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    SubjectRe: Problem in Page Cache Replacement
    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

    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?

    Regards,
    Jaegeuk

    >
    > 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



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