[mpich-discuss] Supporting > 64K ranks in CH4/UCX netmod

M Xie xmxmxie at gmail.com
Thu Apr 8 00:29:59 CDT 2021


I am using MPICH on CH4/UCX netmod, the version is mpich-3.4.1.

I noticed that there is a configure parameter "--with-ch4-rank-bits" which
can set the value of CH4_RANK_BITS, but seems CH4_RANK_BITS is not used in
the code.

And I also find in the netmod/ucx/ucx_impl.h,
_UCX_init_tag()/_UCX_recv_tag() use only 16 bits to set MPI rank in the
ucp_tag, but this cannot differentiate correct ucp_tag when MPI ranks
exceed 64K.

In Open MPI, 20 bits is used in pml/ucx module to set rank in ucp_tag, 20
bits for context, 24 bits for MPI tag, thus the maximum ranks in Open MPI
can be 1M.

Is there any plan to support > 64K ranks in MPICH/CH4/UCX?


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