Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Tue, 16 Feb 2010 10:21:37 -0600

Hi Marty,

Thanks for reminding me...

On Tuesday 16 February 2010 04:28:38 Marty Kraimer wrote:
> In the st.cmd file call
>
> scanOnceSetQueueSize(size)

That sets the buffer for the scanOnce task; Mark's problem is with the queue 
for the callback tasks, which have their own ring buffer.  The routine and 
iocsh command for setting the callback buffer sizes is:

int callbackSetQueueSize(int size);

This must be run before iocInit, and I believe can be used to solve Mark's 
problem at the expense of some RAM which is only really needed at start-up.

Should we consider making those two void routines return an error status?  We 
can't really do that in scanIoRequest() because if necessary it can push an 
io_scan_list from each IOSCANPVT onto all three priority queues, and there's 
have no easy way to report a failure for a specific callback queue, or to re-
do a request for a subset of its queues.  Therefore I don't think there's much 
point in flagging this kind of failure to the caller, the user has to be 
involved in fixing it.

- Andrew

> Mark Rivers wrote:
> > Folks,
> >
> > I have run into a fairly serious problem with asyn and EPICS base.
> >
> > Standard asyn device support allows for I/O Intr scanned records.  The
> > driver does a callback to device support with a new value, and device
> > support in turn calls scanIoRequest to process the record.  When the
> > record processes it reads the "cached" data that was passed to the
> > callback, rather than actually reading from the device.
> >
> > This generally works fine.  However, I  have just run into a serious
> > problem when the number of callback requests is very large.  This is
> > happening at iocInit in a system with lots of records that are I/O Intr
> > scanned.  The problem is a ring buffer overflow in the cbLow task.
> >
> > The sequence of calls is
> >
> > driver->asyn device support->scanIoRequest->callbackRequest.
> >
> > This is the code from EPICS base for callback request.
> >
> > void callbackRequest(CALLBACK *pcallback)
> > {
> >     int priority;
> >     int pushOK;
> >     int lockKey;
> >     if (!pcallback) {
> >         epicsPrintf("callbackRequest called with NULL pcallback\n");
> >         return;
> >     }
> >     priority = pcallback->priority;
> >     if (priority < 0 || priority >= NUM_CALLBACK_PRIORITIES) {
> >         epicsPrintf("callbackRequest called with invalid priority\n");
> >         return;
> >     }
> >     if (ringOverflow[priority]) return;
> >     lockKey = epicsInterruptLock();
> >     pushOK = epicsRingPointerPush(callbackQ[priority], pcallback);
> >     epicsInterruptUnlock(lockKey);
> >     if (!pushOK) {
> >         errlogPrintf("callbackRequest: %s ring buffer full\n",
> >             threadName[priority]);
> >         ringOverflow[priority] = TRUE;
> >     }
> >     epicsEventSignal(callbackSem[priority]);
> > }
> >
> > Note that it is a void function, and thus does not return an error status
> > when the ring buffer is full.  Why not?
> >
> > This is the code for scanIoRequest:
> >
> > void scanIoRequest(IOSCANPVT pioscanpvt)
> > {
> >     int prio;
> >     if (scanCtl != ctlRun) return;
> >     for (prio = 0; prio < NUM_CALLBACK_PRIORITIES; prio++) {
> >         io_scan_list *piosl = &pioscanpvt[prio];
> >         if (ellCount(&piosl->scan_list.list) > 0)
> >             callbackRequest(&piosl->callback);
> >     }
> > }
> >
> > It is also a void function, and so cannot return an error from
> > callbackRequest even if it could return one.
> >
> > The ring pointer buffer is created with this line in callbackInitOnce:
> >         callbackQ[i] = epicsRingPointerCreate(callbackQueueSize);
> >
> > callbackQueueSize is defined with this line:
> > static int callbackQueueSize = 2000;
> >
> > Thus, if there are more than 2000 unprocessed scanIoRequest calls there
> > will be an error which cannot be caught.
> >
> > This is a serious problem for me.  My XIA detector driver is doing
> > callbacks to device support for >4000 records shortly after iocInit. 
> > This will soon grow to 20,000 when a 100 element detector at the
> > Australian Synchrotron starts to use this driver.  I am now getting these
> > errors when I start a system with 16 detectors:
> >
> > callbackRequest: cbLow ring buffer full
> >
> > This very large number of callbacks will typically only happen right
> > after iocInit.  In this case I would be willing to change the asyn device
> > support as follows:
> >
> > - Test if scanIoRequest succeeded
> > - If it failed because of ring buffer full then do a short sleep and try
> > again, up to some maximum number of retries
> >
> > But I cannot implement this logic because scanIoRequest does not return a
> > failure status, so I don't know if it worked or not.
> >
> > What should be done?
> >
> > Thanks,
> > Mark

-- 
The best FOSS code is written to be read by other humans -- Harald Welte

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System