You are welcome to stick barriers in - doesn't hurt anything other
than performance.
On Nov 11, 2009, at 3:00 AM, Glembek Ondřej wrote:
Thanx for your reply...
My coll_sync_priority is set to 50. See the dump of ompi_info --
param coll sync below...
Does sticking barriers hurt anything or is it just a cosmetic
thing??? I'm fine with this solution...
Thanx
Ondrej
$ompi_info --param coll sync
MCA coll: parameter "coll" (current value: <none>,
data source: default value)
Default selection set of components for the
coll framework (<none> means use all components that can be found)
MCA coll: parameter "coll_base_verbose" (current
value: "0", data source: default value)
Verbosity level for the coll framework (0 =
no verbosity)
MCA coll: parameter "coll_sync_priority" (current
value: "50", data source: default value)
Priority of the sync coll component; only
relevant if barrier_before or barrier_after is > 0
MCA coll: parameter
"coll_sync_barrier_before" (current value: "1000", data source:
default value)
Do a synchronization before each Nth
collective
MCA coll: parameter
"coll_sync_barrier_after" (current value: "0", data source: default
value)
Do a synchronization after each Nth
collective
Quoting "Ralph Castain" <r...@open-mpi.org>:
Yeah, that is "normal". It has to do with unexpected messages.
When you have procs running at significantly different speeds, the
various operations get far enough out of sync that the memory
consumed by recvd messages not yet processed grows too large.
Instead of sticking barriers into your code, you can have OMPI do
an internal sync after every so many operations to avoid the
problem. This is done by enabling the "sync" collective component,
and then adjusting the number of operations between forced syncs.
Do an "ompi_info --params coll sync" to see the options. Then set
the coll_sync_priority to something like 100 and it should work for
you.
Ralph
On Nov 10, 2009, at 2:45 PM, Glembek Ondřej wrote:
Hi,
I am using MPI_Reduce operation on 122880x400 matrix of doubles.
The parallel job runs on 32 machines, each having different
processor in terms of speed, but the architecture and OS is the
same on all machines (x86_64). The task is a typical map-and-
reduce, i.e. each of the processes collects some data, which is
then summed (MPI_Reduce w. MPI_SUM).
Having different processors, each of the jobs comes to the
MPI_Reduce in different time.
The *first problem* came when I called MPI_Reduce on the whole
matrix. The system ended up with *MPI_ERR_OTHER error*, each time
on different rank. I fixed this problem by chunking up the matrix
into 2048 submatrices, calling MPI_Reduce in cycle.
However *second problem* arose --- MPI_Reduce hangs up... It
apparently gets stuck in some kind of dead-lock or something like
that. It seems that if the processors are of similar speed, the
problem disappears, however I cannot provide this condition all
the time.
I managed to get rid of the problem (at least after few non-
problematic iterations) by sticking MPI_Barrier before the
MPI_Reduce line.
The questions are:
1) is this a usual behavior???
2) is there some kind of timeout for MPI_Reduce???
3) why does MPI_Reduce die on large amount of data if the system
has enough address space (64 bit compilation)
Thanx
Ondrej Glembek
--
Ondrej Glembek, PhD student E-mail: glem...@fit.vutbr.cz
UPGM FIT VUT Brno, L226 Web: http://www.fit.vutbr.cz/
~glembek
Bozetechova 2, 612 66 Phone: +420 54114-1292
Brno, Czech Republic Fax: +420 54114-1290
ICQ: 93233896
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--
Ondrej Glembek, PhD student E-mail: glem...@fit.vutbr.cz
UPGM FIT VUT Brno, L226 Web: http://www.fit.vutbr.cz/~glembek
Bozetechova 2, 612 66 Phone: +420 54114-1292
Brno, Czech Republic Fax: +420 54114-1290
ICQ: 93233896
GPG: C050 A6DC 7291 6776 9B69 BB11 C033 D756 6F33 DE3C
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