gpl: slowdown when activating multi-threading

[gudeh]

Description

This issue was discovered while addressing non-determinism with GPL when multi-threading was activated (https://github.com/The-OpenROAD-Project/OpenROAD/issues/5360).

I tested the GPL runtime using secure-CI (multiple designs) and locally via a GCP server with Nangate45/Swerv. The tests indicate that activating multi-threading results in the same runtime or, in some cases, even slows down GPL.

I believe the slowdown is due to too many context switches with minimal gain in parallelism, as the loops that use multi-threading typically perform simple mathematical operations.

• It was mentioned that the OMP parallel loops inserted in GPL were implemented by Antmicro developers, who claimed they improved runtime. TODO: Investigate the code added using GitHub blame; are there any runtime reports available?
• Investigate which loops are causing the slowdown. Measure runtime.

Suggested Solution

No response

Additional Context

No response

[maliberty]

I suggest you look at the PR as they often include data there. There may have been multiple changes.

[gudeh]

This is the PR: https://github.com/The-OpenROAD-Project/OpenROAD/pull/4580.

Hi @kbieganski, do you still see improvements in runtime for gpl with MT?

[QuantamHD]

It could be the case that the MT only helps on larger designs, and is a net negative on designs below a certain size threshold.