LP parallel mode config (via CLI)

[NPC]

Version: v1.15.0
Call method: highs.exe
Solver: LP
OS: Windows 11

Hello, I'm trying to configure and toggle LP parallel mode via command-line, using guidance from this comment, and have the following observations:

1. simplex_strategy — no CLI parameter for this, only the settings file, right?

2. When I set simplex_strategy = 3 (corresponding to kSimplexStrategyDualMulti) in the options file, the command line parameter --parallel on seems to be ignored whether it's present, absent, or "off". The timing is the same (parallel is faster than serial LP for me), and the log shows Using parallel dual simplex solver - PAMI with concurrency of 8 in both cases.

   My main question is how to properly toggle parallel/serial mode via the command-line now. If --parallel off (or its omission) worked it would be clear, but for now I seem to be locked into parallel via simplex_strategy=3, and no way to affect it from CLI.

   I need to toggle these programmatically, so editing the options file on the fly is my very last resort.

3. I also tested the new --threads 1 parameter — this is great, THANK YOU! The performance is affected (as expected), but, a minor feedback, the log may be misleading, as it gives a warning but then still states concurrency of 8 afterwards:

WARNING: Number of threads available = 1 < 8 = Simplex concurrency to be used: Parallel performance may be less than anticipated
Using parallel dual simplex solver - PAMI with concurrency of 8

PS May I suggest a minor improvement to Highs.log — when adding to an already non-empty file, it would be useful to add a divider (e.g. a few empty rows) before each new run. And add a timestamp (date-time) around the run title (Running HiGHS… etc).

[jajhall]

1. simplex_strategy — no CLI parameter for this, only the settings file, right?

Correct

2. When I set simplex_strategy = 3 (corresponding to kSimplexStrategyDualMulti) in the options file, the command line parameter --parallel on seems to be ignored whether it's present, absent, or "off". The timing is the same (parallel is faster than serial LP for me), and the log shows Using parallel dual simplex solver - PAMI with concurrency of 8 in both cases.

Yes, you've selected a parallel simplex algorithm, so the parallel option is redundant

My main question is how to properly toggle parallel/serial mode via the command-line now. If --parallel off (or its omission) worked it would be clear, but for now I seem to be locked into parallel via simplex_strategy=3, and no way to affect it from CLI.
I need to toggle these programmatically, so editing the options file on the fly is my very last resort.

For simplex we decided not to switch to one of the parallel algorithms when parallel was "on", requiring the user to select a parallel algorithm explicitly. Thanks to the parallel MIP solver, more people will be using the parallel option, and we didn't want the parallel dual simplex solver to be used inadvertently since it's less robust.

3. I also tested the new --threads 1 parameter — this is great, THANK YOU! The performance is affected (as expected), but, a minor feedback, the log may be misleading, as it gives a warning but then still states concurrency of 8 afterwards:

WARNING: Number of threads available = 1 < 8 = Simplex concurrency to be used: Parallel performance may be less than anticipated
Using parallel dual simplex solver - PAMI with concurrency of 8

Concurrency refers to the nature of the parallel dual simplex algorithm, not its execution. Hence, when using --threads=1 you'll get the same numerical behaviour as with the default value for threads, but the algorithm will be executed serially so, typically, will be slower than the pure serial algorithm. I agree that this is unclear, but it's hard to express succinctly in the log. I'll add more to the documentation.

PS May I suggest a minor improvement to Highs.log — when adding to an already non-empty file, it would be useful to add a divider (e.g. a few empty rows) before each new run. And add a timestamp (date-time) around the run title (Running HiGHS… etc).

I see that this is done in commercial solvers, so we'll discuss adding it.

[NPC]

@jajhall thank you for the answers! I'm trying to understand what's the best approach in my situation (LP, control via CLI, parallel preferred but single-thread sometimes required for stability).

My current plan is:

• Permanently set simplex_strategy = 3 in the options file (since I can't easily toggle it anymore)
• When single-thread is required, pass --threads 1 in CLI

From your comment above, this “faux serial” approach can be slower than pure serial, but since we only use it by exception (as needed to avoid crashes only on some VMs), it shouldn't be a big problem.

Does this sound correct?

I would like to still have a CLI way to switch to serial (former --parallel off), so I don't have to fake it, but I guess you already decided against it.