Eugene Loh wrote:
Shaun Jackman wrote:
For my MPI application, each process reads a file and for each line
sends a message (MPI_Send) to one of the other processes determined by
the contents of that line. Each process posts a single MPI_Irecv and
uses MPI_Request_get_status to test for a received message. If a
message has been received, it processes the message and posts a new
MPI_Irecv. I believe this situation is not safe and prone to deadlock
since MPI_Send may block. The receiver would need to post as many
MPI_Irecv as messages it expects to receive, but it does not know in
advance how many messages to expect from the other processes. How is
this situation usually handled in an MPI appliation where the number
of messages to receive is unknown?
...
Each process posts an MPI_Irecv to listen for in-coming messages.
Each process enters a loop in which it reads its file and sends out
messages. Within this loop, you also loop on MPI_Test to see if any
message has arrived. If so, process it, post another MPI_Irecv(), and
keep polling. (I'd use MPI_Test rather than MPI_Request_get_status
since you'll have to call something like MPI_Test anyhow to complete the
receive.)
Once you've posted all your sends, send out a special message to
indicate you're finished. I'm thinking of some sort of tree
fan-in/fan-out barrier so that everyone will know when everyone is finished.
Keep polling on MPI_Test, processing further receives or advancing your
fan-in/fan-out barrier.
So, the key ingredients are:
*) keep polling on MPI_Test and reposting MPI_Irecv calls to drain
in-coming messages while you're still in your "send" phase
*) have another mechanism for processes to notify one another when
they've finished their send phases
Hi Eugene,
Very astute. You've pretty much exactly described how it works now,
particularly the loop around MPI_Test and MPI_Irecv to drain incoming
messages. So, here's my worry, which I'll demonstrate with an example.
We have four processes. Each calls MPI_Irecv once. Each reads one line
of its file. Each sends one message with MPI_Send to some other
process based on the line that it has read, and then goes into the
MPI_Test/MPI_Irecv loop.
The events fall out in this order
2 sends to 0 and does not block (0 has one MPI_Irecv posted)
3 sends to 1 and does not block (1 has one MPI_Irecv posted)
0 receives the message from 2, consuming its MPI_Irecv
1 receives the message from 3, consuming its MPI_Irecv
0 sends to 1 and blocks (1 has no more MPI_Irecv posted)
1 sends to 0 and blocks (0 has no more MPI_Irecv posted)
and now processes 0 and 1 are deadlocked.
When I say `receives' above, I mean that Open MPI has received the
message and copied it into the buffer passed to the MPI_Irecv call,
but the application hasn't yet called MPI_Test. The next step would be
for all the processes to call MPI_Test, but 0 and 1 are already
deadlocked.
Cheers,
Shaun
