New capabilities in UPC++ and GASNet-EX

[Paul H. Hargrove]

On behalf of the developers of UPC++ and GASNet-EX, I am pleased to announce the availability of some exciting new capabilities.

Recent work in both libraries can enable client software to avoid stalling in communication injection when GASNet-EX detects network backpressure.  In the case of GASNet-EX, existing "immediate-mode" API support has been enhanced to optionally expose backpressure at the Commit stage of NPAM via new API calls. Additionally, the ibv-conduit implementation of "immediate-mode" AM injection has been refined to be more precise.  In the case of UPC++, new experimental immediate-mode RPC APIs can throw an exception when backpressure would otherwise stall at injection.

For users of GASNet-EX, the new capabilities are available in the "stable" branch[*] in git (or "develop" if you prefer).

For users of UPC++, one should use the "develop" branch in git (in which case use of GASNet-EX's stable branch is the default).

For more information, including performance results on Slingshot-11 and InfiniBand networks, please see our LBNL Technical Report:

• Hargrove P, Bonachea D. Investigation into the Performance Benefits of Exposing Network Backpressure in UPC++ and GASNet-EX Lawrence Berkeley National Laboratory Technical Report LBNL-2001668, May 2025. doi:10.25344/S4088R

-Paul H. Hargove

[*]  Recent updates to the GASNet-EX stable branch include

• Experimental new `gex_AM_Cancel{Request,Reply}{Medium,Long}()` APIs enable a client to abandon a Prepare (called instead of the matching Commit).

• Experimental new `gex_AM_Commit*_v2()` APIs extend the `Commit` API   family with a `commit_flags` argument and an `int` return type.  This enables use of `GEX_FLAG_IMMEDIATE` at commit time.

• ibv-conduit:

• Support for `GEX_FLAG_IMMEDIATE` with AM injection (both FPAM and NPAM) has been improved to detect full send queues and completion queues, conditions which previously could have resulted in stalls rather than immediate return.

• NEW: Support for network offload of atomic operations.  See the ibv-conduit README for more information, such as supported op/type pairs.

• Improved tracing and statistics support for the `gex_AD_*()` APIs

• Fixes for:

• bug4772 - regression in low-memory error handling with single-process runs

• bug4773 - (partial fix) improved error handling in segment allocation

• bug4774 - Add argument-checking to GASNet object query accessors

The complete list of changes since 2024.5.0 appears under the PENDING heading of the ChangeLog