[mvapich-discuss] question on environment variable in MVAPICH

[David Lowenthal]

Hello,

I have a question about environment variable VIADEV_R3_NOCACHE_THRESHOLD.
The description of this variable states:

# Default value: 1048576
# This is the message size (in bytes) which will be sent using
# the R3 mode if the registration cache is turned off, i.e.
VIADEV_USE_DREG_CACHE=0

And,

# - R3 : Sends messages without registering memory by using a copy-based
approach

I am not clear on what protocol is being used on either side of the
threshold.  It appears that a copy-based approach is used above the
threshold?  If so, what is being used below the threshold?

Thanks in advance for any help anyone can provide.

Dave
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