On Tue, 25 Aug 2015 11:47:06 -0700, you wrote:
>Harvey,
>
>Yeah, wow semaphores will *not* work! heh.
>
>So using sem_wait() / sem_post() introduces incredibly long stalls. Even
>when using a single process. The webserver which uses libmongoose wont even
>function. Hell I put a printf() in the front of my control loop, and that
>doesn't even work :/
Then you're not getting to the printf.....
Ok, one possibility is that the system calls are taking lots of time.
Another possibility is that the wait time is giving you a problem.
If you understand the mechanism of a semaphore, then you can implement
your own in code.
Not sure what's going on.
Harvey
>
>Going to do some more digging, and see if I can somehow make this work.
>
>On Mon, Aug 24, 2015 at 6:32 PM, Harvey White <ma...@dragonworks.info>
>wrote:
>
>> On Mon, 24 Aug 2015 18:19:14 -0700, you wrote:
>>
>> >>
>> >> *I know the feeling.*
>> >>
>> >> * Good luck and don't hesitate to ask for more help, or at least
>> advice,*
>> >> * for whatever I can do. Linux I can't really talk about, the*
>> >> * fundamentals I think I can.*
>> >>
>> >
>> >I think Linux in this context was not so important. I mean it is / was,
>> but
>> >I generally do ok with high level on topic discussions. So long as I know
>> >what my options are, everything is good.
>>
>> It's more of a general operating system issue, and that is fundamental
>> knowledge. Personally, I don't see a problem, but the list moderators
>> haven't seemed to have a problem either, so that's ok.
>>
>> >
>> >*Ask either on the list or private email if you want.*
>> >>
>> >
>> >I don't mind asking here if that is fine with everyone. Technically, I
>> felt
>> >a little funny posting here, as it was semi off topic( in relation to the
>> >beaglebone ), but maybe the discussion helps someone else too ? If there
>> is
>> >a problem, then I have no issues moving to another forum.
>>
>> True, except it *is* to get the beaglebone working, and *is* an issue
>> that can bite people writing somewhat more complicated projects.
>>
>> I'd hope that it will help others, and for that matter, if someone
>> disagrees with what I've said, I'd welcome the discussion.
>>
>> Hopefully, the concepts will help with the more complicated projects
>> using any sort of beagle....
>>
>> Harvey
>>
>> >
>> >On Mon, Aug 24, 2015 at 6:00 PM, Harvey White <ma...@dragonworks.info>
>> >wrote:
>> >
>> >> On Mon, 24 Aug 2015 17:40:33 -0700, you wrote:
>> >>
>> >> >Hey Harvey, and Walter
>> >> >
>> >> >Just kind of an update. Last night after our discussion I found a
>> really
>> >> >good resource / discussion of what fork() is and the different ways it
>> can
>> >> >be used. So with this information in mind along with our discussion
>> >> >yesterday it seems that what I want to do can indeed be done without
>> using
>> >> >POSIX shared memory( I had little doubt ) - *and* seemingly more
>> simple.
>> >>
>> >> That sounds good
>> >> >
>> >> >I'd still have to use a Semaphore - I think to keep the web server
>> >> callback
>> >> >from stalling my canbus routines. But I think that seems fairly
>> >> reasonable.
>> >> >
>> >>
>> >> That also sounds quite reasonable to do. As your programs get more
>> >> complicated, you'll have to figure out how to interlock/protect/manage
>> >> resources.
>> >>
>> >> I have a project that manages a graphics engine (software), I2C slave
>> >> (ditto), heartbeat/errortask, I2C error reporting task, and the like;
>> >> and uses a FIFO, semaphores, queues and the like to protect resources
>> >> and manage memory.
>> >>
>> >> Probably a bit too complex, but it kinda grew that way.
>> >>
>> >>
>> >> >Still I may just implement semaphores into my current code to check it
>> >> out,
>> >> >but not sure when. Been a semi rough day, and I'm whooped . . .
>> >>
>> >> I know the feeling.
>> >>
>> >> Good luck and don't hesitate to ask for more help, or at least advice,
>> >> for whatever I can do. Linux I can't really talk about, the
>> >> fundamentals I think I can.
>> >>
>> >> Ask either on the list or private email if you want.
>> >>
>> >> Harvey
>> >>
>> >> >
>> >> >On Sun, Aug 23, 2015 at 9:44 PM, William Hermans <yyrk...@gmail.com>
>> >> wrote:
>> >> >
>> >> >> OK have a good one, thanks for the discussion.
>> >> >>
>> >> >> On Sun, Aug 23, 2015 at 9:11 PM, Harvey White <
>> ma...@dragonworks.info>
>> >> >> wrote:
>> >> >>
>> >> >>> On Sun, 23 Aug 2015 20:18:26 -0700 (PDT), you wrote:
>> >> >>>
>> >> >>> >
>> >> >>> >>
>> >> >>> >> *Well, you're certainly right that the callback is messing*
>> >> >>> >> * things up. If I assume the same callback, then the callback
>> is*
>> >> >>> >> * certainly changing data. If you can set the right breakpoint,
>> you
>> >> >>> can*
>> >> >>> >> * tag the situation *if* the breakpoint also knows that the
>> process
>> >> is*
>> >> >>> >> * reading from the CAN bus.*
>> >> >>> >>
>> >> >>> >> * Had you considered disabling that callback function until the
>> >> read*
>> >> >>> >> * from the CANbus is finished? Would it be practical? That's
>> where
>> >> >>> the*
>> >> >>> >> * semaphore might help a lot.*
>> >> >>> >>
>> >> >>> >> * what variables could be common between the two routines?*
>> >> >>> >>
>> >> >>> >> * Harvey*
>> >> >>> >>
>> >> >>> >
>> >> >>> >Well this is where previous experience fails me. I've pretty much
>> >> avoided
>> >> >>> >code related to threading in software. In the past. I do know of
>> >> fork()
>> >> >>> and
>> >> >>> >roughly what it is capable of, and I know about threads, but not to
>> >> >>> >implement them in C on Linux. Or what can be done with them. Lets
>> talk
>> >> >>> code
>> >> >>> >a minute.
>> >> >>>
>> >> >>> OK, as well as I can follow it.
>> >> >>>
>> >> >>> >
>> >> >>> >*IPC - Server - Reads from canbus*
>> >> >>> >int main(){
>> >> >>> > struct can_frame frame;
>> >> >>> > int sock = InitializeCAN("vcan0");
>> >> >>> >
>> >> >>> > statistics_t *stats = NULL;
>> >> >>> >
>> >> >>> > const long shm_size = sysconf(_SC_PAGESIZE);
>> >> >>> >
>> >> >>> > int shm_fd = shm_open("acme", O_CREAT | O_RDWR, FILE_PERMS);
>> >> >>>
>> >> >>> **NOTE: the problem may be "acme", since we know that acme products
>> >> >>> are not effective against roadrunners.....
>> >> >>>
>> >> >>> > if(shm_fd == -1)
>> >> >>> > HandleError(strerror(errno));
>> >> >>> >
>> >> >>> > const int retval = ftruncate(shm_fd, shm_size);
>> >> >>> > if(retval == -1)
>> >> >>> > HandleError(strerror(errno));
>> >> >>> >
>> >> >>> > shared_memory = InitializeShm(shm_size * sizeof(char), shm_fd);
>> >> >>> > close(shm_fd);
>> >> >>> >
>> >> >>> > while(1){
>> >> >>> > frame = ReadFrame(sock);
>> >> >>> > if(frame.can_dlc == FRAME_DLC)
>> >> >>> > stats = ProcessFastpacket(frame);
>> >> >>>
>> >> >>> right at this point, you have no protection against access and no
>> >> >>> interlocking.
>> >> >>>
>> >> >>> I'll have to give you pseudocode, because I don't know how to do
>> this
>> >> >>> in Linux.
>> >> >>>
>> >> >>> In the init routine, before you set up either main as a
>> >> >>> process (I assume you do this). Declare a semaphore:
>> >> >>>
>> >> >>> semaphore_handle shared_access; // create semaphore
>> >> >>> handle accessible to both processes.
>> >> >>> semaphore_create (shared_access); // create
>> >> >>> semaphore
>> >> >>>
>> >> >>>
>> >> >>> then modify this next section to:
>> >> >>>
>> >> >>> if(stats != NULL){
>> >> >>> if (semaphore_take(shared_access), <wait forever>)
>> >> >>> {
>> >> >>> WriteToShm(shared_memory, stats);
>> >> >>> semaphore_give (shared_access);
>> >> >>> }
>> >> >>> stats = NULL;
>> >> >>> printf("%s", ReadFromShm(shared_memory));
>> >> >>> }
>> >> >>> task_delay(n);
>> >> >>>
>> >> >>> NOTE: Process A hangs until it can "get" the semaphore; if
>> Process B
>> >> >>> has it, B can keep it only long enough to send the packet
>> >> >>> >
>> >> >>> > if(stats != NULL){
>> >> >>> > WriteToShm(shared_memory, stats);
>> >> >>> > stats = NULL;
>> >> >>> > printf("%s", ReadFromShm(shared_memory));
>> >> >>> > }
>> >> >>> > }
>> >> >>> >}/* main() */
>> >> >>> >
>> >> >>> >
>> >> >>> >
>> >> >>> >*IPC - Client / webserver*
>> >> >>> >
>> >> >>> >int main(void) {
>> >> >>> > struct mg_server *server = mg_create_server(NULL,
>> ev_handler);
>> >> >>> >
>> >> >>> > mg_set_option(server, "listening_port", "8000");
>> >> >>> > mg_set_option(server, "document_root", "./web");
>> >> >>> >
>> >> >>> > printf("Started on port %s\n", mg_get_option(server,
>> >> >>> >"listening_port"));
>> >> >>> >
>> >> >>> > // POSIX IPC - shared memory
>> >> >>> > const long shm_size = sysconf(_SC_PAGESIZE);
>> >> >>> > int shm_fd = shm_open("file", O_CREAT | O_RDWR,
>> FILE_PERMS);
>> >> >>> > if(shm_fd == -1)
>> >> >>> > HandleError(strerror(errno));
>> >> >>> >
>> >> >>> > const int retval = ftruncate(shm_fd, shm_size);
>> >> >>> > if(retval == -1)
>> >> >>> > HandleError(strerror(errno));
>> >> >>> >
>> >> >>> > shared_memory = InitializeShm(shm_size * sizeof(char),
>> >> shm_fd);
>> >> >>> >
>> >> >>> > close(shm_fd);
>> >> >>> >
>> >> >>> > char id = 0x00;
>> >> >>> > for (;;) {
>> >> >>> > mg_poll_server(server, 10);
>> >> >>> >
>> >> >>> then do the same here
>> >> >>>
>> >> >>> if (semaphore_take(shared_access), <wait forever>)
>> >> >>> {
>> >> >>> if(shared_memory->sdata.data[19] != id){
>> >> >>> push_message(server,shared_memory->sdata.data);
>> >> >>> id =
>> >> >>> shared_memory->sdata.data[19];
>> >> >>> }
>> >> >>> semaphore_give (shared_access);
>> >> >>> }
>> >> >>> task_delay (n clock ticks);
>> >> >>>
>> >> >>> semaphore_take gets the semaphore if and only if it's available. It
>> >> >>> does so in a thread safe manner. the <wait_forever> is whatever
>> value
>> >> >>> the system uses to tell the process to hang. You don't want the
>> >> >>> process to wait and then just go.
>> >> >>>
>> >> >>> Because each example here releases the semaphore (semaphore_give) if
>> >> >>> and only if it could get it, and since giving and taking the
>> semaphore
>> >> >>> is thread safe, the two threads should be fine.
>> >> >>>
>> >> >>> So your "consumer" thread can't check for valid data until there's
>> >> >>> something there. When it first starts up, it has to get bad (null)
>> >> >>> data and throw that away, since you can't guarantee that one thread
>> >> >>> starts before the other (unless you block the thread using a
>> suspend,
>> >> >>> but that's not really the best thing to do), so you have to consider
>> >> >>> that you have two parallel and independent threads.
>> >> >>>
>> >> >>> The consumer thread can access shared memory only when it's not been
>> >> >>> actively written to. It has to figure out if data is good and what
>> to
>> >> >>> do with it. However, once written, that data will remain
>> uncorrupted
>> >> >>> until the consumer has read and processed it (because the consumer
>> has
>> >> >>> the semaphore and doesn't give it up until then).
>> >> >>>
>> >> >>> The producer thread checks to see if the data is there to send,
>> >> >>> accesses shared memory by getting the semaphore (when the consumer
>> is
>> >> >>> not reading it), and then writes that shared memory. It then
>> releases
>> >> >>> the semaphore, goes idle (because the task switcher has to have a
>> time
>> >> >>> to start up the other task unless you have multiple cores), and then
>> >> >>> checks for data, and waits to see when it can write that data.
>> >> >>>
>> >> >>> The typical task clock is either 1 ms or 10 ms, and the clock tick
>> is
>> >> >>> that (1 ms or 10 ms per tick). You play with the values for best
>> >> >>> throughput on the n delays.
>> >> >>>
>> >> >>>
>> >> >>> > if(shared_memory->sdata.data[19] != id){
>> >> >>> > push_message(server,
>> >> shared_memory->sdata.data);
>> >> >>> > id = shared_memory->sdata.data[19];
>> >> >>> > }
>> >> >>> > }
>> >> >>> >
>> >> >>> > mg_destroy_server(&server);
>> >> >>> > return 0;
>> >> >>> >}
>> >> >>> >
>> >> >>> >In the context of whats interesting where threading is concerned.
>> The
>> >> >>> loops
>> >> >>> >in each executable here might be useful. If somehow each, or even
>> just
>> >> >>> the
>> >> >>> >for loop in the IPC client could somehow use objects in memory from
>> >> the
>> >> >>> IPC
>> >> >>> >server.
>> >> >>>
>> >> >>> That was the shared memory, right?
>> >> >>>
>> >> >>> >That is let us suppose for a minute IPC was removed entirely, then
>> >> >>> >somehow I could turn off the callback in the IPC client. This is
>> what
>> >> I'm
>> >> >>> >having a problem imagining. How could this be done ?
>> >> >>>
>> >> >>> You may possibly be able to schedule *when* the callback happens.
>> >> >>>
>> >> >>> What causes the callback, sending a CAN message?
>> >> >>>
>> >> >>> > In the context of
>> >> >>> >libmongoose I'm not sure. In the context of threading or using
>> fork()
>> >> I'm
>> >> >>> >also not sure.
>> >> >>>
>> >> >>> Fork creates a separate process which can be controlled or killed as
>> >> >>> needed, running as a sub-process (IIRC).
>> >> >>>
>> >> >>> you're dealing with creating two processes (really two programs) and
>> >> >>> interprocess communication.
>> >> >>>
>> >> >>> >But if I could somehow through using threading context to
>> >> >>> >disable the callback I think that would be ideal. That way I could
>> >> simply
>> >> >>> >disable that whole thread for a fraction of a second, and then
>> resume
>> >> it
>> >> >>> >once a fastpacket is constructed.
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>> Well, synchronizing the two tasks with semaphores says that if the
>> >> >>> callback happens and you can turn off that callback, then the data
>> is
>> >> >>> ok as long as you can schedule the callback. No idea when that
>> >> >>> happens.
>> >> >>>
>> >> >>> So you maybe able to
>> >> >>> 1) produce data
>> >> >>> 2) keep from overwriting it
>> >> >>> 3) enable the consumer to read data
>> >> >>> 4) have it send data (and I assume the callback happens here)
>> >> >>> 5) data is clobbered in the shared area, but we don't care since
>> it's
>> >> >>> sent already
>> >> >>> 6) give the semaphore back allowing new data to be written
>> >> >>> 7) that data can't be clobbered by the callback (assuming) until
>> after
>> >> >>> it's read and in the send process
>> >> >>>
>> >> >>> May solve the problem...
>> >> >>>
>> >> >>>
>> >> >>> >
>> >> >>> >Anyway, a little information that might be needed. socketCAN reads
>> >> data
>> >> >>> in
>> >> >>> >8 byte lengths for each frame..fastpackets are several frames in
>> >> length,
>> >> >>> >and with the only current one I'm tracking being 11 frames long.
>> Or 88
>> >> >>> >total bytes, not discounting the initial char from each frame which
>> >> is a
>> >> >>> >sequence number. If there is a way, and I'm sure there is, I am all
>> >> for
>> >> >>> >changing from an IPC model to a threaded model. But I still have
>> some
>> >> >>> >doubts. Such as will it be fast enough to track multiple
>> fastpackets a
>> >> >>> >second ? Past that how complex will it be ?
>> >> >>>
>> >> >>> Won't be all that complex, I think
>> >> >>> the processes are written as two parts
>> >> >>> one is a system call to set up a process
>> >> >>> the other is the process itself which looks like
>> >> >>>
>> >> >>> void processA(void* arguments if any)
>> >> >>> {
>> >> >>> // declarations and inits the first time through
>> >> >>> while (1)
>> >> >>> {
>> >> >>> basic process loop;
>> >> >>> }
>> >> >>> }
>> >> >>>
>> >> >>> not complicated at all, how to create the process ought to be well
>> >> >>> documented
>> >> >>>
>> >> >>> you just need to make sure that the two processes have access to
>> >> >>> shared memory
>> >> >>>
>> >> >>> assuming 1000 us available per process, a context switching time of
>> 50
>> >> >>> us (may be shorter, but it's a number)
>> >> >>>
>> >> >>> You have 950 us to send a complete message without it having a delay
>> >> >>> you have that same 950 us to detect and build a message.
>> >> >>>
>> >> >>> that gives you 500 message cycles/second
>> >> >>>
>> >> >>> taking twice as long gives you 250 message cycles/second and about
>> >> >>> 1950 us to compose and send a message, that's with a 2 ms clock
>> tick.
>> >> >>> All that clock tick does is control task switching. The processor
>> >> >>> clock controls the speed of operations otherwise.
>> >> >>>
>> >> >>> >
>> >> >>> >I have given multiple approaches consideration, just having a hard
>> >> time
>> >> >>> >imaging how to work this out using a threading model.
>> >> >>>
>> >> >>> perhaps this might help
>> >> >>>
>> >> >>> Harvey
>> >> >>>
>> >> >>> (off to bed, have to be in training for 8 am classes in a week).
>> >> >>>
>> >> >>> --
>> >> >>> For more options, visit http://beagleboard.org/discuss
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>> >> >>>
>> >> >>
>> >> >>
>> >>
>> >> --
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