The ICL ULFM team is happy to announce ULFM 2.0 a new implementation of the MPI extension handling faults in sync with the current Open MPI master. Innumerable new features have been added both to Open MPI and to ULFM, we will focus on this announce on the ULFM ones. For information about what is new in Open MPI please read www.open-mpi.
This implementation conforms to the User Level Failure Mitigation (ULFM) MPI Standard draft proposal. The ULFM proposal is developed by the MPI Forum’s Fault Tolerance Working Group to support the continued operation of MPI programs after crash (node failures) have impacted the execution. The key principle is that no MPI call (point-to-point, collective, RMA, IO, …) can block indefinitely after a failure, but must either succeed or raise an MPI error. This implementation produces the three supplementary error codes and five supplementary interfaces defined in the communicator section of the ULFM chapter standard draft document.
- MPIX_ERR_PROC_FAILED when a process failure prevents the completion of an MPI operation.
- MPIX_ERR_PROC_FAILED_PENDING when a potential sender matching a non-blocking wildcard source receive has failed.
- MPIX_ERR_REVOKED when one of the ranks in the application has invoked the MPI_Comm_revoke operation on the communicator.
- MPIX_Comm_revoke(MPI_Comm comm) Interrupts any communication pending on the communicator at all ranks.
- MPIX_Comm_shrink(MPI_Comm comm, MPI_Comm* newcomm) creates a new communicator where dead processes in comm were removed.
- MPIX_Comm_agree(MPI_Comm comm, int *flag) performs a consensus (i.e. fault tolerant allreduce operation) on flag (with the operation bitwise or).
- MPIX_Comm_failure_get_acked(MPI_Comm, MPI_Group*) obtains the group of currently acknowledged failed processes.
- MPIX_Comm_failure_ack(MPI_Comm) acknowledges that the application intends to ignore the effect of currently known failures on wildcard receive completions and agreement return values.
There are four main MPI network models available in Open MPI: “ob1”, “cm”,”yalla”, and “ucx”. Only “ob1” is adapted to support fault tolerance. “ob1” uses BTL (“Byte Transfer Layer”) components for each supported network. “ob1” supports a variety of networks that can be used in combination with each other. Collective operations (blocking and non-blocking) use an optimized implementation on top of “ob1”.
- Loopback (send-to-self)
- OpenFabrics: InfiniBand, iWARP, and RoCE
- uGNI (Cray Gemini, Aries)
- Shared memory Vader (FT supported w/CMA, XPmem, KNEM untested)
- Tuned, and non-blocking collective communications
A full list of supported, untested and disabled components is provided below.
If you are looking for, or want to cite a general reference for ULFM, please use
Wesley Bland, Aurelien Bouteiller, Thomas Herault, George Bosilca, Jack J. Dongarra: Post-failure recovery of MPI communication capability: Design and rationale. IJHPCA 27(3): 244-254 (2013). Available from: http://journals.sagepub.com/doi/10.1177/1094342013488238.
Building ULFM Open MPI
./configure --with-ft [...options...]
# use --with-ft (default) to enable ULFM,
make [-j N] all install
# use an integer value of N for parallel builds
There are many available configure options (see ./configure --help for a full list); a summary of the more commonly used ones is included in the upstream Open MPI README file. The following paragraph gives a summary of ULFM Open MPI specific options behavior.
- --with-ft=TYPE Specify the type of fault tolerance to enable. Options: mpi (ULFM MPI draft standard). Fault tolerance support is enabled by default(as if --with-ft=mpi were implicitly present on the configure line).You may specify --without-ft to compile an almost stock Open MPI.
- --with-platform=FILE Load configure options for the build from FILE. When --with-ft=mpi is set, the file contrib/platform/ft_mpi_ulfm is loaded by default. This file disables components that are known to not be able to sustain failures, or are insufficiently tested. You may edit this file and/or force back these options on the command line to enable these components.
- --enable-mca-no-build=LIST Comma-separated list of – pairs that will not be built. For example, --enable-mca-no-build=btl-portals,oob-ud will disable building the portals BTL and the ud OOB component. When --with-ft=mpi is set, this list is populated with the content of the aforementioned platform file. You may override the default list with this parameter.
- --with-pmi and --with-slurm Force the building of SLURM scheduler support. Slurm with fault tolerance is tested. Do not use srun, otherwise your application gets killed by the RM upon the first failure. Instead, use mpirun in an salloc/sbatch.
- --with-sge This is untested with fault tolerance.
- --with-tm Force the building of PBS/Torque scheduler support. PBS is tested with fault tolerance. Use mpirun in a qsub allocation.
- --disable-mpi-thread-multiple Disable the MPI thread level MPI_THREAD_MULTIPLE (it is enabled by default). Multiple threads with fault tolerance is lightly tested.
- --disable-oshmem Disable building the OpenSHMEM implementation (by default, it is enabled). ULFM Fault Tolerance does not apply to OpenSHMEM.
Modified, Untested and Disabled Components
Frameworks and components which are not listed in the following list are unmodified and support fault tolerance. Listed frameworks may be modified(and work after a failure), untested (and work before a failure, but may malfunction after a failure), or disabled (they prevent the fault tolerant components from operating properly).
- pml MPI point-to-point management layer
- “ob1” modified to handle errors
- “monitoring”, “v” unmodified, untested
- “bfo”, “cm”, “crcpw”, “ucx”, “yalla” disabled
- btl Point-to-point Byte Transfer Layer
- “openib”, “tcp”, “vader(+cma)” modified to handle errors (removed unconditional abort on error, expect performance similar to upstream)
- “portals4”, “scif”, “smcuda”, “usnic”, “vader(+knem,+xpmem)” unmodified, untested (may work properly, please report)
- mtl Matching Transport Layer used for MPI point-to-point messages on some types of networks
- All “mtl” components are disabled
- coll MPI collective algorithms
- “tuned”, “basic”, “nbc” modified to handle errors
- “cuda”, “fca”, “hcoll”, “portals4” unmodified, untested (expect unspecified post-failure behavior)
- osc MPI one-sided communications
- Unmodified, untested (expect unspecified post-failure behavior)
- io MPI I/O and dependent components
- fs File system functions for MPI I/O
- fbtl File byte transfer layer: abstraction for individual read/write operations for OMPIO
- fcoll Collective read and write operations for MPI I/O
- sharedfp Shared file pointer operations for MPI I/O
- All components in these frameworks are unmodified, untested (expect clean post-failure abort)
- vprotocol Checkpoint/Restart components
- “pml-v”, “crcp” unmodified, untested
- wait_sync Multithreaded wait-synchronization object
- modified to handle errors (added a global interrupt to trigger all wait_sync objects)
Running ULFM Open MPI
Building your application
As ULFM is an extension to the MPI standard, you will need to #include "mpi-ext.h" in C, or use mpi_ext in Fortran to access the supplementary error codes and functions. Compile your application as usual, using the provided mpicc, mpif90, or mpicxx wrappers.
Running your application
You can launch your application with fault tolerance by simply using the provided mpiexec. Beware that your distribution may already provide a version of MPI, make sure to set your PATH and LD_LIBRARY_PATH properly. Note that fault tolerance is enabled by default in ULFM Open MPI; you can disable all fault tolerance capabilities by launching your application with mpiexec --disable-recovery.
Running under a batch scheduler
ULFM can operate under a job/batch scheduler, and is tested routinely with both PBS and Slurm. One difficulty comes from the fact that many job schedulers will “cleanup” the application as soon as a process fails. In order to avoid this problem, it is preferred that you use mpiexec within an allocation (e.g. salloc, sbatch, qsub) rather than a direct launch (e.g. srun).
Run-time tuning knobs
ULFM comes with a variety of knobs for controlling how it runs. The default parameters are sane and should result in good performance in most cases. You can change the default settings with --mca mpi_ft_foo.
- orte_enable_recovery <true|false> (default: true) controls automatic cleanup of apps with failed processes within mpirun. The default differs from upstream Open MPI.
- mpi_ft_enable <true|false> (default: true) permits turning off fault tolerance without recompiling. Failure detection is disabled. Interfaces defined by the fault tolerance extensions are substituted with corresponding non-fault tolerant implementations (e.g. MPI_Allreduce is substituted to MPIX_Comm_agree).
- mpi_ft_verbose (default: 0) increases the output of the fault tolerance activities. A value of 1 will report detected failures.
- mpi_ft_detector <true|false> (default: false) controls the activation of the OMPI level failure detector. When this detector if turned off, all failure detection is delegated to ORTE, which may be slow and/or incomplete.
- mpi_ft_detector_thread <true|false> (default: false) controls the use of a thread to emit and receive failure detector’s heartbeats. Setting this value to “true” will also set MPI_THREAD_MULTIPLE support, which has a noticeable effect on latency. You may want to enable this option if you experience false positive processes incorrectly reported as failed.
- mpi_ft_detector_period (default: 1e-1 seconds) heartbeat period. Recommended value is 1/3 of the timeout. Values lower than 100us may impart a noticeable effect on latency (typically a 3us increase).
- mpi_ft_detector_timeout (default: 3e-1 seconds) heartbeat timeout (i.e. failure detection speed). Recommended value is 3 times the heartbeat period.
Known Limitations in ULFM-2.0-rc (2e75c73):
- TOPO, FILE, RMA have little support for fault tolerance mechanisms.
- There is a tradeoff between failure detection accuracy and performance. The current default is to favor performance. Users that experience detection accuracy issues may enable a more precise mode.
- The failure detector operates on MPI_COMM_WORLD exclusively. Processes connected from MPI_COMM_CONNECT/ACCEPT and MPI_COMM_SPAWN may occasionally not be detected when they fail.
- Failures during NBC collective may not be recovered properly in some cases.
- Return of OpenIB credits spent toward a failed process can take several seconds. Until the
btl_openib_ib_retry_countcontrolled timeout triggers, lack of send credits may cause a temporary stall under certain communication patterns. Impacted users can try to adjust the value of these mca parameters.
Focus has been toward integration with current Open MPI master, performance, and stability.
- ULFM is now based upon Open MPI master branch (#689f1be9). It will be regularly updated until eventually merged.
- Fault Tolerance is enabled by default and is controlled with MCA variables.
- Added support for multithreaded modes (MPI_THREAD_MULTIPLE, etc.)
- Added support for non-blocking collective operations (NBC).
- Added support for CMA shared memory transport (Vader).
- Added support for advanced failure detection at the MPI level. Implements the algorithm described in “Failure detection and propagation in HPC systems.” https://doi.org/10.1109/SC.2016.26.
- Removed the need for special handling of CID allocation.
- Non-usable components are automatically removed from the build during configure
- RMA, FILES, and TOPO components are enabled by default, and usage in a fault tolerant execution warns that they may cause undefined behavior after a failure.
- Code cleanup and performance cleanup in non-FT builds; –without-ft at configure time gives an almost stock Open MPI.
- Code cleanup and performance cleanup in FT builds with FT runtime disabled; –mca ft_enable_mpi false thoroughly disables FT runtime activities.
- Some error cases would return ERR_PENDING instead of ERR_PROC_FAILED in collective operations.
- Some test could set ERR_PENDING or ERR_PROC_FAILED instead of ERR_PROC_FAILED_PENDING for ANY_SOURCE receptions.
Focus has been toward improving stability, feature coverage for intercomms, and following the updated specification for MPI_ERR_PROC_FAILED_PENDING.
- Forked from Open MPI 1.5.5 devel branch
- Addition of the MPI_ERR_PROC_FAILED_PENDING error code, as per newer specification revision. Properly returned from point-to-point, non-blocking ANY_SOURCE operations.
- Alias MPI_ERR_PROC_FAILED, MPI_ERR_PROC_FAILED_PENDING and MPI_ERR_REVOKED to the corresponding standard blessed -extension- names MPIX_ERR_xxx.
- Support for Intercommunicators:
- Support for the blocking version of the agreement, MPI_COMM_AGREE on Intercommunicators.
- MPI_COMM_REVOKE tested on intercommunicators.
- Disabled completely (.ompi_ignore) many untested components.
- Changed the default ORTE failure notification propagation aggregation delay from 1s to 25ms.
- Added an OMPI internal failure propagator; failure propagation between SM domains is now immediate.
- SendRecv would not always report MPI_ERR_PROC_FAILED correctly.
- SendRecv could incorrectly update the status with errors pertaining to the Send portion of the Sendrecv.
- Revoked send operations are now always completed or remote cancelled and may not deadlock anymore.
- Cancelled send operations to a dead peer will not trigger an assert when the BTL reports that same failure.
- Repeat calls to operations returning MPI_ERR_PROC_FAILED will eventually return MPI_ERR_REVOKED when another process revokes the communicator.
Focus has been toward improving performance, both before and after the occurrence of failures. The list of new features includes:
- Support for the non-blocking version of the agreement, MPI_COMM_IAGREE.
- Compliance with the latest ULFM specification draft. In particular, the MPI_COMM_(I)AGREE semantic has changed.
- New algorithm to perform agreements, with a truly logarithmic complexity in number of ranks, which translates into huge performance boosts in MPI_COMM_(I)AGREE and MPI_COMM_SHRINK.
- New algorithm to perform communicator revocation. MPI_COMM_REVOKE performs a reliable broadcast with a fixed maximum output degree, which scales logarithmically with the number of ranks.
- Improved support for our traditional network layer:
- TCP: fully tested
- SM: fully tested (with the exception of XPMEM, which remains unsupported)
- Added support for High Performance networks
- Open IB: reasonably tested
- uGNI: reasonably tested
- The tuned collective module is now enabled by default (reasonably tested), expect a huge performance boost compared to the former basic default setting
- Back-ported PBS/ALPS fixes from Open MPI
- Back-ported OpenIB bug/performance fixes from Open MPI
- Improve Context ID allocation algorithm to reduce overheads of Shrink
- Miscellaneous bug fixes
Version Numbers and Binary Compatibility
Starting from ULFM Open MPI version 2.0, ULFM Open MPI is binary compatible with the corresponding Open MPI master branch and compatible releases (see the binary compatibility and version number section in the upstream Open MPI README). That is, applications compiled with a compatible Open MPI can run with the ULFM Open MPI mpirun and MPI libraries. Conversely, as long as the application does not employ one of the MPIX functions, which are exclusively defined in ULFM Open MPI, an application compiled with ULFM Open MPI can be launched with a compatible Open MPI mpirun and run with the non-fault tolerant MPI library.
Contacting the Authors
Found a bug? Got a question? Want to make a suggestion? Want to contribute to ULFM Open MPI? Working on a cool use-case? Please let us know! The best way to report bugs, send comments, or ask questions is to sign up on the user’s mailing list: firstname.lastname@example.org Because of spam, only subscribers are allowed to post to these lists (ensure that you subscribe with and post from exactly the same e-mail address — email@example.com is considered different firstname.lastname@example.org!). Visit these pages to subscribe to the lists: https://groups.google.com/forum/#!forum/ulfm When submitting questions and problems, be sure to include as much information as possible. This web page details all the information that we request in order to provide assistance: http://www.open-mpi.org/community/help/
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