Update OpenMPI documentation #301
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preview available: https://docs.tds.cscs.ch/301 |
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preview available: https://docs.tds.cscs.ch/301 |
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preview available: https://docs.tds.cscs.ch/301 |
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Not ready (open TODOs) but open to feedback already. |
| !!! info "CXI provider does all communication through the network interface cards (NICs)" | ||
| When using the libfabric CXI provider, all communication goes through NICs, including intra-node communication. | ||
| This means that intra-node communication can not make use of shared memory optimizations and the maximum bandwidth will not be severely limited. | ||
| This means that intra-node communication can not make use of shared memory optimizations and the maximum bandwidth will be severely limited. |
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I'd move this note higher up (at the beginning of the section), so that the limitation is clear from the start, and add a link to the next section in this note. Something like "For an experimental workaround to this limitation, see 'Using the external LINKx provider'".
| The experimental [LINKx](https://ofiwg.github.io/libfabric/v2.3.1/man/fi_lnx.7.html) libfabric provider allows composing multiple libfabric providers for inter- and intra-node communication. | ||
| The CXI provider can be used for inter-node communication while the shared memory (`shm`) provider can be used to take advantage of xpmem for CPU-CPU communication and GDRCopy for GPU-GPU communication. | ||
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| !!! warning "The LINKx provider is experimental and may contain bugs, in particular for intra-node communication" |
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| !!! warning "The LINKx provider is experimental and may contain bugs, in particular for intra-node communication" | |
| !!! danger "The LINKx provider is experimental and may contain bugs, in particular for intra-node communication" |
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I may actually go the opposite direction... The LINKx provider is experimental and it was buggy until very recently. In my latest tests, however, it's actually been very good. I may be too naive in thinking that the LINKx provider is working well for most applications though. My tests are primarily OSU...
In principle I'd like to promote LINKx as the default/primary choice and point users to CXI-only as a fallback in case they encounter issues.
In the user's shoes, how upset would you be if we recommend LINKx as the first option, only for you to find out that your application crashes with it? Or conversely, how upset would you be if you play it safe with CXI as the default choice, and then you figure out later you've been leaving a lot of performance on the table by not using LINKx even though it works just fine?
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Starting to update the docs with my latest notes as the OpenMPI uenv getting closer to usable (eth-cscs/alps-uenv#263).
Still a few TODOs.