Thank you, jeff,
"motion is supposed to:* increment "head"* set all the other felds in the
emcmot_command_t* increment "tail"if it does this, then task is supposed to be
able to:* make a complete copy of the emcmot_command_t* compare head and tail*
if they are equal, it has a consistent copyand it can act on the contained
command* if they are unequal, then it is an inconsistentcopy and it will try
again after a short time (typically 1servo period or 1ms) -- this is a "split
read""1) In the explaination above ,do you make "motion" and "task"
reversally?2) From your description, can i conclude that the change of head++
and tail++ are made by hardware not by soft code?
> On Sun, Mar 29, 2015 at 12:02:43AM +0000, matic bie wrote:
> > Hi,friends:
> > The source codes showed in the following picture, my puzzling question :
> > 1) I looked up all the project's codes ,but i did not find anywhere
> > the "emcmotCommand->head" and "emcmotCommand->tail"changes. Can
> > anyone know ?2)"emcmotDebug->split++", here what's the meaning of the
> > word "split",and what does it represent?
> >
> >
>
> ->head and ->tail are a very simple synchronization method between task
> (userspace) and motion (realtime).
>
> Here's how it's supposed to work: task and motion have a shared memory
> area which contains an emcmot_command_t structure. task writes a
> command to it, and motion reads out the command and acts on it.
>
> The structure's first field is "head" and its last field is "tail".
> emcmot_command_t is too large an object to be copied atomically, so
> there is the possibility that motion sees a partially-updated version of
> the structure.
>
> motion is supposed to:
> * increment "head"
> * set all the other felds in the emcmot_command_t
> * increment "tail"
> if it does this, then task is supposed to be able to:
> * make a complete copy of the emcmot_command_t
> * compare head and tail
> * if they are equal, it has a consistent copy
> and it can act on the contained command
> * if they are unequal, then it is an inconsistent
> copy and it will try again after a short time (typically 1
> servo period or 1ms) -- this is a "split read"
>
> However, I agree that searching the source code does not find the place
> where emcmotCommand->head or ->tail are incremented.
>
> The same scheme is used for other structured data passed between task
> and motion. Some of these do have head++ operations, e.g.,:
> emc/motion/command.c: emcmotStatus->head++;
>
> Additionally, this scheme was designed for simple CPUs of the 90s, and
> it would not hurt to reevaluate whether the scheme still works on
> today's SMP machines with their memory ordering promises ('char' is not
> typically a good type for memory operations that are desired to be
> "atomic" on non-x86 systems either); and whether there are now
> better-recognized synchronization primitives that could be used instead.
>
> As far as I know, emcmotDebug->split is supposed to be a monitor of the
> number of times a split read actually occurs.
>
> Jeff
>
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