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Prevent parallel simplex when solving MIPs - #3042

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Prevent parallel simplex when solving MIPs#3042
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@jajhall jajhall commented May 26, 2026 •
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If the parallel option is set "on" by a user in an attempt to run the parallel MIP solver, don't allow this to cause a switch from kSimplexStrategyDual to kSimplexStrategyDualMulti (in HEkk::chooseSimplexStrategyThreads) otherwise the MIP solver may use parallel dual simplex - which is less robust.

A couple of un-used options have been deleted

@jajhall
Don't allow parallel="on" to cause switch from kSimplexStrategyDual t…
dd33e6e 
…o kSimplexStrategyDualMulti so the MIP solver only ever uses serial simplex
@jajhall
jajhall requested a review from Opt-Mucca May 26, 2026 22:42
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@jajhall jajhall changed the title Prevent parallel simplex when soving MIPs Prevent parallel simplex when solving MIPs May 26, 2026
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Very happy with removing the unused options!

I'm assuming this call-path never affects any LP solve that's unrelated to a MIP solve? If so then I'm happy with the change!

@jajhall

jajhall commented May 27, 2026

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I'm assuming this call-path never affects any LP solve that's unrelated to a MIP solve? If so then I'm happy with the change!

It will if someone wants to invoke the parallel simplex solver by setting parallel="on" and simplex_strategy=kSimplexStrategyDual, but I'll document this in the release notes. However,

  • there are two parallel dual simplex solvers, so it's better to force users to select the one they want, rather than only using PAMI
  • the use of parallel dual simplex is niche
  • it's preferable to having the MIP solver fail due to parallel simplex being used

@jajhall
jajhall merged commit 41ac58d into latest May 27, 2026
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@jajhall
jajhall deleted the no-pami-in-mip branch May 27, 2026 09:21
This was referenced Jun 28, 2026
Merged
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social4hyq pushed a commit to social4hyq/homebrew-core that referenced this pull request Sep 20, 2026
HarmonybrewBot HarmonybrewBot
!12881 merge bump-highs-1.15.0 into main
491e312 
highs 1.15.0

Created-by: HarmonybrewBot
Commit-by: HarmonybrewBot
Merged-by: HarmonybrewBot
Description: Created by `brew bump`

---

Created with `brew bump-formula-pr`.<details>
  <summary>release notes</summary>
  <pre>The highlights of v1.15 are the first variant of the parallel MIP solver
and the addition of HiPO in Python.

## Code changes

Following [PR #2886](ERGO-Code/HiGHS#2886), our
prototype multithreaded MIP solver is available, refactoring
the worker/node-search logic of the branch-and-cut solver to run
across multiple threads. This is the first release of the parallel MIP
solver.

A fix to the parallel scheduler was merged in
[PR #3087](ERGO-Code/HiGHS#3087).

Following [PR #2994](ERGO-Code/HiGHS#2994),
HiPO can now be used to solve LPs and QPs from `highspy`, with
documentation added in
[PR #3024](ERGO-Code/HiGHS#3024) and
[PR #3060](ERGO-Code/HiGHS#3060). HiPO
performance was further improved by faster handling of free variables
([PR #3013](ERGO-Code/HiGHS#3013)) and faster
triangular solves
([PR #3014](ERGO-Code/HiGHS#3014)).

Prompted by [#2957](ERGO-Code/HiGHS#2957),
square Hessians that are only slightly asymmetric are now accepted by
`Highs::qFormatOk`, rather than being rejected outright
([PR #2965](ERGO-Code/HiGHS#2965),
[PR #2984](ERGO-Code/HiGHS#2984) and
[PR #3003](ERGO-Code/HiGHS#3003)). Identification
of non-convexity in the QP solvers was also improved, with extended
documentation added in
[PR #3056](ERGO-Code/HiGHS#3056), and a further
bug affecting some QPs was fixed in
[PR #3069](ERGO-Code/HiGHS#3069). QP hot start
was added in [PR #3089](ERGO-Code/HiGHS#3089).

Following [PR #2982](ERGO-Code/HiGHS#2982),
several bugs in the computation of an Irreducible Infeasible Subsystem
(IIS) via `Highs::getIis` were fixed.

Following [PR #3046](ERGO-Code/HiGHS#3046),
zero-cost singleton columns are no longer fixed to an infinite bound
during `HPresolve::dualFixing`, and the size of shifts applied during
reduced-cost fixing was reduced in
[PR #2986](ERGO-Code/HiGHS#2986). Strengthened
variable bounds found during presolve are now retained
([PR #3010](ERGO-Code/HiGHS#3010)), a bug in
the computation of the fractional value of a variable with finite
lower and upper bounds was fixed
([PR #3005](ERGO-Code/HiGHS#3005)), and the
handling of infeasibilities for semi-continuous and semi-integer
variables was corrected
([PR #3018](ERGO-Code/HiGHS#3018)).

Following [PR #3042](ERGO-Code/HiGHS#3042),
parallel simplex is no longer used when solving the LP relaxations
that arise within the MIP solver, since it isn't safe to do so when
the MIP solver is itself running in parallel.

Following [PR #2971](ERGO-Code/HiGHS#2971),
`highspy` now supports scalar division, and significant improvements
were made to `highspy`'s static typing in
[PR #2983](ERGO-Code/HiGHS#2983). Context
manager support was added to `highspy` in
[PR #3093](ERGO-Code/HiGHS#3093).

Following [PR #3039](ERGO-Code/HiGHS#3039),
`Highs::setBasis` and `Highs::setLogicalBasis` have been implemented,
and additional methods were added to the C# wrapper in
[PR #3041](ERGO-Code/HiGHS#3041).

Prompted by [#3044](ERGO-Code/HiGHS#3044),
a further bug was fixed in
[PR #3092](ERGO-Code/HiGHS#3092), and
[PR #3091](ERGO-Code/HiGHS#3091) corrects the
re-checking of an implied bound, fixing
[#3090](ERGO-Code/HiGHS#3090).

Following [PR #2981](ERGO-Code/HiGHS#2981),
C-heap memory leaks in solver state cleanup were fixed, and
[PR #3081](ERGO-Code/HiGHS#3081) fixes a bug
in column stuffing.

Following [PR #3016](ERGO-Code/HiGHS#3016),
`Highs::presolve()` now logs and returns failure appropriately when
called inconsistently with the state of the incumbent model.

Prompted by [#3007](ERGO-Code/HiGHS#3007), a
potential typo in `FactorHiGHSSolver::chooseNla()` was corrected, and
several other minor fixes were collected in
[PR #3017](ERGO-Code/HiGHS#3017) and
[PR #3004](ERGO-Code/HiGHS#3004) (the latter
for `HighsCliqueTable`).

Following [PR #3057](ERGO-Code/HiGHS#3057),
lines in HiGHS options files that contain only spaces are now ignored,
and the text of the first line containing an error is printed in the
resulting message.

## Build changes

The Python build has been updated and an additional python package is
available. The HiPO dependencies are linked via the optional
`highspy-extras`, e.g. Metis, for all platforms, and OpenBLAS, for
Windows and Linux. The `highspy-extras` package is automatically
consumed by `highspy` and does not need to be imported manually. Note,
that `highspy-extras` is distributed under the Apache 2.0 license, due
to the dependencies' licensing.

On Linux, `libblas` is no longer supported, in favour of OpenBLAS. MKL
support would be considered at a later stage.

Prompted by [#3000](ERGO-Code/HiGHS#3000),
[PR #3027](ERGO-Code/HiGHS#3027) fixes a
recent `alpine:edge` compilation issue.

Following [PR #3025](ERGO-Code/HiGHS#3025),
all remaining thread sanitizer data race warnings are cleared.

GPU support was added for local Python pip installs in
[PR #3071](ERGO-Code/HiGHS#3071) and the BLAS
library used by HiPO is now checked at runtime
([PR #3080](ERGO-Code/HiGHS#3080)).

The `cibuildwheel` workflow and `pyproject` configuration were updated
to `cibuildwheel` 4.0.0
([PR #3061](ERGO-Code/HiGHS#3061)), and the
NuGet publishing workflow now uses trusted publishing rather than an
API key
([PR #3062](ERGO-Code/HiGHS#3062),
following [PR #2959](ERGO-Code/HiGHS#2959)).

The macOS GitHub Actions build times were significantly improved in
[PR #3033](ERGO-Code/HiGHS#3033), and the C#
wrapper build was fixed to statically link the MSVC STL and correctly
install on win32
([PR #3070](ERGO-Code/HiGHS#3070)).

Following [PR #3001](ERGO-Code/HiGHS#3001),
the install location of the readme and license files was corrected,
and explicit `-fexceptions` copts were added to the Bazel config
([PR #3072](ERGO-Code/HiGHS#3072) and
[PR #3075](ERGO-Code/HiGHS#3075)).
</pre>
  <p>View the full release notes at <a href="https://github.com/ERGO-Code/HiGHS/releases/tag/v1.15.0">https://github.com/ERGO-Code/HiGHS/releases/tag/v1.15.0</a>.</p>
</details>
<hr>

See merge request: Harmonybrew/homebrew-core!12881
social4hyq pushed a commit to social4hyq/homebrew-core that referenced this pull request Sep 20, 2026
HarmonybrewBot HarmonybrewBot
!13385 merge bump-highs-1.15.1 into main
39c71cb 
highs 1.15.1

Created-by: HarmonybrewBot
Commit-by: HarmonybrewBot
Merged-by: HarmonybrewBot
Description: Created by `brew bump`

---

Created with `brew bump-formula-pr`.<details>
  <summary>release notes</summary>
  <pre>Patch release v1.15.1 fixes:

Apache release binaries not including HiPO fixed in [PR #3112](ERGO-Code/HiGHS#3112).

Build for users with C++20 fixed in [PR #3114](ERGO-Code/HiGHS#3114).

OpenBLAS setup issue affecting performance fixed in [PR #3122](ERGO-Code/HiGHS#3122).

Values passed for string option values (except for file names) are
converted to lower case [PR #3108](ERGO-Code/HiGHS#3108).

---


The highlights of v1.15 are the first variant of the parallel MIP solver
and the addition of HiPO in Python.

## Code changes

Following [PR #2886](ERGO-Code/HiGHS#2886), our
prototype multithreaded MIP solver is available, refactoring
the worker/node-search logic of the branch-and-cut solver to run
across multiple threads. This is the first release of the parallel MIP
solver.

A fix to the parallel scheduler was merged in
[PR #3087](ERGO-Code/HiGHS#3087).

Following [PR #2994](ERGO-Code/HiGHS#2994),
HiPO can now be used to solve LPs and QPs from `highspy`, with
documentation added in
[PR #3024](ERGO-Code/HiGHS#3024) and
[PR #3060](ERGO-Code/HiGHS#3060). HiPO
performance was further improved by faster handling of free variables
([PR #3013](ERGO-Code/HiGHS#3013)) and faster
triangular solves
([PR #3014](ERGO-Code/HiGHS#3014)).

Prompted by [#2957](ERGO-Code/HiGHS#2957),
square Hessians that are only slightly asymmetric are now accepted by
`Highs::qFormatOk`, rather than being rejected outright
([PR #2965](ERGO-Code/HiGHS#2965),
[PR #2984](ERGO-Code/HiGHS#2984) and
[PR #3003](ERGO-Code/HiGHS#3003)). Identification
of non-convexity in the QP solvers was also improved, with extended
documentation added in
[PR #3056](ERGO-Code/HiGHS#3056), and a further
bug affecting some QPs was fixed in
[PR #3069](ERGO-Code/HiGHS#3069). QP hot start
was added in [PR #3089](ERGO-Code/HiGHS#3089).

Following [PR #2982](ERGO-Code/HiGHS#2982),
several bugs in the computation of an Irreducible Infeasible Subsystem
(IIS) via `Highs::getIis` were fixed.

Following [PR #3046](ERGO-Code/HiGHS#3046),
zero-cost singleton columns are no longer fixed to an infinite bound
during `HPresolve::dualFixing`, and the size of shifts applied during
reduced-cost fixing was reduced in
[PR #2986](ERGO-Code/HiGHS#2986). Strengthened
variable bounds found during presolve are now retained
([PR #3010](ERGO-Code/HiGHS#3010)), a bug in
the computation of the fractional value of a variable with finite
lower and upper bounds was fixed
([PR #3005](ERGO-Code/HiGHS#3005)), and the
handling of infeasibilities for semi-continuous and semi-integer
variables was corrected
([PR #3018](ERGO-Code/HiGHS#3018)).

Following [PR #3042](ERGO-Code/HiGHS#3042),
parallel simplex is no longer used when solving the LP relaxations
that arise within the MIP solver, since it isn't safe to do so when
the MIP solver is itself running in parallel.

Following [PR #2971](ERGO-Code/HiGHS#2971),
`highspy` now supports scalar division, and significant improvements
were made to `highspy`'s static typing in
[PR #2983](ERGO-Code/HiGHS#2983). Context
manager support was added to `highspy` in
[PR #3093](ERGO-Code/HiGHS#3093).

Following [PR #3039](ERGO-Code/HiGHS#3039),
`Highs::setBasis` and `Highs::setLogicalBasis` have been implemented,
and additional methods were added to the C# wrapper in
[PR #3041](ERGO-Code/HiGHS#3041).

Prompted by [#3044](ERGO-Code/HiGHS#3044),
a further bug was fixed in
[PR #3092](ERGO-Code/HiGHS#3092), and
[PR #3091](ERGO-Code/HiGHS#3091) corrects the
re-checking of an implied bound, fixing
[#3090](ERGO-Code/HiGHS#3090).

Following [PR #2981](ERGO-Code/HiGHS#2981),
C-heap memory leaks in solver state cleanup were fixed, and
[PR #3081](ERGO-Code/HiGHS#3081) fixes a bug
in column stuffing.

Following [PR #3016](ERGO-Code/HiGHS#3016),
`Highs::presolve()` now logs and returns failure appropriately when
called inconsistently with the state of the incumbent model.

Prompted by [#3007](ERGO-Code/HiGHS#3007), a
potential typo in `FactorHiGHSSolver::chooseNla()` was corrected, and
several other minor fixes were collected in
[PR #3017](ERGO-Code/HiGHS#3017) and
[PR #3004](ERGO-Code/HiGHS#3004) (the latter
for `HighsCliqueTable`).

Following [PR #3057](ERGO-Code/HiGHS#3057),
lines in HiGHS options files that contain only spaces are now ignored,
and the text of the first line containing an error is printed in the
resulting message.

## Build changes

The Python build has been updated and an additional python package is
available. The HiPO dependencies are linked via the optional
`highspy-extras`, e.g. Metis, for all platforms, and OpenBLAS, for
Windows and Linux. The `highspy-extras` package is automatically
consumed by `highspy` and does not need to be imported manually. Note,
that `highspy-extras` is distributed under the Apache 2.0 license, due
to the dependencies' licensing.

On Linux, `libblas` is no longer supported, in favour of OpenBLAS. MKL
support would be considered at a later stage.

Prompted by [#3000](ERGO-Code/HiGHS#3000),
[PR #3027](ERGO-Code/HiGHS#3027) fixes a
recent `alpine:edge` compilation issue.

Following [PR #3025](ERGO-Code/HiGHS#3025),
all remaining thread sanitizer data race warnings are cleared.

GPU support was added for local Python pip installs in
[PR #3071](ERGO-Code/HiGHS#3071) and the BLAS
library used by HiPO is now checked at runtime
([PR #3080](ERGO-Code/HiGHS#3080)).

The `cibuildwheel` workflow and `pyproject` configuration were updated
to `cibuildwheel` 4.0.0
([PR #3061](ERGO-Code/HiGHS#3061)), and the
NuGet publishing workflow now uses trusted publishing rather than an
API key
([PR #3062](ERGO-Code/HiGHS#3062),
following [PR #2959](ERGO-Code/HiGHS#2959)).

The macOS GitHub Actions build times were significantly improved in
[PR #3033](ERGO-Code/HiGHS#3033), and the C#
wrapper build was fixed to statically link the MSVC STL and correctly
install on win32
([PR #3070](ERGO-Code/HiGHS#3070)).

Following [PR #3001](ERGO-Code/HiGHS#3001),
the install location of the readme and license files was corrected,
and explicit `-fexceptions` copts were added to the Bazel config
([PR #3072](ERGO-Code/HiGHS#3072) and
[PR #3075](ERGO-Code/HiGHS#3075)).
</pre>
  <p>View the full release notes at <a href="https://github.com/ERGO-Code/HiGHS/releases/tag/v1.15.1">https://github.com/ERGO-Code/HiGHS/releases/tag/v1.15.1</a>.</p>
</details>
<hr>

See merge request: Harmonybrew/homebrew-core!13385
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