Re: mmap vs fs cache

Ric Mason
2013-03-09T03:28:43Z

Hi Johannes,
On 03/09/2013 12:16 AM, 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?
>
> What could happen is that previously used and activated pages do not
> get evicted anymore since there is a constant supply of younger

If a user process exit, its file pages and anonymous pages will be freed
immediately or go through page reclaim?

> 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

Why you emphasize a *significant* amount of active file pages?

> 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.
>
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