By default, the device driver executes the PUT and GET procedures in the same sequence, the procedures appear in the the driver file. Procedures which are not ready to run are skipped, but the overall sequence of execution is given by the order in which the procedure appear in the driver.
For PUT procedures this has the implication, that they are not executed immediately after an operator (or commanding device) has sent a new value for a parameter. The procedure has to wait it's turn. If there are a lot of GET procedures ready to run, this may take some time, specially with complex devices which are quite slow in response.
By specifying the PUTPRIORITY mode in the header of the device file, you can speed this up, making the device driver for a slow device more responsive:
You enable the PUTPRIORITY mode for a driver by adding PUTPRIORITY as a top level keyword to the driver file. This changes the order of execution of procedures in the following way:
If at least one PUT procedure of the driver is ready to run, all GET procedures are skipped. GET procedures only are executed if no PUT procedure is ready to run. The outcome of this is, that the PUT procedure is executed not later than the device thread's IDLE time after the new value has been commanded.
But as with most good things there is one hitch with this: If you change a parameter once every driver cycle of faster, GET procedures will never get a chance to run. The driver will respond quickly and execute any settings you command, but state information like alarms or meter readings will never be updated. This is the reason why PUTPRIORITY is not the default behavior of a sat-nms device driver.