System Model (i.e. what to buy and configure)

    We look at three models

        Workstations (zero cost soln)

        Clusters (aka NOW aka COW, aka LAMP, aka Beawolf, aka pool)

        Hybrid--likely "winner"

Workstation model

    Connect workstations in department via LAN

    Includes personal workstations and public ones

    Often have file servers

    The workstations can be diskless

        Tannenbaum seems to like this

        Most users don't

        Not so popular anymore (disks are cheap)

        Maintenence is easy

        Must have some startup code in rom

    If have disk on workstation can use it for

        1. Paging and temp files

        2. 1 + (some) system executables

        3. 2 + file caching

        4. full file system

    Case 1 is often called dataless

        Just as easy to (software) maintain as diskless

        Still need startup code in rom

        Serious reduction in load on network and file servers
    
    Case 2

        Reduces load more and speeds up program start time

        Adds maintenance since new releases of programs must be
        loaded onto the workstations

HOMEWORK 12-7, 12-8

    Case 3

        Can have very few executables permanently on the disk

        Must keep the caches consistent

            Not trivial for data files with multiple writers

                This issue comes up for NFS as well and is discussed
                (much) later

            Should you cache whole files or blocks?

    Case 4

        You can work if just your machine is up

        Lose location transparancy

        Most maintenance

    Using Idle workstations

        Early systems did this manually via rsh

            Still used today

        Newer systems like Condor (Univ Wisc ?) try to automate this

            How find idle workstations?

                Idle = no mouse or keybd activity and low load avg

                Workstation can announce it is idle and this is
                recorded by all

                Job looking for machine can inquire

                Must worry about race conditions

HOMEWORK 12-10

                Some jobs want a bunch of machines so look for many
                idle machines

                Can also have centralized soln, processor server

                    Usual tradeoffs
            
            What about local environment?

                Files on servers are no problem

                Requests for local files must be sent home

                    ... but not needed for tmp files

                Syscalls for mem or proc mgt probably need to be
                executed on the remote machine

                Time is a bit of a mess unless have time synchronized
                by a system like ntp

                If program is interactive, must deal with devices

                    mouse, keybd, display

HOMEWORK 12-11

            What if machine becomes non-idle (i.e. owner returns)?

                Detect presence of user.

                Kill off the guest processes.

                    Helpful if made checkpoints (or ran short jobs)

                Erase files, etc.

                Some NYU research unifies this entire evacuation
                procedure with other failures (transaction oriented).

                Could try to migrate the guest processes to other
                hosts but this must be very fast or the owner will
                object (at least I would).

                Goal is to make owner not be aware of your presence.

                    May not be possible since you may have paged out
                    his basic environment (shell, editor, X server,
                    window manager) that s/he left running when s/he
                    stopped using the machine.