rt_fifo's controlled by semaphore's latency

[Steven Krekels]

Hi,

I'm trying to improve some throughput over the ISA bus on a 
PC104 (Pentium MMX 266) system running debian, kernel 2.4.18, rtai 24.1.9.
As the ISA bus is quite slow ( on 8MHz one 16bit access takes about
800ns) I'm looking for optimization in the code...

Based on timings measured with a logic analyser on the ISA bus, I've
got the impression that rt_fifo's controlled by semaphores is 
taking more time the I would expect.
I'm having a hard time finding out if there is room for improvement or 
that I just have to accept the current timings. 
Of course improvement is my goal :-)

Here's what it's about:

 Task 1 Irq:
  - signals task1 semaphore (indicating input data available on isa)
 Task 2 Irq:
  - signals task2 semaphore (allowing output data to be written on isa)

 Task 1: input processing
  a. rt_sem_wait() on task1 semaphore 
  b. read data from isa
  c. process data
  d. write data to rt_pipe
  e. rt_sem_signal() data available semaphore 

 Task 2: output processing
  a. rt_sem_wait() on data available semaphore 
  b. read data from rt_pipe
  c. rt_sem_wait() on semaphore 2
  d. write to isa 

And here are the timings (relative time values): 
    0ns  -- Task2 irq is fired
 4370ns  -- Task2 irq handler
 2185ns  -- Task1 irq is fired
 4410ns  -- Task1 irq handler
 3615ns  -- Task1 starts reading input
 9005ns  -- Task2 starts to write output

The irq latency is good, i.e. below 5 us.
I'm surprised by the time it takes to write to the pipe, signal a
semaphore and read from the pipe especially since only 36 bytes of data
is handled. 
so the time I'm concerned about is between task1,part a and task 2,part d.
The very first time it takes ~50us, but on average it takes 10us 
between task1,part a and task 2,part d.

Are these reasonable figures? I really don't know. 
I don't know what is happening behind the scenes of rt_pipes 
and rt_semaphores. But tens of us looks a lot to me. 


Any hints/suggestions/remarks/...  ?


Cheers,
	
	Steven. 


-- 
Steven Krekels               Email: steven.krekels at alcatel.be
Alcatel Bell Space N.V.      Phone: +32 3 829 51 75
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