Re: [TCLCORE] Feature request: event priorities?

[Brian G. <bgr...@mo...>]

Alexandre Ferrieux wrote:
>
>> In gitk (my git commit visualizer, written in Tcl/Tk) I currently have
>> some moderately complicated code to work around this problem, which
>> mostly works but is not completely satisfactory.  What I do is to
>> remove the fileevent handler every time it gets called, and add an
>> idle handler which re-adds the fileevent handler on the channel.  Even
>> with this, and with a limit on how much data the handler processes
>> (which keeps the time taken by the handler to around 50ms) the UI is
>> still somewhat laggy and jerky.
>>     
>
> Are you sure no hidden blocking operation occurs in the handler ? 50ms
> shouldn't be that jerky...
>
> Now of course, there're various overheads, like registering the
> handler back and forth, and also the internal [after 0] used for
> fileevents. To smooth all this, what about replacing the fileevent by
> a simple periodic task and non-blocking reads ? It also gives you more
> control over the maximum bandwidth.
>   

Turning it into a polling situation is certainly one way to solve the 
problem.  That will give the reader more control, but it can starve the 
writer. 

I've solved this problem differently by using an intermediate file.  The 
writer writes to a temporary file, so assuming enough disk space is 
available, it will never block.  The reader can then read the same file 
at it's leisure, breaking the speed dependencies of the 2 tasks.  This 
works extremely well, letting the work horse data generator to run at 
full batch speeds while having a GUI that appears to keep up.  I also 
have another control channel that allows to 2 processes to re-sync the 
temporary file if needed, for example, if I want to limit the temporary 
file size, then the two can handshake and when the reader gets caught 
up, the file is zeroed and processing resumes.

In plumbing, the water flow is limited by the smallest pipe!

-Brian

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