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psing51
Beginner
201 Views

significance/meaning of zero byte MPI messages (APS message profiling data)

Hi,
I recently tried the APS tool to capture the message details (size and amount) for the WRF application on the intel 8280 (opa).
We launch 1 process per core, and here is the data - 
nodes,Message_size(B),Volume(MB),Volume(%),Transfers,Time(sec),Time(%)
1,0,0,0,58099903,3988.14,97.05
2,0,0,0,219491539,7554.45,96.19
4,0,0,0,850730419,15073.44,96.02

It seems that significant amount of time is spent in the transfer of these 0 byte messages and with more number of nodes, the amount of messages increases. Could you please help me in understanding following-

Q: The significance of these 0 bytes messages and How are they related to MPI communication protocol? 

I guess aps collects messages transferred between all processes (inter node + intra node), so 
Q:Is there a way to check (from aps) that how much of these messages were transmitted to the network? (inter node messages - for 2 and 4 node runs)

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4 Replies
PrasanthD_intel
Moderator
201 Views

Hi Puneet,

Thanks for reaching out to us. We are working on your issue. we will get back to you soon.

-Prasanth

psing51
Beginner
201 Views

Thank you for the reply. 
WRF version 3.9.1.1 and the dataset was conus 2.5km.

jimdempseyatthecove
Black Belt
201 Views

Messages to a target include a Tag in addition to data (if any). Thus you can pass status (information) via Tag as opposed to in the data blob. If I were to guess, I suspect that this is the cause of 0-byte messages. A guessed-at example might be a SYNC or Watchdog tickle, but the user application can do this as well.

Jim Dempsey

201 Views

Hi Punnet,

Actually APS just shows correct message sizes for senders only. For receiving ranks APS currently shows 0 byte messages. Engineering is working on a fix.

Klaus-Dieter