HSF Software Forum on Potential Gains from Modern Hardware, 10 October, 2018
Meeting Agenda and Slides
- There is a free forum slot in 2 weeks time (24 October). Contact
Graeme if there is interest in presenting something.
- Otherwise we should start to consider presentations for next year.
Potential Gains for Software from Modern Hardware
- FullCMS example for AutoFDO is Geant4 example with the full CMS
geometry, but not an actual full simulation
- Static build is probably really needed for some of the
- Getting a large static G4 library incorporated into an ATLAS
simulation build has not been easy
- AutoFDO project itself only seems to develop slowly
- Small allocations/deallocations
- Initialisation and destruction is costly, especially for more
- Optimisations can be to use size knowledge (
arenas, simpler objects
- Try to avoid problem of breaking hotspots into multiple smaller
- Does HEPSpec model our codes well?
- Not at the instruction level!
- Geant4 is measured not to be bound by memory access speeds
- This is interaction with memory subsystem, not any new/free
- Both simulation and digi/reco show a lot of memory “port” access
and much less arithmetic than expected
- Trident can be started/stopped, but not programatically at the
- Overheads are measured and usually ~1%
- Haswell is much better in code page access, which is why the single
library shows so much less gain on this architecture than Ivy
- Jakob - scope of R&D and gradual improvements is quite different,
R&D might only target one aspect, limiting the overall impact;
where as the gradual improvements might be very generic to the
whole code base
- David L - HL-LHC simulation for CMS is projected to be only 5% of
- Reconstruction scales in a very non-linear way, but simulation
- Stefan R - but for LHCb 90% is simulation!
- ALICE clarification: x15 is speed up on a single CPU, then x10 for
GPU on top
- What is “exploiting modern CPU arch”, +100%?
- It’s re-coding for vectorisation
- Pere - but GeantV has put a lot of effort into doing this and does
not even get x2 speed up.
- Other sciences can have different problems, more amenable to
- Working often with software engineers at centres, but our
code has been quite optimised already
- LHCb got a perfect speed-up in RICH reco (x8), but this is only
one piece of the code base, so overall impact for the application
is much less
- Data structures are important
- For GeantV, need to go beyond 1-D binning
- However, data access patterns can vary through the processing
- LHC detectors are running, so we need to keep things working without
breaking as well, stopping and rewriting everything is not going
- Stewart MH has been looking at LTO in ATLAS - anyone else? Could
be an interesting topic for a future meeting