11.23.2.Configuration Parameters for Vectoring and Parallelization

The following virtuoso.ini [Parameters] section entries concern query parallelization and vectoring:

Note: These settings only apply to Virtuoso 7.0 and later.

  • AsyncQueueMaxThreads :

    Sets the number of threads in a pool that is used for getting extra threads for running queries and for aq_request. Each running statement has at least one thread that is not allocated from this pool plus zero or more threads from this pool.

    Setting the pool size to the number of cores plus a few is a reasonable default. On platforms with core multithreading, one can count a core thread as a core for purposes of this parameter.

    If one expects to run many slow aq_requests() (see async_queue() , aq_request() , etc.), then the number of threads should be increased by the number of slow threads one expects.

    Slow threads are typically I/O bound threads used for web crawling or similar long-latency, low-CPU activity.

    Note: Only effective with Virtuoso 7.0 and later.

  • ThreadsPerQuery :

    This is maximum number of threads that can be claimed from the thread pool by a single query. A value of one means that no query parallelization will take place, and all queries will run single threaded.

    The number of cores on the machine is a reasonable default if running large queries.

    Note that since every query is served by at least one thread, a single query taking all the extra threads will not prevent other queries from progressing.

    Note: Only effective with Virtuoso 7.0 and later.

  • VectorSize This the number of simultaneous sets of query variable bindings processed at one time. The default is 10,000, which is good for most cases.

    If we are evaluating the query:

    FROM t1 a
    WHERE a.row_no + 1 = b.row_no

    with vector size of 10,000, then 10,000 rows of t1 a will be fetched first; 1 will be added to the 10,000 row_no values; and then the corresponding row of t1 b will be fetched for the 10,000 row_no of t1 a. This process will repeat until enough batches of t1 a have been fetched to come to its end.

    Note: Only effective with Virtuoso 7.0 and later.

  • AdjustVectorSize :

    Using a larger vector size when evaluating large queries with indexed random-access can yield up to a 3x speed-up relative to using the default vector size. However, always using a large vector size will prohibitively increase the overhead of running small queries. For this reason, there is the option to adaptively select the vector size. Set AdjustVectorSize = 1 to enable this feature. The SQL execution engine will increase the vector size when it sees an index lookup that does not get good locality, (e.g., after sorting the keys to look for, too few consecutive lookups fall on the same page). Having more keys to look up increases the chance that consecutive keys should be found on the same page, thus eliminating much of the index lookup cost.

    Note: Only effective with Virtuoso 7.0 and later.

  • MaxVectorSize :

    When AdjustVectorSize is on, this setting gives the maximum vector size. The default is 1,000,000 and the largest allowed value is about 3,500,000.

    Note: Only effective with Virtuoso 7.0 and later.