= Adaptive replication =

BOINC's current replication policy replicates a job even if
one of the hosts is known to be highly reliable.
The overhead of replication is high - at least 50% of total CPU time
is spent checking validity.

'''Adaptive replication''' is an optional policy that avoids replicating a job
if it has been sent to a highly reliable host.
The goal of this policy is to provide a target level of confidence
with minimal overhead - perhaps only 5% or 10% of total CPU time.

== Policy ==

BOINC maintains an estimate E(H) of host H's recent error rate.
This is maintained as follows:

 * It is initialized to 0.1
 * It is multiplied by 0.95 when H reports a correct (replicated) result.
 * It is incremented by 0.05 when H reports an incorrect (replicated) result.

Thus, it takes a long time to earn a good reputation
and a short time to lose it.

The adaptive replication policy is as follows.

 * Each job is initially marked as unreplicated.
 * On each request, the scheduler decides whether to trust the host as follows:
  * If E(H) > A, don't trust the host.
  * Otherwise, trust the host with probability 1 - E(H)/A.
 * If we decide to trust the host, preferentially send it unreplicated jobs.
 * Otherwise, preferentially send it replicated jobs.  If we have to send it an unreplicated job, mark it as replicated and create new instances accordingly.

== Implementation ==

Database:
 * Add "target_nresults" field to app table.  Default is zero (app doesn't use adaptive replication).

Scheduler:
 * Decide whether to trust host as described above.
 * If we send an unreplicated job (i.e., target_nresults=1 and app.target_nresults>1) to an untrusted host, set wu.target_nresults = app.target_nresults and flag the WU for transitioning.

Validator:
 * Don't update host.error_rate for unreplicated results (i.e., wu.target_nresults=1 and app.target_nresults>1).