Re: [pcp] suitability of PCP for event tracing

["Frank Ch. Eigler" <fche@redhat.com>]

Hi -

> [...]

Makes sense.

> So, can anyone mount a convincing argument that the requirements would 
> demand changes to allow asynchronous behaviour between PMAPI clients 
> <---> PMCD <---> PMDAs?

Not without more data, so your plan sounds good.

> If not, I strongly suggest we work to flesh out the changes needed
> to make a variable length array of structured event records
> available through the existing poll-based APIs.

OK.  This suggests two separate steps: allowing more than one
separately-timestamped metric tuple to come back from the new
array-fetch functions; and to allow structured data through some new
metric data type.

For the former, it seems we'd need at least three functions in pmapi.h
to be forwarded to the pmda: one to start watching metrics of
interest, one to poll any values collected since last time, and one to
shut down the watch.

For the pmda side, since main thread control is to stay elsewhere,
asynchronous data collection would seem to require multithreading in
the typical cases, but perhaps not initially.

- FChE