Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Wed, 15 Jun 2005 11:15:43 -0500

Jeff Hill wrote:

I revised my comments on the wiki as follows.

I think email (via core talk) is probably a far simpler medium to use

for discussions, as we can always use the core-talk archive to review

past dicussions. The Wiki's talk pages don't work terribly well in

practice for live conversations (they're not event driven), as you've

obviously realized since you're emailing your comments as well as

posting them.

2) You have created interface classes for buffers, arrays, enumerated,
strings etc. All of these interfaces are already in data access. Was there a
good justification to create new ones? In any case, it seems that we should
have the goal of using the same interfaces for each of the fundamental data
types?

Agreed about the goal, but your interfaces don't necessarily take into

account all of our requirements, and we think some of your interfaces

are stricter than they need to be, which harms performance. I suspect

we would have few problems implementing most of your interfaces on top

of ours, but adopting ours should be more efficient since you don't have

to do everything one character or element at a time - we expose a whole

segment at once.

3) These interfaces seem to require contiguous storage of random sized
blocks of characters. Doesn't this preclude use of fixed length buffer based
(free list based) memory allocation, and therefore almost guarantee
fragmentation of memory. Note that fragmentation isnt just a binary
condition (memory of a particular size is or isnt available). Fragmentation
is also an efficency isssue because most random sized block memory
allocators require searching for the best sized free block. Note that one of
my goals for V4 is to no longer require EPICS_CA_MAX_ARRAY_BYTES.

Go back and read the section on the EpicsBuffer interface again; none of

our classes require contiguous buffer storage. Any segmented

implementation has to provide segments that are (presented as) an exact

multiple of the basic element size as given by the parameter to

EpicsBufferCreator::create(), but the EpicsBuffer interface and the

classes that use it were explicitly designed to allow both contiguous

and segmented buffer implementations to be used interchangably without

having to know which you are using.

Since we never really write down and agree what all the detailed

requirements are, but just present each other with code snippets that

implement our view of them, I think we're going to continue to have

disagreements like these. It would speed things up immensely if we

could first agree in writing on what we are going to accomplish *before*

we actually start implementing it (and I plead as guilty as everyone

else on that charge).

- Andrew
--
Podiabombastic: The tendency to shoot oneself in the foot.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System