Coarray article for the upcoming GCC Summit.

[Bill Long]

Janne Blomqvist wrote:
> Bill Long wrote:
>> MOENE Toon wrote:
>>> Is there some difficulty with coarrays here that I am overlooking ?
>> The Rice University group has had two problems in this area, though 
>> neither affect our (Cray's) implementation.  As background, our 
>> general implementation of coarrays on our vector systems works like 
>> this:  Coarrays are placed in a separate "symmetric" heap that starts 
>> at the same base address on each image and contains only coarrays.  
>> Because of the restrictions on allocatable coarrays, it is always 
>> possible to store coarrays such that the base address for a 
>> particular coarray is the same on each image.  This allows you to 
>> know the address of a remote coarray reference using only the local 
>> address information for the same coarray.   For ordinary and 
>> allocatable coarrays this is pretty straightforward, and Rice seems 
>> to have no problem addressing static coarrays.
>>
>
>> The second problem seems to have multiple names, one of which is 
>> "pinning of memory" on the images. Even if you handle the symmetric 
>> heap in some special way, the targets of pointer components and the 
>> actual memory for allocatable components can be anywhere in the local 
>> memory of each node.  Some hardware DMA protocols evidently require 
>> that remotely accessed memory has to be "registered" or "pinned" 
>> somehow so the hardware in the network can access it.   The Cray 
>> vector implementation gets around this issue by doing two things:  1) 
>> we disable demand paging on any node running a coarray (or UPC) 
>> image, and 2) we (effectively) pin/register all of the physical 
>> memory on the node by using large pages and remote address 
>> translation tables.  This results in very good performance, but is 
>> more restrictive than a generic implementation.  Considering that 
>> libraries like MPI need to get around this same issue, I assume 
>> gfortran will have some solution available.  But, I think it is 
>> important to be aware of it from the start, and think about the best 
>> solution when doing the basic design work.
>
> This could be tricky, if we want something portable, performant and 
> robust (pick one, ha ha). Here's one article ranting about RDMA that 
> got quite a lot of press a few years ago:
>
> http://www.hpcwire.com/hpc/815242.html
>
> and the response
>
> http://www.hpcwire.com/hpc/885757.html
>
> I think the only sensible solution here would be to use some 
> appropriate abstraction layer like gasnet or armci. 

Yes, I agree.  I believe Rice is using armci, though my current bias is 
for gasnet (see below).
 

> Do these also solve the first problem you mention?

The first problem is really internal to the compiler rather than a 
network issue.  I don't see that as a problem for gfortran as long as 
allocatable and pointer components are represented as some sort of dope 
vector in the parent structure.  Rice's problem is that they had no way 
to control the data layout of the structure or the dope vector.


>
> Is Cray planning to help out with coarray gfortran on portals? 

I can't commit to anything other than my free commentary. :)  However, I 
would point out that we have cooperated on the portals implementation of 
gasnet  (mainly for a UPC customer).  So, if gfortran also targeted 
gasnet, it should port to the XT systems pretty easily.  (We currently 
ship UPC for the XT systems using gasnet over portals.)  The 
implementation for the vector systems is embedded in the compiler code 
generator since the hardware memory load and store instructions can 
specify addresses on other nodes directly by ORing the image number into 
the upper bits of the address.  I think it would not be worthwhile for 
gfortran to target that architecture, at least for now.

> I suppose most gfortran contributors have experience with programming 
> and using MPI applications, but MPP systems programming is somewhat 
> outside our experience. So I think any help in this are would be very 
> welcome.

I'm not really a systems programmer, but more often than not these days, 
the nodes on an MPP system are running Linux (possibly stripped down).  
So, I suspect you have more experience than you think.   Most of the 
work comes in "start.c" - the user mode code that does initial setup 
before calling the main program.  In there you define the local image 
number and the number of images, and initialize any data structures 
needed for synchronizations or maintaining the symmetric heap.  Probably 
not all that different from MPI_Initialize(), only it is executed 
automatically before the user code starts.

Cheers,
Bill

-- 
Bill Long                                   longb@cray.com
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