mirror of https://github.com/BOINC/boinc.git
132 lines
4.9 KiB
PHP
132 lines
4.9 KiB
PHP
<?
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require_once("docutil.php");
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page_head("Redundancy and errors");
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echo "
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A BOINC 'result' abstracts an instance of a computation,
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possibly not performed yet.
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Typically, a BOINC server sends 'results' to clients,
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and the clients perform the computation and replies to the server.
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But many things can happen to a result:
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<ul>
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<li> The client computes the result correctly and returns it.
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<li> The client computes the result incorrectly and returns it.
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<li> The client fails to download or upload files.
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<li> The application crashes on the client.
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<li> The client never returns anything because it breaks
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or stops running BOINC.
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<li> The scheduler isn't able to send the result because it
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requires more resources than any client has.
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</ul>
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<p>
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BOINC provides a form of redundant computing
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in which each computation is performed on multiple clients,
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the results are compared,
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and are accepted only when a 'consensus' is reached.
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In some cases new results must be created and sent.
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<p>
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BOINC manages most of the details;
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however, there are two places where the application developer gets involved:
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<ul>
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<li> <b>Validation:</b>
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This performs two functions.
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First, when a sufficient number (a 'quorum') of successful results
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have been returned, it compares them and sees if there is a 'consensus'.
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The method of comparing results (which may need to take into
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account platform-varying floating point arithmetic)
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and the policy for determining consensus (e.g., best two out of three)
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are supplied by the application.
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If a consensus is reached, a particular result is designated
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as the 'canonical' result.
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Second, if a result arrives after a consensus has already been reached,
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the new result is compared with the canonical result;
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this determines whether the user gets credit.
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<li> <b>Assimilation:</b>
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This is the mechanism by which the project is notifed of the completion
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(success or unsuccessful) of a work unit.
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It is performed exactly once per work unit.
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If the work unit was completed successfully
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(i.e. if there is a canonical result)
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the project-supplied function reads the output file(s)
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and handles the information, e.g. by recording it in a database.
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If the workunit failed,
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the function might write an entry in a log,
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send an email, etc.
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</ul>
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<hr>
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In the following example,
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the project creates a workunit with
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<br>
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min_quorum = 2
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<br>
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target_nresults = 3
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<br>
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max_delay = 10
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<p>
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BOINC automatically creates three results,
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which are sent at various times.
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At time 8, two successful results have returned
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so the validater is invoked.
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It finds a consensus, so the work unit is assimilated.
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At time 10 result 3 arrives;
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validation is performed again,
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this time to check whether result 3 gets credit.
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<pre>
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time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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created validate; assimilate
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WU x x x
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created sent success
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result 1 x x---------------x
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created sent success
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result 2 x x-------------------x
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created sent success
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result 3 x x-----------------------x
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</pre>
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<hr>
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In the next example,
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result 2 is lost (i.e., there's no reply to the BOINC scheduler).
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When result 3 arrives a consensus is found
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and the work unit is assimilated.
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At time 13 the scheduler 'gives up' on result 2
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(this allows it to delete the canonical result's output files,
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which are needed to validate late-arriving results).
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<pre>
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time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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created validate; assimilate
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WU x x x
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created sent success
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result 1 x x---------------x
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created sent lost giveup
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result 2 x x-------- x
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created sent success
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result 3 x x-----------------------x
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</pre>
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<hr>
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In the next example,
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results 2 returns an error at time 5.
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This reduces the number of outstanding results to 2;
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because target_nresults is 3, BOINC creates another result (result 4).
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A consensus is reached at time 9, before result 4 is returned.
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<pre>
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time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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created validate; assimilate
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WU x x x
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created sent success
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result 1 x x---------------x
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created sent error
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result 2 x x-------x
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created sent success
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result 3 x x-------------------x
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created sent success
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result 4 x x----------------------x
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</pre>
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";
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page_tail();
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?>
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