Is there a limit to the number of compare processes that can be run at a given time? (4300551)

Usually it is recommended not to run too many compare processes at a given time. The question arises as to whether there is a limit imposed in Shareplex for the number of concurrent compare that can be run.

General information.

Shareplex does not impose any direct limit on the number of compare that can be run at a given time. However, indirectly there is a limit imposed due to some mechanism described as follows. There is a maximum of 20 SharePlex processes that can read from, and/or write to, the post queue at the same time, including the replication processes and the compare processes. There is no practical way to determine how many compare processes can be running without exceeding this limit. So it is recommended to limit the number of compare processes to be run concurrently, usually to between 3 and 5.

There is another angle to this. When running compare on a very large table, there may be a resource issue due to sorting and other actions that go on behind the scene. This is another reason it is recommended to limit the number of compare run at the same time.

The following output shows how Compare processes on target features in Shareplex queues as a Reader to the Post queue and is a factor when determining the # of processes that read from or write to a Post queue:
qview> list
The following queues exist:
  usalodb-vcs+P+o.SID1-o.SID2
    subqueues range from 0 to 1.19
        WRITER  +PI+usalodb-vcs+sp_mport+0xcaaaaaaa      (nnn.nn.nn.nnn)

        READER  +PP+usalodb-vcs+sp_opst+o. SID1-o. SID2
        READER  +U+usalodb-vcs+sp_declt+o. SID1-o. SID2-2578268
        READER  +U+usalodb-vcs+sp_declt+o. SID1-o. SID2-2527476