaim shrinks slapd down from
> >>25GB to 18-19GB, while the page cache still contains ~7GB of
> >>unmapped pages. Ideally I'd like a tuning knob so I can say to keep
> >>no more than 2GB of unmapped pages in the cache. (And the desired
> >>effect of that would be to allow user processes to grow to 30GB
> >>total, in this case.)
> >
> >We should find out where the unmapped page cache is coming from if you
> >are only accessing mapped file cache and disabled readahead.
> >
> >How do you arrive at this number of unmapped page cache?
>
> This number is pretty obvious. When slapd has grown to 25GB, the
This 25G is presumably from /proc/pid/statm, right?

> page cache has grown to 32GB (less about 200MB, the minfree). So:
And this value is from where? /proc/meminfo - Cached line?

> 7GB unmapped in the cache.
>
> >What could happen is that previously used and activated pages do not
> >get evicted anymore since there is a constant supply of younger
> >reclaimable cache that is actually thrashing. Whenever you drop the
> >caches, you get rid of those stale active pages and allow the
> >previously thrashing cache to get activated. However, that would
> >require that there is already a significant amount of active file
> >pages before your workload starts (check the nr_active_file number in
> >/proc/vmstat before launching slapd, try sync; echo 3 >drop_caches
> >before launching to eliminate this option) OR that the set of pages
> >accessed during your workload changes and the combined set of pages
> >accessed by your workload is bigger than available memory -- which you
> >claimed would not happen because you only access the 30GB file area on
> >that system.
>
> There are no other active pages before the test begins. There's
> nothing else running. caches have been dropped completely at the
> beginning.
>
> The test clearly is accessing only 30GB of data. Once slapd reaches
> this process size, the test can be stopped and restarted any number
> of times, run for any number of hours continuously, and memory use
> on the system is unchanged, and no pageins occur.
Interesting. It might be worth trying what happens if you do
madvise(..., MADV_DONTNEED) on the data file instead of dropping caches
with /proc/sys/vm/drop_caches. That way we can establish whether the extra
cached data is in the data file (things will look the same way as with
drop_caches) or somewhere else (there will be still unmapped page cache).

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR