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    SubjectRe: Process migration and load balancing in clusters

    Nathan Bryant wrote:
    > I've always been fascinated with distributed systems (such as Sprite) that
    > can handle transparent process migration and load balancing in a cluster
    > of workstations. The kernel modifications for doing this on Linux are not
    > exactly trivial, of course; it would probably require changing a lot of
    > internal interfaces. But I've been toying with the idea of experimenting
    > with this, either on Linux or some other system, perhaps Mach-based.
    > Anybody given any thought to this sort of thing?
    Actually, the problems of process migration in a homogeneous network are
    very similar to the "transparent checkpoint and resume" issues which
    were discussed a few days ago. Migrating from CPU to CPU in an SMP
    environment shouldn't be hard at all, ditto for suspending and resuming
    across a time period not long enough to cause timeouts and not crossing
    a reboot. Migratable programs under Linux could be done--perhaps even
    at the user level--with the following restrictions:

    * Open files, and the state thereof, must be preserved during the
    suspend/migration; for migration the entire filesystem would probably
    have to be or appear at least largely identical.
    * Network connections are even harder to migrate, as they must be
    either bounced off of some form of masquerading at their original host
    and/or need to run some sort of protocol to tear them down and recreate
    them later after the migration/resume completes.
    * Processes using resources peculiar to the local machine (e.g. doom or
    anything else running with MITSHM; svgalib apps; dosemu; X at all if
    migrating across a slow link) shouldn't migrate.
    * Many things won't be suitable for migration, then, in a distributed
    semi-homogeneous UN*X environment, and many semi-portable apps will need
    to be migratable only at certain points.
    * The way Linux handles shared libraries, restoration of clean code
    pages from disk, etc., leads one to believe that migrating or
    suspend/resuming a process would only entail migrating/saving the dirty
    pages, pcb/vm data, and resource states; if migration merits being done,
    it can be done quickly.


    The priests and the friars/Behold me in dread
    Because I still love you,/My love, and you're dead.
    ---Dead Can Dance, "I Am Stretched On Your Grave", based on King/S.
    O'Connor's rewrite of "The Unquiet Grave", trad. Irish folk.

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