Re: Problem in Page Cache Replacement

Fengguang Wu
2012-11-22T18:53:09Z

Hi Jaegeuk,

Sorry for the delay. I'm traveling these days..

On Wed, Nov 21, 2012 at 05:42:33PM +0800, Jaegeuk Hanse wrote:
> On 11/21/2012 05:02 PM, Fengguang Wu wrote:
> >On Wed, Nov 21, 2012 at 04:34:40PM +0800, Jaegeuk Hanse wrote:
> >>Cc Fengguang Wu.
> >>
> >>On 11/21/2012 04:13 PM, metin d wrote:
> >>>> Curious. Added linux-mm list to CC to catch more attention. If you run
> >>>>echo 1 >/proc/sys/vm/drop_caches does it evict data-1 pages from memory?
> >>>I'm guessing it'd evict the entries, but am wondering if we could run any more diagnostics before trying this.
> >>>
> >>>We regularly use a setup where we have two databases; one gets used frequently and the other one about once a month. It seems like the memory manager keeps unused pages in memory at the expense of frequently used database's performance.
> >>>My understanding was that under memory pressure from heavily
> >>>accessed pages, unused pages would eventually get evicted. Is there
> >>>anything else we can try on this host to understand why this is
> >>>happening?
> >We may debug it this way.
> >
> >1) run 'fadvise data-2 0 0 dontneed' to drop data-2 cached pages
> > (please double check via /proc/vmstat whether it does the expected work)
> >
> >2) run 'page-types -r' with root, to view the page status for the
> > remaining pages of data-1
> >
> >The fadvise tool comes from Andrew Morton's ext3-tools. (source code attached)
> >Please compile them with options "-Dlinux -I. -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE"
> >
> >page-types can be found in the kernel source tree tools/vm/page-types.c
> >
> >Sorry that sounds a bit twisted.. I do have a patch to directly dump
> >page cache status of a user specified file, however it's not
> >upstreamed yet.
>
> Hi Fengguang,
>
> Thanks for you detail steps, I think metin can have a try.
>
> flags page-count MB symbolic-flags long-symbolic-flags
> 0x0000000000000000 607699 2373
> ___________________________________
> 0x0000000100000000 343227 1340
> _______________________r___________ reserved

We don't need to care about the above two pages states actually.
Page cache pages will never be in the special reserved or
all-flags-cleared state.

> But I have some questions of the print of page-type:
>
> Is 2373MB here mean total memory in used include page cache? I don't
> think so.
> Which kind of pages will be marked reserved?
> Which line of long-symbolic-flags is for page cache?

The (lru && !anonymous) pages are page cache pages.

Thanks,
Fengguang

> >>>On Tue 20-11-12 09:42:42, metin d wrote:
> >>>>I have two PostgreSQL databases named data-1 and data-2 that sit on the
> >>>>same machine. Both databases keep 40 GB of data, and the total memory
> >>>>available on the machine is 68GB.
> >>>>
> >>>>I started data-1 and data-2, and ran several queries to go over all their
> >>>>data. Then, I shut down data-1 and kept issuing queries against data-2.
> >>>>For some reason, the OS still holds on to large parts of data-1's pages
> >>>>in its page cache, and reserves about 35 GB of RAM to data-2's files. As
> >>>>a result, my queries on data-2 keep hitting disk.
> >>>>
> >>>>I'm checking page cache usage with fincore. When I run a table scan query
> >>>>against data-2, I see that data-2's pages get evicted and put back into
> >>>>the cache in a round-robin manner. Nothing happens to data-1's pages,
> >>>>although they haven't been touched for days.
> >>>>
> >>>>Does anybody know why data-1's pages aren't evicted from the page cache?
> >>>>I'm open to all kind of suggestions you think it might relate to problem.
> >>> Curious. Added linux-mm list to CC to catch more attention. If you run
> >>>echo 1 >/proc/sys/vm/drop_caches
> >>> does it evict data-1 pages from memory?
> >>>
> >>>>This is an EC2 m2.4xlarge instance on Amazon with 68 GB of RAM and no
> >>>>swap space. The kernel version is:
> >>>>
> >>>>$ uname -r
> >>>>3.2.28-45.62.amzn1.x86_64
> >>>>Edit:
> >>>>
> >>>>and it seems that I use one NUMA instance, if you think that it can a problem.
> >>>>
> >>>>$ numactl --hardware
> >>>>available: 1 nodes (0)
> >>>>node 0 cpus: 0 1 2 3 4 5 6 7
> >>>>node 0 size: 70007 MB
> >>>>node 0 free: 360 MB
> >>>>node distances:
> >>>>node 0
> >>>> 0: 10