Re: GM-2, send/recv and size of messages

Hi Thomas,

Thomas Traub wrote:
> I am working with GM-2. After reading the docu:
> http://www.myri.com/scs/GM-2/doc/html/Receiving_Messages.html#receiving
> it says that the size of the incomming message and the size of the buffer provided for receiving must match EXACTLY.
> This is very strange and uncomfortable.

Well, this is the world of the GM API :-)
It took me a while to get it right, but the catch is that the "size" is 
not the "length". The size in the GM API can be seen as a tag, something 
to use to match incoming messages with posted receive buffers. GM used 
to enforce a complicate relationship between length and size (the length 
of a receive buffer should be at least the power of 2 of the size, aka 
gm_max_length_for_size(), but it was dropped in the implementation 
because it was confusing, evil and useless IMHO). But there is one 
remaining catch, this minimum length for the size if still needed 
somehow for small messages. To be safe and to not follow the weird 
restrictions on the length, I use the GM size as a tag between the value 
9 and 31 and I don't have to deal with minimum lengths.

> #define GMPI_PKT_GM_SIZE 30

That means I am using the tag 30 for the data packets (whatever their 
length is). I used to use another tag, 31, for the initialization packet 
but not anymore.

As you can guess, a tag range as small as 9 to 31 is pretty useless for 
MPI that uses much more complex matching rules. So I pick one GM size 
for everything and do the matching myself in the MPICH-GM lib.

In MX, the matching space is 64 bit maskable, and there is no 
relationship between matching information and length of the send or the 
receive. It's night and day.

Hope it helps

Patrick
-- 

Patrick Geoffray
Myricom, Inc.
http://www.myri.com