I've been doing some tuning and noticed a confusing thing.
I've tried different types of Allreduce and Reduce for particular message size and noticed that in the output my setting influenced the message size 4 times bigger than what I've specified. For exaple if I say use this type for message size 128 actually message size 512 would be affected. For other collectives everything works as expected.
Documentation states that MINMSGLOG for Allreduce and Reduce is sizeof(float) which is 4 in my case instead of 1 for other cases by default. So perhaps the actual message size starts from 1 byte but the value printed is just 4 times bigger. Is it possible?