Messages in this thread | | | Date | Sat, 09 Mar 2013 11:28:35 +0800 | From | Ric Mason <> | Subject | Re: mmap vs fs cache |
| |
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. > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
| |