Using UPC++ on OLCF Summit

The UPC++ team does not anticipate having access to Summit in 2024.
Consequently, the installs made in December 2023 will not be maintained.
Reports regarding the status of UPC++ on Summit may be submitted to the issue tracker (see below), but may only result in updates to this page.

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 compiler and CUDA.

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 compiler environment module.

Environment Modules

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

$ module load ums 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-cuda
  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::cuda_device>. When using this module, copy operations on cuda_device memory leverage GPUDirect RDMA ("native" memory kinds). This module supports the gcc and pgi compiler families.
• upcxx
  This module omits support for constructing an active upcxx::device_allocator<upcxx::cuda_device> object, resulting in a small potential speed-up for applications which do not require a "CUDA-aware" build of UPC++. This module supports the gcc and pgi compiler families.

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

Caveats

No support for IBM XL compilers

Please note that UPC++ does not yet work with the IBM XL compilers (the default compiler family on Summit).

GPU modes and process GPU sharing

ORNL defaults summit GPUs to EXCLUSIVE mode, unlike most other sites. In this mode a given physical GPU can only be opened by one process at a time, and concurrent open attempts of the same GPU by other processes will lead to a runtime error. This policy can sometimes be helpful to enforce a job layout designed for a 1:1 mapping between processes and GPUs, but can be problematic for other layouts.

If you ever want to deliberately share GPUs between processes we recommend using bsub -alloc_flags gpudefault to restore NVIDIA's default behavior that allows multiple processes to share one GPU. Another possibility is bsub -alloc_flags gpumps which activates NVIDIA's MPS service that performs more fine-grained interleaving of kernels from multiple processes to improve GPU throughput, but beware the MPS service has occasionally been implicated in stability problems (e.g. leading to job hangs).

Module name conflicts with E4S SDK

Currently the default MODULEPATH on Summit 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-cuda 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 gcc 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 environment module, as follows. A command such as module --loc show upcxx/2022.9.0 will show the full path which would be loaded (without making changes to ones environment). If the result does not begin with /sw/summit/modulefiles/ums/ums014, then one should repeat module load ums-ums014 (or simply ml ums-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 around the jsrun job launcher on Summit. The majority of the resource allocation/placement capabilities of jsrun have no equivalent in upcxx-run. So due to the complexity of a Summit compute node, we strongly discourage use of upcxx-run for all but the simplest cases. This is especially important when using GPUs, since it is impractical to coerce upcxx-run to pass the appropriate arguments to jsrun on your behalf.

Instead of using upcxx-run or jsrun for job launch on Summit, we recommend use of the upcxx-jsrun script we have provided. This script wraps jsrun to set certain environment variables appropriate to running UPC++ applications, and to accept additional (non-jsrun) options which are specific to UPC++ or which automate otherwise error-prone settings. Other than --help, which upcxx-jsrun acts on alone, all jsrun options are available via upcxx-jsrun with some caveats noted in the paragraphs which follow.

Here are some of the most commonly used upcxx-jsrun command-line options.
Run upcxx-jsrun --help for a more complete list.

• --shared-heap VAL
  Behaves just as with upcxx-run

• --1-hca
  Binds each process to one HCA (default)

• --2-hca
  Binds each process to two HCAs

• --4-hca
  Binds each process to all four HCAs

• --high-bandwidth
  Binds processes to the network interfaces appropriate for highest bandwidth

• --low-latency
  Binds processes to the network interfaces appropriate for lowest latency

• --by-gpu[=N]
  Create/bind processes into Resource Set by GPU Creates N processes (default 7), bound to 7 cores of one socket, with 1 GPU

• --by-socket[=N]
  Create/bind processes into Resource Set by socket Creates N processes (default 21), bound to one socket, with 3 GPUs

The section Network ports on Summit provides a description of the four HCAs on a Summit node, and how they are connected to the two sockets. The "hca" and "latency/bandwidth" options are provided to simplify the process of selecting a good binding of processes to HCAs. This is probably the most important role of upcxx-jsrun, because there are no equivalent jsrun options.

With --1-hca (the default), each process will be bound to a single HCA which is near to the process.

When --2-hca is passed, each process will be bound to two HCAs. The --high-bandwidth and --low-latency options determine which pairs of HCAs are selected. Between these two option, the high-bandwidth option is the default because it corresponds to the most common case in which use of two HCAs per process is preferred over one (as will be described below).

When --4-hca is passed, each process is bound to all four HCAs. This option is included only for completeness and generally provides worse performance than the alternatives.

The default of --1-hca has been selected because our experience has found the use of a single HCA per process to provide the best latency and bandwidth for a wide class of applications. The only notable exception is applications which desire to saturate both network rails from a single socket at a time, such as due to communication in "bursts" or use of only one socket (or process) per node. In such a case, we recommend passing --2-hca (with the default --high-bandwidth) in order to enable each process to use both I/O buses and network rails. However, this can increase latency and reduce the peak aggregate bandwidth of both sockets communicating simultaneously.

Of course, "your mileage may vary" and you are encouraged to try non-default options to determine which provide the best performance for your application.

For many combinations of the options above, there are multiple equivalent bindings available (such as two HCAs near to each socket in the --1-hca case). When multiple equivalent bindings exist, processes will be assigned to them round-robin.

In addition to the options described above for HCA binding, there are --by-gpu and --by-socket options to simplify construction of two of the more common cases of resource sets. Use of either is entirely optional, but in their absence be aware that jsrun defaults to a single CPU core per resource set. If you do choose to use them, be aware that they are mutually exclusive and that they are implemented using the following jsrun options: --rs_per_host, --cpu_per_rs, --gpu_per_rs, --tasks_per_rs, --launch_distribution and -bind. So, use of the --by-* options may interact in undesired ways with explicit use of those options and with any options documented as conflicting with them.

To become familiar with use of jsrun on Summit, you should read the Summit User Guide. Other than --help, which upcxx-jsrun acts on alone, all jsrun options are available via upcxx-jsrun with the caveats noted above.

Advanced use of upcxx-jsrun

If you need to use upcxx-run options not accepted by upcxx-jsrun, then it may be necessary to set environment variables to mimic those options. To do so, follow the instructions on launch of UPC++ applications using a system-provided "native" spawner, in the section Advanced Job Launch in the UPC++ Programmer's Guide.

If you need to understand the operation of upcxx-jsrun, the --show and --show-full options may be of use. Passing either of these options will echo (a portion of) a jsrun command rather than executing it. The use of --show will print the jsrun command and its options, eliding the UPC++ executable and its arguments. This is sufficient to understand the operation of the --by-* options.

The --*-hca and --shared-heap options are implemented in two steps where the second is accomplished by specifying upcxx-jsrun as the process which jsrun should launch. The "front end" instance of upcxx-jsrun passes arguments to the multiple "back end" instances of upcxx-jsrun using environment variables. Use of --show-full adds the relevant environment settings to the --show output. However, be advised that there is no guarantee this environment-based internal interface will remain fixed.

If you wish to determine the actual core bindings and GASNET_IBV_PORTS assigned to each process for a given set of [options] one can run the following:

upcxx-jsrun --stdio_mode prepended [options] \
  -- sh -c 'echo HCAs=$GASNET_IBV_PORTS host=$(hostname) cores=$(hwloc-calc --whole-system -I core $(hwloc-bind --get))' | sort -V

1: 0: HCAs=mlx5_0+mlx5_3 host=d04n12 cores=0,1,2,3,4,5,6
1: 1: HCAs=mlx5_1+mlx5_2 host=d04n12 cores=7,8,9,10,11,12,13
1: 2: HCAs=mlx5_0+mlx5_3 host=d04n12 cores=14,15,16,17,18,19,20
1: 3: HCAs=mlx5_1+mlx5_2 host=d04n12 cores=22,23,24,25,26,27,28
1: 4: HCAs=mlx5_0+mlx5_3 host=d04n12 cores=29,30,31,32,33,34,35
1: 5: HCAs=mlx5_1+mlx5_2 host=d04n12 cores=36,37,38,39,40,41,42
1: 6: HCAs=mlx5_0+mlx5_3 host=h35n08 cores=0,1,2,3,4,5,6
1: 7: HCAs=mlx5_1+mlx5_2 host=h35n08 cores=7,8,9,10,11,12,13
1: 8: HCAs=mlx5_0+mlx5_3 host=h35n08 cores=14,15,16,17,18,19,20
1: 9: HCAs=mlx5_1+mlx5_2 host=h35n08 cores=22,23,24,25,26,27,28
1: 10: HCAs=mlx5_0+mlx5_3 host=h35n08 cores=29,30,31,32,33,34,35
1: 11: HCAs=mlx5_1+mlx5_2 host=h35n08 cores=36,37,38,39,40,41,42

Single-node runs

On a system configured as Summit has been, there are multiple complications related to launch of executables compiled for -network=smp such that no use of jsrun (or simple wrappers around it) can provide a satisfactory solution in general. Therefore, the provided installations on Summit do not support -network=smp. We recommend that for single-node (shared memory) application runs on Summit, one should compile for the default network (ibv). It is also 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 Summit inherit both $PATH and the $MODULEPATH from the environment at the time the job is submitted using bsub. So, no additional steps are needed in batch jobs using upcxx-jsrun if a upcxx or upcxx-cuda environment module was loaded when the job was submitted.

Interactive example

NOTE: this example assumes module load ums ums-ums014 is performed in one's shell startup files.

summit$ module load gcc   # since default `xl` is not supported

summit$ module load upcxx-cuda

summit$ upcxx -V
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

g++ (GCC) 9.1.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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

summit$ bsub -W 5 -nnodes 2 -P [project] -Is bash
Job <714297> is submitted to default queue <batch>.
<<Waiting for dispatch ...>>
<<Starting on batch2>>

bash-4.2$ upcxx-jsrun --by-socket=1 ./hello-world.x
Hello world from process 0 out of 4 processes
Hello world from process 2 out of 4 processes
Hello world from process 3 out of 4 processes
Hello world from process 1 out of 4 processes

CMake

A UPCXX CMake package is provided in the UPC++ install on Summit, as described in README.md. CMake is available on Summit via module load cmake. With the upcxx and cmake environment modules both loaded, CMake will additionally require either CXX=mpicxx in the environment or -DCMAKE_CXX_COMPILER=mpicxx on the command line.

Support for GPUDirect RDMA

The default version of the upcxx-cuda environment module (but not the upcxx one) includes support for the GPUDirect RDMA (GDR) capabilities of the GPUs and InfiniBand hardware on Summit. This enables communication to and from GPU memory without use of intermediate buffers in host memory. This delivers significantly faster GPU memory transfers via upcxx::copy() than previous releases without GDR support. However, there are currently some outstanding known issues.

The upcxx-cuda environment module will initialize your environment with settings intended to provide correctness by default, compensating for the known issues in GDR support. This is true even where this may come at the expense of performance. At this time we strongly advise against changing any GASNET_* or UPCXX_* environment variables set by the upcxx-cuda environment module unless you are certain you know what you are doing. (Running module show upcxx-cuda will show what it sets).

Network ports on Summit

Node Architecture of OLCF Summit, highlighting component connections
Image credit: OLCF Summit User Guide

Each Summit compute node has two CPU sockets, each with its own I/O bus. Each I/O bus has a connection to the single InfiniBand Host Channel Adapter (HCA). The HCA is connected to two "rails" (network ports). This combination of two I/O buses and two network rails results in four distinct paths between memory and network. The software stack exposes these paths as four (logical) HCAs named mlx5_0 through mlx5_3.

Which HCAs are used in a UPC++ application is determined at run time by the GASNET_IBV_PORTS family of environment variables. Which ports are used can have a measurable impact on network performance, but unfortunately there is no "one size fits all" optimal setting. For instance, the lowest latency is obtained by having each process use only one HCAs on the I/O bus of the socket where it is executing. Meanwhile, obtaining the maximum bandwidth of a given network rail from a single socket requires use of both I/O buses.

More information can be found in slides which describe the node layout from the point of view of running MPI applications on Summit. However, the manner in which MPI and UPC++ use multiple HCAs differs, which accounts for small differences between those recommendations and the settings used by upcxx-jsrun and described below.

Use of the appropriate options to the upcxx-jsrun script will automate setting of the GASNET_IBV_PORTS family of environment variables to use the recommended HCA(s). However, the following recommendations may be used if for some reason one cannot use the upcxx-jsrun script.

Similar to the MPI environment variables described in those slides, a _1 suffix on GASNET_IBV_PORTS specifies the value to be used for processes bound to socket 1. While one can set GASNET_IBV_PORTS_0 for processes bound to socket 0, below we will instead use the un-suffixed variable GASNET_IBV_PORTS because it specifies a default to be used not only for socket 0 (due to the absence of a GASNET_IBV_PORTS_0 setting), but for unbound processes as well.

• Processes each bound to a single socket -- latency-sensitive.
  To get the best latency from both sockets requires use only one HCA, attached to the I/O bus nearest to the socket.

  • GASNET_IBV_PORTS=mlx5_0
  • GASNET_IBV_PORTS_1=mlx5_3

• Processes each bound to a single socket -- bandwidth-sensitive.
  How to get the full bandwidth from both sockets depends on the communication behaviors of the application. If both sockets are communicating at the same time, then the latency-optimized settings immediately above are typically sufficient to achieve peak aggregate bandwidth. However, if a single communicating socket (at a given time) is to achieve the peak bandwidth a different pair of process-specific settings is required (which comes at the cost of slightly increased mean latency).

  • GASNET_IBV_PORTS=mlx5_0+mlx5_3
  • GASNET_IBV_PORTS_1=mlx5_1+mlx5_2

• Processes each bound to a single socket -- mixed or unknown behavior.
  In general, the use of a single HCA is the best option in terms of the minimum latency and peak aggregate (per-node) bandwidth. For this reason the latency-optimizing settings (presented first) are the nearest thing to a "generic" application recommendation.

• Processes unbound or individually spanning both sockets.
  In this case is it difficult to make a good recommendation since any given HCA has a 50/50 chance of being distant from the socket on which a given process is executing. The best average performance comes from "splitting the pain" using two of the available paths per process with one near to each socket, and together spanning both network rails. This leads to the same settings (presented second) as recommended for achieving peak bandwidth from a single socket at a time.

Correctness with multiple HCAs

The use of multiple HCAs per node will typically open the possibility of a corner-case correctness problem, for which the recommended work-around is to set GASNET_USE_FENCED_PUTS=1 in ones environment. This is done by default in the upcxx and upcxx-cuda environment modules. However, if you launch UPC++ applications without the module loaded, we recommend setting this yourself at run time.

The issue is that, by default, the use of multiple network paths may permit an rput which has signaled operation completion to be overtaken by a subsequently issued rput, rget or rpc. When an rput is overtaken by another rput to the same location, the earlier value may be stored rather than the latter. When an rget overtakes an rput targeting the same location, it may fail to observe the value stored by the rput. When an rpc overtakes an rput, CPU accesses to the location targeted by the rput is subject to both of the preceding problems. Setting GASNET_USE_FENCED_PUTS=1 prevents this overtaking behavior, in exchange for a penalty in both latency and bandwidth. However, the bandwidth penalty is tiny when compared to the increase due to using multiple HCAs to access both network rails and/or I/O buses.

If you believe your application is free of the X-after-rput patterns described above, you may consider setting GASNET_USE_FENCED_PUTS=0 in your environment at run time. However, when choosing to do so one should be prepared to detect the invalid results which may result if such patterns do occur.

For more details, search for GASNET_USE_FENCED_PUTS in the ibv-conduit README

Information about UPC++ installs on other production systems

Please report any errors or omissions in the issue tracker.