# Agenda for XALT Sept 22, 2022 at 10:00 U.S. Central (15:00 UTC)

- Q & A
- Based on Discussion at last meeting the rule about system user accounts vs. regular users has changed
- Now using UID_MIN from /etc/login.defs or 500 otherwise
- Changed the way that setting LD_PRELOAD works inside sh_src/ld.in
- Removed unsetting LD_PRELOAD at top of sh_src/ld.in
- Now adding LD_PRELOAD= in front of every command *EXCEPT* the true ld linker commands
- How subprocesses from MPI executions work
- Presentation on how XALT finds the compiler name

## Attending

- Robert McLay (TACC)
- Kenneth Hoste (HPC-UGent)
- Ben Fulton (Indiana University)
- Brian Toonen (Argonne National Lab)
- Lisa Childers (Argonne National Lab)
- Amit Ruhela (TACC)

## Notes

- How subprocesses from MPI executions work?
- subprocesses ran by rank 0 are always recorded
- unclear how this can be avoided
- Ben: any run with more than 2 MPI processes was being recorded (default in XALT)
- TACC sets MPI_ALWAYS_RECORD to 128 to only record large MPI jobs
- all processes being run by interactive sessions with 4 ranks were being recorded by XALT because of this
- Brian: Any experience with MPMD (?) workloads, where some ranks run one binary, others run another binary?
- Brian: Is XALT trapping mpiexec, or just the processes being forked by it?
- The latter.
- XALT does "see" mpiexec being run (but it's usually filtered out via XALT configuration)
- could figure out more by parsing mpirun arguments, but then that'll cost time and add to overhead of XALT
- or could leverage MPI debugger interface (at least available in MPICH)
- Yes, XALT does work with Rust (and Robert knows why)
- Presentation by Amit
- slides available at https://github.com/xalt/xalt/blob/master/my_docs/22/xalt_monthly_mtg_2022_09_22/presentation.pdf
- what program that calls the linker determines what XALT thinks the programming language is (C, C++, Fortran, ...)
- extract_linker tool should also be aware of
- ld.bfd
- aarch64 linker command (ARM)
- mold (see https://github.com/rui314/mold)

## Next meeting

- Thu 27 Oct 2022 at 10:00 Central US time