# vtkHyperTreeGridEvaluateCoarse: data race in vtkThreadedTaskQueue produces short output array
**URL:** https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369
**Category:** Development
**Created:** [April 2, 2026, 1:01am UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369 "2026-04-02T01:01:49Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@omdaniel](https://discourse.vtk.org/u/omdaniel)
#### Post date: [April 2, 2026, 1:01am UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/1 "2026-04-02T01:01:49Z")
I am running into an issue of non-deterministic output when using vtkHyperTreeGridEvaluateCourse
when I run my vtkhdf with a few hundred time steps, in about 1-5% of the times steps the resulting cell arrays have off by one array sizes after EvaluateCoarse, but which time steps have the off by 1 differ on repeat runs
I extracted 40 time steps more likely to trigger the issue into the vtkhdf file attached for testing.
```python
from vtkmodules.vtkIOHDF import vtkHDFReader
from vtkmodules.vtkFiltersHyperTree import vtkHyperTreeGridEvaluateCoarse
from vtkmodules.vtkCommonExecutionModel import vtkStreamingDemandDrivenPipeline
from vtkmodules.vtkCommonCore import vtkSMPTools
vtkhdf = "evaluatecoarse_reproducer.vtkhdf"
num_runs = 5
print(f"File: {vtkhdf}")
print(f"SMP backend: {vtkSMPTools.GetBackend()}")
print(f"Runs: {num_runs}")
print()
for run in range(1, num_runs + 1):
reader = vtkHDFReader()
reader.SetFileName(vtkhdf)
reader.Update()
info = reader.GetOutputInformation(0)
sddp = vtkStreamingDemandDrivenPipeline
n_steps = info.Length(sddp.TIME_STEPS()) if info.Has(sddp.TIME_STEPS()) else 1
mismatches = []
for step in range(n_steps):
reader.SetStep(step)
reader.Update()
htg = reader.GetOutput()
ec = vtkHyperTreeGridEvaluateCoarse()
ec.SetInputData(htg)
ec.SetOperator(vtkHyperTreeGridEvaluateCoarse.OPERATOR_AVERAGE)
ec.Update()
out = ec.GetOutput()
expected = out.GetNumberOfCells()
cd = out.GetCellData()
for i in range(cd.GetNumberOfArrays()):
arr = cd.GetArray(i)
if arr.GetNumberOfTuples() != expected:
mismatches.append(
f" step {step}: {arr.GetName()} "
f"{arr.GetNumberOfTuples()} vs {expected} "
f"(diff={expected - arr.GetNumberOfTuples()})"
)
del ec
if mismatches:
print(f"Run {run}: {len(mismatches)} mismatches")
for m in mismatches:
print(m)
else:
print(f"Run {run}: OK")
del reader
```
**Environment:**
- VTK 9.6.0 (pip wheel), SMP backend: Sequential
- Python 3.13.9, Linux 4.18.0 (RHEL 8), x86\_64
- Source file: `Filters/HyperTree/vtkHyperTreeGridEvaluateCoarse.cxx` (v9.6.0)
[evaluatecoarse\_reproducer.vtkhdf](https://discourse.vtk.org/uploads/short-url/70zfiNrRfvTr6eYvCuTtZSSPPEV.vtkhdf) (6.5 MB)
---
### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [April 2, 2026, 12:38pm UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/2 "2026-04-02T12:38:43Z")
Hello @omdaniel, thanks for the report, I can reproduce (unreliably) the issue
```auto
~/dev/ParaView-6.1.20260319-MPI-Linux-Python3.12-x86_64/bin/pvpython scripts/eval.py
File: evaluatecoarse_reproducer.vtkhdf
SMP backend: TBB
Runs: 5
Run 1: 3 mismatches
step 36: scl 10937 vs 10940 (diff=3)
step 36: texp 10937 vs 10940 (diff=3)
step 36: var 10939 vs 10940 (diff=1)
Run 2: OK
Run 3: OK
Run 4: OK
Run 5: 3 mismatches
step 13: scl 10919 vs 10920 (diff=1)
step 13: texp 10919 vs 10920 (diff=1)
step 13: var 10919 vs 10920 (diff=1)
```
Intuitively, this means that either the reader or the filter reads/write uninitialized or out-of-bounds memory somewhere, which is not a good sign.
---
### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [April 2, 2026, 12:55pm UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/3 "2026-04-02T12:55:19Z")
This might be caused by [https://gitlab.kitware.com/vtk/vtk/-/commit/4a93b9c5a6ae94c09b5b73b8cbe5db34cc0c8885](https://gitlab.kitware.com/vtk/vtk/-/commit/4a93b9c5a6ae94c09b5b73b8cbe5db34cc0c8885) , can you try building VTK after reverting the commit and see if it still happens?
---
### Author:  [@omdaniel](https://discourse.vtk.org/u/omdaniel)
#### Post date: [April 2, 2026, 1:01pm UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/4 "2026-04-02T13:01:47Z")
I will be on vacation for the next week, but after that I should be able to test that.
---
### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [April 2, 2026, 1:12pm UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/5 "2026-04-02T13:12:40Z")
Same, keep me updated!
---
### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [April 3, 2026, 1:39pm UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/6 "2026-04-03T13:39:22Z")
@omdaniel [https://gitlab.kitware.com/vtk/vtk/-/merge\_requests/13108](https://gitlab.kitware.com/vtk/vtk/-/merge_requests/13108) should fix the race issue and the bad performance.
---
### Author:  [@omdaniel](https://discourse.vtk.org/u/omdaniel)
#### Post date: [April 13, 2026, 8:32pm UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/7 "2026-04-13T20:32:32Z")
Will this be in a VTK 9.6.x patch
---
### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [April 14, 2026, 6:55am UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/8 "2026-04-14T06:55:51Z")
yes we’ll backport it to 9.6.2
---
### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [April 17, 2026, 7:54am UTC](https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/9 "2026-04-17T07:54:44Z")
Fixed in `master` and `release` branches