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    SubjectRe: [Ksummit-2010-discuss] checkpoint-restart: naked patch


    On 11/05/2010 01:17 PM, Gene Cooperman wrote:
    > On Fri, Nov 05, 2010 at 04:57:33AM -0700, Luck, Tony wrote:
    >>> Oren noted that sometimes it's important to stop the process only
    >>> for a few milliseconds while one checkpoints. In DMTCP, we do that
    >>> by configuring with --enable-forked-checkpointing. This causes us
    >>> to fork a child process taking advantage of copy-on-write and then
    >>> checkpoint the memory pages of the child while the parent continues
    >>> to execute.
    >>
    >> Interesting ... but while the process is only stopped for the duration
    >> of the fork, it may be taking COW faults on almost every page it
    >> touches. I think this will not work well for large HPC applications
    >> that allocate most of physical memory as anonymous pages for the
    >> application. It may even result in an OOM kill if you don't complete
    >> the checkpoint of the child and have it exit in a timely manner.
    >>
    >> -Tony
    >>
    >
    > I agree with you that forked checkpointing is probably not what you
    > want in the middle of an HPC computation. But isn't that part of
    > the nature of COW? Whether the COW is invoked within the kernel,
    > or from outside the kernel via fork --- in either case, when you have
    > mostly dirty pages, you will have to copy most of the pages.
    > Do I understand your point correctly? Thanks,
    > - Gene

    COW is one way of reducing down time (whether through fork or
    in-kernel checkpoint). However, it is possible to avoid using
    it (and thus avoid extra page faults and memory overload) by
    using the page-table "dirty" bit to track dirty pages. This way
    one can "pre-copy" the checkpoint image while the application is
    running, without additional overhead (the idea is similar to how
    live-migration is done).

    Oren.


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