Re: double buffered waveformRecord

[Michael Davidsaver <mdavidsaver@bnl.gov>]

On 1/4/2013 5:36 PM, Andrew Johnson wrote:
> ...
> I wouldn't regard setting *no_elements and *offset as "playing games", I think
> it's perfectly reasonable for a record to implement multiple-buffering by
> dividing the data buffer into a series of smaller segments and using *offset
> to select the current segment within the buffer.

I was hoping to avoid the need for one big allocation.  For just two 
buffers this isn't an issue, but if one wants a deeper buffer then 
problems will eventually arise.

Also, I know what I am doing breaks with the arr (array slicing) channel 
plugin, which assumes the offset will always be [0, NELM).

> ...
> If I've understood your code correctly, it's relying on *undefined behavior*
> because you're subtracting pointers to two differently allocated buffers to
> calculate the *offset value.  If the C compiler detects this it is quite at
> liberty to do anything it likes at that point.

Like I said, abuse.

I am curious though about what 'differently allocated buffers' mean wrt. 
the C standard?

> ...
> You will need to rename the record type before it has any chance of getting
> into Base BTW, but you probably guessed that already...

All of this is highly experimental at this point.

> ...
> I think there may be problems trying to change the pointer, because that's the
> fundamental value that the server code uses to identify the field that was
> subscribed to — it's the only field-specific parameter the record passes to
> db_post_events() so it must match the pointer given out when the subscription
> was created.

I was thinking that the replacement pointer would have the same lifetime 
as the 'offset' provided by get_array_info.  So it would be fetched each 
time, used once, and then discarded.

For the pointer which would live permenently in DBADDR and be used for 
db_post_events() and similar, I was considering using 
'(void*)&prec->bptr'.  If it isn't being dereferenced then it is just an 
identifier.


> Interesting; have you done any bench-marking of that optimization? - 
> Andrew 

So far I have only done some semi-realistic test.  I have a single 
waveform record which has a CP input link to itself and measure the 
processing rate.  This shows a 2x improvement for waveform lengths 
between 100k and 1M.  This drops off for smaller and larger lengths.

The real test will be trying this with a waveform digitizer.  We have a 
SIS3302 VME ADC which I have been trying to push.


Michael