Specifying the Number of FIFO Virtual Processors

The database server uses first in, first out (FIFO) virtual processors to manage high-performance loads and unloads through named pipes. By default, two FIFO virtual processors are started initially. The NUMFIFOVPS configuration parameter specifies the number of FIFO virtual processors that the database server brings up initially on each coserver.

To increase parallel execution when you load and unload data through named pipes, add a FIFO virtual processor for each table fragment on each coserver to which data is loaded. Use the onstat -g ioq command to monitor FIFO virtual processors during data loading. Use onmode -p to start additional FIFO virtual processors while the database server is in online mode. You cannot drop FIFO virtual processors in online mode.

For information about using named pipes to load and unload tables through external tables, refer to the IBM Informix: Extended Parallel Server Administrator's Guide.

Limiting the number of scan threads that a query can use might reduce the time that a large query waits in the ready queue, particularly when many large queries are submitted concurrently. However, if the number of scan threads is less than nfrags, the query takes longer once it starts. For example, if a query needs to scan 20 fragments in a table, but the scan_threads formula lets the query begin when only 10 scan threads are available, each scan thread will scan two fragments serially, making query execution approximately twice as long as if 20 scan threads were used.