We have seen this issue with Intel MPI as well and the solution was to set the following two variables. export MPIR_CVAR_CH4_OFI_RANK_BITS=17 export MPIR_CVAR_CH4_OFI_TAG_BITS=$((41 - MPIR_CVAR_CH4_OFI_RANK_BITS)) The total length is 41 and can be adjusted for desired ranks and tags count. Thanks, Amit Ruhela ________________________________ From: Zhou, Hui via discuss <[email protected]> Sent: Thursday, April 8, 2021 10:18 AM To: [email protected] <[email protected]> Cc: Zhou, Hui <[email protected]> Subject: Re: [mpich-discuss] Supporting > 64K ranks in CH4/UCX netmod Hi Min, I think we can do something about it. We’ll follow-up when we have updates. -- Hui Zhou From: M Xie via discuss <[email protected]> Date: Thursday, April 8, 2021 at 12:30 AM To: [email protected] <[email protected]> Cc: M Xie <[email protected]> Subject: [mpich-discuss] Supporting > 64K ranks in CH4/UCX netmod Hi, 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? Thanks. Min