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Re: cyg_thread_delay vs cyg_flag_wait behavior
- From: Andrew Lunn <andrew at lunn dot ch>
- To: "Chase, Tom" <tomc at dtccom dot com>
- Cc: ecos-discuss at ecos dot sourceware dot org
- Date: Mon, 19 Jun 2006 19:20:09 +0200
- Subject: Re: [ECOS] cyg_thread_delay vs cyg_flag_wait behavior
- References: <F7F756E5ED50F345959AE893AD2F156608C6EA@dtcsrvr09.dtccom.com>
On Mon, Jun 19, 2006 at 09:07:26AM -0400, Chase, Tom wrote:
> Hello,
>
> I am working on my fourth ecos project. Historically I have had a thread
> monitoring a serial port. The thread has the lowest priority and sits at a
> getc until some data arrives then deals with it. This has worked well for
> debugging and configuration allowing a PC application to talk to the device
> and set up frequencies or what not.
>
> On the latest project this isn't working reliably. It will work for a few
> bytes of data then stop responding. The change in performance appears to be
> related to cyg_thread_delay vs. cyg_flag_wait. For the first time, I have a
> higher priority thread that has no cyg_thread_delays in it. It sits at a
> cyg_flag_wait most of the time (servicing flags every 100 to 250 ms). If I
> add a cyg_thread_delay to that thread (putting it just before the flag_wait
> for example) then the serial behavior is as I expect.
>
> I am working on an OMAP5912 and using the interrupt driven serial. I have
> verified that the thread is indeed waking up from the wait as I expect (10
> times a second) and not always running (I.E. there is time available for
> lower priority threads). I have tried using various settings (buffered, non
> buffered, stdin, TTY etc.) with no change in performance. I have a
> cyg_thread_delay that is called when the unit prepares to power down (to
> lock out the keyboard) and any buffered serial inputs are handled then
> (which is one clue to why this was happening).
>
> I can add a delay but I would like to understand the differences between
> these two functions while they are blocking. Conceptually I think of the
> delay and the wait doing the same thing and do not understand why they
> behave differently while they are in effect. Can anyone advise?
It sounds like the thread that is waiting is actually spinning inside
the wait function. From the sources:
// this loop allows us to deal correctly with spurious wakeups
while ( result && (0 == saveme.value_out) ) {
self->set_sleep_reason( Cyg_Thread::WAIT );
self->sleep();
// keep track of myself on the queue of waiting threads
queue.enqueue( self );
// Allow other threads to run
Cyg_Scheduler::reschedule();
So there is a while loop there making a spin possible. I would take a
closer look at this and see if it really is spinning rather than
sleeping, and if so why.
Andrew
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