Understanding ASYN_CANBLOCK

[Marco Filho via Tech-talk <tech-talk at aps.anl.gov>]

Hi all. I'm writing this e-mail to ask for help understanding ASYN_CANBLOCK.

I have an asyn driver that communicates via a thread-safe API with the hardware. It tries to read thousands of parameters and I noticed that the IOC was freezing periodically as I added more parameters to be scanned.

I also noticed that reducing the SCAN time from 1 second to 5 seems to make the IOC shell freeze every couple of seconds or so (instead of freezing constantly), and guessed - just a guess - that this could be due to asyn trying to scan all parameters at once, no matter the period of 5 seconds between each reading.

Searching for a way to distribute the readings in time and reading the documentation, I found ASYN_CANBLOCK, which seems to create a separate thread for managing the queue requests:

"If queueRequest is called for a port that can block the request is queued to a thread dedicated to the port."

I expected then that putting the ASYN_CANBLOCK flag in the driver constructor would solve my problem, but to my surprise the behavior was the opposite: I found out that the constructor was already using this flag:

asynPortDriver(portName, 1,
      asynInt8ArrayMask | asynInt32Mask | asynInt32ArrayMask | asynInt64Mask | asynFloat64Mask | asynOctetMask | asynDrvUserMask,          // Interfaces that we implement
      asynInt8ArrayMask | asynInt32Mask | asynInt32ArrayMask | asynInt64Mask | asynFloat64Mask | asynOctetMask,       // Interfaces that do callbacks
      ASYN_MULTIDEVICE | ASYN_CANBLOCK, 1, /* ASYN_CANBLOCK=1, ASYN_MULTIDEVICE=1, autoConnect=1 */
      0, 0),  /* Default priority and stack size */

And removing the flag caused the OPI and shell to not freeze. By visually and manually inspecting I also have the feeling that the parameters are being properly scanned and that I'm not leaving anything behind.

The question is: by removing ASYN_CANBLOCK am I actually distributing the readings in time or am I doing something else? Are the requests actually being executed in parallel?

Thanks for any elucidations,

Marco