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<title>Scheduler RPC Timing and Retry Policies</title>
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<body bgcolor=ffffff>
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<h2>Scheduler RPC Timing and Retry Policies</h2>
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<p>
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Each scheduler RPC reports results, gets work, or both.
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The client's <b>scheduler RPC policy</b> has several components:
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when to make a scheduler RPC, which project to contact,
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which scheduling server for
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that project, how much work to ask for, and what to do if the RPC fails.
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<p>
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The scheduler RPC policy has the following goals:
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<ul>
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<li> Make as few scheduler RPCs as possible.
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<li> Use random exponential backoff if a project's scheduling servers are down.
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This avoids an RPC storm when the servers come back up.
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<li> Eventually re-read a project's master URL file in case its set
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of schedulers changes.
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<li> Report results before or soon after their deadlines.
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</ul>
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<h3>Resource debt</h3>
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<p>
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The client maintains an exponentially-averaged sum of the CPU time
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it has devoted to each project.
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The constant EXP_DECAY_RATE determines
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the decay rate (currently a factor of e every week).
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<p>
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Each project is assigned a <b>resource debt</b>, computed as
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<p>
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resource_debt = resource_share / exp_avg_cpu
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<p>
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Resource debt is a measure of how much work the client owes the
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project, and in general the project with the greatest resource debt is
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the one from which work should be requested.
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<h3>Minimum RPC time</h3>
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<p>
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The client maintains a <b>minimum RPC time</b> for each project.
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This is the earliest time at which a scheduling RPC should be done to
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that project (if zero, an RPC can be done immediately).
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The minimum RPC time can be set for various reasons:
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<ul>
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<li> Because of a request from the project, i.e. a
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<request_delay> element in a scheduler reply message.
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<li> Because RPCs to all of the project's scheduler has failed.
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An exponential backoff policy is used.
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<li> Because one of the project's computations has failed (the
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application crashed, or a file upload or download failed).
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An exponential backoff policy is used to prevent a cycle of rapid failures.
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</ul>
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<h3>Scheduler RPC sessions</h3>
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<p>
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Communication with schedulers is organized into <b>sessions</b>,
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each of which may involve many RPCs.
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There are two types of sessions:
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</p>
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<ul>
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<li> <b>Get-work</b> sessions, whose goal is to get a certain amount of work.
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Results may be reported as a side-effect.
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<li>
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<b>Report-result</b> sessions, whose goal is to report results.
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Work may be fetched as a side-effect.
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</ul>
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The internal logic of scheduler sessions is encapsulated in the class
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SCHEDULER_OP.
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This is implemented as a state machine, but its logic
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expressed as a process might look like:
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<pre>
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get_work_session() {
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while estimated work < high water mark
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P = project with greatest debt and min_rpc_time < now
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for each scheduler URL of P
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attempt an RPC to that URL
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if no error break
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if some RPC succeeded
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P.nrpc_failures = 0
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else
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P.nrpc_failures++
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P.min_rpc_time = exponential_backoff(P.min_rpc_failures)
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if P.nrpc_failures mod MASTER_FETCH_PERIOD = 0
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P.fetch_master_flag = true
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for each project P with P.fetch_master_flag set
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read and parse master file
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if error
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P.nrpc_failures++
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P.min_rpc_time = exponential_backoff(P.min_rpc_failures)
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if got any new scheduler urls
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P.nrpc_failures = 0
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P.min_rpc_time = 0
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}
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report_result_session(project P) {
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for each scheduler URL of project
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attempt an RPC to that URL
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if no error break
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if some RPC succeeded
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P.nrpc_failures = 0
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else
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P.nrpc_failures++;
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P.min_rpc_time = exponential_backoff(P.min_rpc_failures)
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}
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</pre>
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The logic for initiating scheduler sessions is expressed in the
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following poll function:
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<pre>
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if a scheduler RPC session is not active
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if estimated work is less than low-water mark
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start a get-work session
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else if some project P has overdue results
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start a report-result session for P;
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is P is the project with greatest resource debt,
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the RPC request should ask for enough work to bring us up
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to the high-water mark
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</pre>
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