Re: [Gomp-discuss] GOMP Requirements v1.1

[Ioannis E. Venetis]

Dear Scott,

This document is really very nice and I agree mostly with it. But, of 
course, I have some concerns, which can be tracked down to the last 
paragraph of the document:

"GOMP will use the threading model specified by the –enable-thread=xxx 
configuration option."

Although I agree that this should be the default behaviour, I would be 
very happy to have a way to change this behaviour during linking of an 
OpenMP program.

It is my understanding that development will proceed more or less along 
the lines of the document that Ross Towle has posted some time ago 
(http://lists.gnu.org/archive/html/gomp-discuss/2004-08/msg00000.html). 
This means that very generic OpenMP helper functions will be used, like 
_OMP_MasterFunction, _OMP_GetNextChunk, etc. These functions implement a 
master-slave parallel programming paradigm, as required by OpenMP, and 
will use more specific function calls according to the threading model 
used, like pthreads_create/join if POSIX threads are to be used.

Here is where my proposal starts to make sense. There are many threading 
libraries out there that implement a master-slave parallel programming 
paradigm and, by the way, I have been working for long on such a 
library, in the context of a project, which has given me some experience 
on these problems. We had modified a compiler to produce code that used 
the API of our library. The first versions of the compiler injected 
directly the whole parallel code, but we moved to a scheme that is 
almost the same as the one proposed in Ross' document. Changing the 
library to support this new scheme was quite easy and I expect that 
other libraries could be changed quite easily too, to support such a scheme.

Allowing to change the threading library that is linked in, will allow a 
great amount of (mostly research?) projects that implement such 
libraries to use GCC as an OpenMP compiler. The only need will be to 
implement the OpenMP helper functions using the API of the specific 
library, which I repeat should be trivial in most of these libraries, 
and link with the new version of libgomp. This might also help GOMP to 
be adopted by a much bigger community, because from my experience, the 
most difficult part in these projects is to have a compiler that 
generates code for your library, so that you can use directly big 
applications and not have to write them from scratch.

I have also seen some concerns about the statement "in some cases, how 
code is reorganized depends on the threading model in use". I tend to 
agree with Ross on this matter, that the correct approach is to use only 
generic functions and implement them in a library. From my experience, I 
am convinced that most (if not all) performance issues with 
multithreaded applications can be effectively solved in the threading 
library.

Some other minor corrections in the document:

1) In the last paragraph under "Justification":
The phrase "will provide a simplified syntax tools" should be either 
"will provide simplified syntax tools" or "will provide a simplified 
syntax tool"

2) In the second paragraph under "Effects":
The phrase "All effort will be made to unify these modification to limit 
duplication of effort." was a bit difficult to me to understand (I had 
to read it 3 times :-). Maybe we could rephrase it as "Special care will 
be taken to make these modifications as generic as possible, in order to 
avoid duplication of effort."

3) In the third paragraph under "Effects":
Should the first word (Change) be "Changes"?

4) In the fifth paragraph under "Goals":
"GOMP will be well documented, such the other developers will understand 
the how OpenMP affects GCC" should be "GOMP will be well documented, 
such that other developers will understand how OpenMP affects GCC"


I apologize for the length of the document and I hope that the 
discussion will continue and some good solutions will emerge.

Ioannis E. Venetis

Scott Robert Ladd wrote:
> Hello,
>
> I've posted the GOMP Requirements document for public review.
>
>     http://www.coyotegulch.com/gomp/gomp_requirements.html
>
> Or, if you prefer PDF format:
>
>     http://www.coyotegulch.com/gomp/gomp_requirements.pdf
>
> This document has also been announced in several newsgroups and other 
> forums, where users and developers may have an interest in this project.
>
> All considered comments are welcome.
>
> ..Scott