Skip to content

Update known Cray MPICH issues #256

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 3 commits into from
Sep 8, 2025
Merged

Update known Cray MPICH issues #256

Changes from 2 commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
937582e
Add alternative workaround for Cray MPICH hangs
msimberg Sep 5, 2025
f2c12c5
Add workaround for slow intra-node communication with Cray MPICH
msimberg Sep 5, 2025
8875a49
Apply suggestion from @msimberg
msimberg Sep 5, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
31 changes: 31 additions & 0 deletions docs/software/communication/cray-mpich.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,12 @@ Cray MPICH may sometimes hang on larger runs.
export FI_MR_CACHE_MONITOR=disabled
```

The option
```bash
export FI_MR_CACHE_MONITOR=userfaultfd
```
may also avoid hangs, and typically performs better than completely disabling the cache monitor.

Performance may be negatively affected by this option.

#### `"cxil_map: write error"` when doing inter-node GPU-aware MPI communication
Expand All @@ -88,6 +94,31 @@ Note that this has a performance impact for small message sizes, so it should on
export FI_CXI_SAFE_DEVMEM_COPY_THRESHOLD=0
```

[](){#ref-communication-cray-mpich-slow-intranode}
#### Slow intra-node host communication with Cray MPICH

Cray MPICH can perform badly when doing intra-node CPU-CPU memory communication.

!!! info "Workaround"
In some situations Cray MPICH can perform better when communication is done over the NICs, even within a node.
To force Cray MPICH to use NICs for all communication, set:

```bash
export MPIR_CVAR_NO_LOCAL=1
```

Whenever possible, prefer using GPU-GPU communication instead of CPU-CPU communication.
It can even be beneficial to transfer data to the GPU only for the communication even if the buffer originally is in CPU memory.

#### `"cxil_map: write error"` when doing inter-node GPU-aware MPI communication

This error message is sometimes triggered by applications that use GPU Direct MPI calls when they trigger a bug in gdrcopy (a low-level library used to copy buffers between GPUs).
Setting the following option will completely disable gdrcopy.
Note that this has a performance impact for small message sizes, so it should only be enabled on a case-by-case basis.
```bash
export FI_CXI_SAFE_DEVMEM_COPY_THRESHOLD=0
```

### Resolved issues

#### `"cxil_map: write error"` when doing inter-node GPU-aware MPI communication
Expand Down
Loading