RE: epicsMessageQueue Linux PREEMPT_RT

[<james.rowland@diamond.ac.uk>]

Hi Andrew, Eric

I'll try that. If we're talking API changes I'd like to suggest priority message support, adding at the head instead of the tail like vxWorks msgQSend MSG_PRI_URGENT. I find it useful for timer messages. epicsMessageQueueSendPriority?

James

> -----Original Message-----
> From: Andrew Johnson [mailto:anj@aps.anl.gov]
> Sent: 28 January 2011 17:30
> To: Rowland, James (DLSLtd,RAL,DIA)
> Cc: EPICS tech-talk
> Subject: Re: epicsMessageQueue Linux PREEMPT_RT
> 
> Hi James,
> 
> On Friday 28 January 2011 10:16:45 you wrote:
> >
> > The PREEMPT_RT patch makes the kernel fully pre-emptive (leading to
> > improved maximum latency) and adds support for priority inheritance
> > mutexes. There are no user space API changes, the code to use
> > _POSIX_THREAD_PRIORITY_SCHEDULING and _POSIX_THREAD_PRIO_INHERIT is
> > already in libCom (if disabled by default). I'd like to use the
> > epicsMessageQueue from a high priority thread but the send and
> receive
> > functions may calloc an eventNode, and that might cause a page fault
> or
> > block my thread on the malloc lock. I'd like to be able to pre-
> allocate
> > the eventNodes because I know the maximum number of threads that may
> > wait on the queue in advance.
> 
> Thanks; after looking at the code I see that could be tricky.  However
> it
> looks like if you could arrange for all your reading threads to wait
> for a
> message at startup (with the queue empty) they will each call
> getEventNode()
> and hence allocate the necessary number of eventNode objects.  After
> they're
> all waiting on the queue you would release them all by sending the
> requisite
> number of messages through the queue from the initializing thread.  I
> think
> that should work with the existing API, but I've not actually tried it.
> 
> We probably should add a routine to the API to allow the eventFreeList
> to be
> pre-allocated, but you probably want to be able to run on existing
> versions of
> Base.
> 
> Eric — unless you stole the code from someone else you were the author
> of all
> the *MessageQueue.* files, 'bzr log' does not lie...
> 
> - Andrew
> --
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