Using UPC++ on OLCF Frontier

This document is a continuous work-in-progress, intended to provide up-to-date information on a public install maintained by (or in collaboration with) the UPC++ team. However, systems are constantly changing. So, please report any errors or omissions in the issue tracker.

Typically installs of UPC++ are maintained only for the current default versions of the system-provided environment modules such as for PrgEnv, ROCm and compiler.

This document is not a replacement for the documentation provided by the centers, and assumes general familiarity with the use of the system.

General

Stable installs are available through environment modules. A wrapper is used to transparently dispatch commands such as upcxx to an install appropriate to the currently loaded PrgEnv-{gnu,cray,amd,gnu-amd,cray-amd} and compiler (gcc, cce, amd, amd-mixed) environment modules.

Environment Modules

In order to access the UPC++ installation on Frontier, one must run

$ module load ums ums014

If not adding the command to one's shell startup files, the module load ... command will be required once per login shell in which you need a upcxx environment module.

Environment modules provide two alternative configurations of the UPC++ library:

• upcxx-hip
  This module supports memory kinds, a UPC++ feature that enables communication to/from GPU memory via upcxx::copy on upcxx::global_ptr<T, memory_kind::hip_device>. When using this module, copy operations on hip_device memory leverage ROCm RDMA ("native" memory kinds).
• upcxx
  This module omits support for constructing an active upcxx::device_allocator<upcxx::hip_device> object, resulting in a small potential speed-up for applications which do not require a "HIP-aware" build of UPC++.

By default each module above will select the latest recommended version of the UPC++ library. One can see the installed versions with a command like module avail upcxx and optionally explicitly select a particular version with a command of the form: module load upcxx/20XX.YY.ZZ.

On Frontier, the UPC++ environment modules select a default network of ofi. You can optionally specify this explicitly on the compile line with upcxx -network=ofi ....

Caveats

The installs provided on Frontier utilize the Cray Programming Environment, and the cc and CC compiler wrappers in particular. It is possible to use upcxx (or CC and upcxx-meta) to link code compiled with the "native compliers" such as g++ and amdclang++ (provided they match the PrgEnv-* module). However, direct use of the native compilers to link UPC++ code is not supported with these installs.

Module name conflicts with E4S SDK

Currently the default MODULEPATH on Frontier includes center-provided E4S SDK installs of UPC++ which are not (yet) as well-integrated as the ones described here. It is currently safe to load upcxx and upcxx-hip if one wishes to use the latest installs described here (the default, and our strong recommendation). However, module load upcxx/[version] may resolve to something different than what one was expecting.

The MODULEPATH may change each time one loads a PrgEnv-* module, among others. This could silently give the E4S SDK installs precedence over the ones documented here. Consequently, it is advisable to check prior to loading a upcxx or upcxx-hip environment module, as follows. A command such as module --redirect show upcxx/2023.3.0 | grep -m1 /sw/frontier will show the full path of the modulefile which would be loaded (without making changes to ones environment). If the output does not begin with /sw/frontier/ums/ums014/modulefiles, then one should repeat module load ums014 (or simply ml ums014) to restore the precedence of the installs provided by the maintainers of UPC++.

Note that these changes to MODULEPATH are only relevant until you have loaded a UPC++ environment module.

Job launch

The upcxx-run utility provided with UPC++ is a relatively simple wrapper, which in the case of Frontier uses srun. To have full control over process placement, thread pinning and GPU allocation, users are advised to consider launching their UPC++ applications directly with srun. However, one should do so only with the upcxx or upcxx-hip environment module loaded to ensure the appropriate environment variable settings.

If you would normally have passed -shared-heap to upcxx-run, then it is particularly important that both UPCXX_SHARED_HEAP_SIZE and GASNET_MAX_SEGSIZE be set accordingly. The values of those and other potentially relevant environment variables set (or inherited) by upcxx-run can be listed by adding -show to your upcxx-run command (which will print useful information but not run anything).
Additional information is available in the Advanced Job Launch chapter of the UPC++ v1.0 Programmer's Guide.

Each Frontier node contains 64 CPU cores and 4 Slingshot-11 NICs (and 8 GPUs). Currently each UPC++ process can use at most one Slingshot NIC. In order for a job to utilize all four NICs on a Frontier node, all of the following are necessary:

1. run at least four processes per node
2. ensure each process is bound to distinct CPU cores out of the 64 available
3. set environment variables directing each process to use the NIC most appropriate to its core binding

Item 1 is up to the user, and item 2 is the default behavior on Frontier. For item 3, one should ensure the following are set in ones environment:

• GASNET_OFI_DEVICE_TYPE=Node
• GASNET_OFI_DEVICE_0=cxi2
• GASNET_OFI_DEVICE_1=cxi1
• GASNET_OFI_DEVICE_2=cxi3
• GASNET_OFI_DEVICE_3=cxi0

Those five variables are set by the upcxx and upcxx-hip environment modules, loading one of which is a "best practice" when running a UPC++ application. Care should be taken to ensure they are set even if not using the environment modules.

Single-node runs

On a system like Frontier, there are multiple complications related to launch of executables compiled for -network=smp such that no use of srun (or simple wrappers around it) can provide a satisfactory solution in general. Therefore, we recommend that for single-node (shared memory) application runs on Frontier, one should compile for the default network (ofi). Note that due to an oddity in the SLURM configuration, single-node runs on Frontier (and Crusher) need to include the non-default option srun --network=single_node_vni when launching single-node jobs.

It is alternatively acceptable to use -network=mpi, such as may be required for some hybrid applications (UPC++ and MPI in the same executable). However, note that in multi-node runs -network=mpi imposes a significant performance penalty.

Batch jobs

By default, batch jobs on Frontier inherit both $PATH and the $MODULEPATH from the environment at the time the job is submitted/requested using sbatch or salloc. So, no additional steps are needed to use upcxx-run if a upcxx environment module was loaded when sbatch or salloc ran.

Interactive example:

NOTE: this example assumes module load ums ums014 has been performed previously, such as in one's shell startup files.

frontier$ module load upcxx

frontier$ upcxx --version
UPC++ version 2023.9.0  / gex-2023.9.0-0-g5b1e532
Citing UPC++ in publication? Please see: https://upcxx.lbl.gov/publications
Copyright (c) 2023, The Regents of the University of California,
through Lawrence Berkeley National Laboratory.
https://upcxx.lbl.gov

Cray clang version 15.0.0  (324a8e7de6a18594c06a0ee5d8c0eda2109c6ac6)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/cray/pe/cce/15.0.0/cce-clang/x86_64/share/../bin

frontier$ upcxx -O hello-world.cpp -o hello-world.x

frontier$ salloc -t 5 --nodes 2
salloc: Granted job allocation 1296496
salloc: Waiting for resource configuration
salloc: Nodes frontier[01449-01450] are ready for job
frontier$ upcxx-run -n 4 -N 2 ./hello-world.x
Hello from 0 of 4
Hello from 1 of 4
Hello from 2 of 4
Hello from 3 of 4

CMake

A UPCXX CMake package is provided in the UPC++ install on Frontier, as described in README.md. Thus with the upcxx environment module loaded, CMake should "just work".

Known Issues

Correctness problems with intensive communication on HPE Slingshot-11

Currently, there are known issues with the vendor's communications software stack below UPC++ and GASNet-EX which may negatively impact certain communication-intensive UPC++ applications (e.g. those concurrently sending large numbers of RPCs to one or more processes).

Impacts observed have included crashes and hangs of correct UPC++ applications. Or course, either of those failure modes can be the result of other issues. If you believe your application is impacted, please follow the steps below.

1. Try running your application on a system with a network other than Slingshot-11 (but not Slingshot-10 which has a similar, but distinct, issue). If the failures persist, then the problem is not the one described here. You should look for defects in your application, or for other defects in UPC++ or external software.
2. If you have observed crashes, but not hangs, then try running your application with GASNET_OFI_RECEIVE_BUFF_SIZE=recv in the environment. This disables use of a feature linked to the known source of crashes, but may result in a small reduction in RPC performance.
3. If you have observed hangs, then try running your application with all of the following environment variable settings:
   GASNET_OFI_RECEIVE_BUFF_SIZE=recv
FI_OFI_RXM_RX_SIZE=8192
FI_CXI_DEFAULT_CQ_SIZE=13107200
FI_MR_CACHE_MONITOR=memhooks
FI_CXI_RX_MATCH_MODE=software
FI_CXI_REQ_BUF_MIN_POSTED=10
FI_CXI_REQ_BUF_SIZE=25165824
   These settings will have negative impact on both performance and on memory use. However, in most cases they have been seen to be sufficient to eliminate the problem(s).

If none of the options above resolves crashes or hangs of your communication-intensive UPC++ application, you can seek assistance using the issue tracker.

Information about UPC++ installs on other production systems

Please report any errors or omissions in the issue tracker.