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Every notebook has a kernel — the runtime that executes your code. The kernel toolbar at the top of each notebook lets you select, control, and monitor the kernel.

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The kernel picker

Click the kernel button (shows the current kernel name) to open the picker.

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Kernel information

The button displays:
  • Status indicator: Colored dot showing kernel state
  • Display name: Language and environment (e.g., “Python 3.11 | /usr/local/bin/python”)

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Selecting a kernel

The picker groups available kernels by language:
  • Python: All Python installations and virtual environments
  • R: R installations
  • Julia: Julia installations
Each entry shows the language version and environment path. Click to switch kernels.
Up to five kernels show per group. Click See more kernels to view all available options.

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Active kernels

If a kernel is already running, it appears separately in the picker — you can attach to an existing kernel session instead of starting a new one.

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When kernels start

A notebook’s kernel starts automatically when you run its first cell, using the selected compute settings.

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Cloud workspaces

In Qualia Cloud, notebooks run on managed compute environments, so the picker lists what the environment provides rather than what’s installed on your machine:
  • Python — supported on all cloud compute types.
  • R — supported on CPU compute. R notebooks start an R session in the same managed environment as Python notebooks. R on GPU compute is not available yet.
  • Julia — supported on CPU compute. Julia notebooks start a Julia 1.11 session in the same managed environment as Python notebooks. Julia on GPU compute is not available yet.
R and Julia packages installed during a cloud session remain available until that compute environment shuts down. Install them again in a fresh environment. Qualia records Python packages installed in a cloud workspace and reinstalls them when compute restarts. If cloud compute reports a cleanup failure, wait briefly and retry stopping or starting it as the error directs. Interrupting a cell leaves its compute environment running.

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Status indicators

The kernel status dot changes color: When code is running, a spinner appears in the toolbar.

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Toolbar actions

The toolbar provides buttons for common kernel operations:

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Execution control

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Kernel management

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Combined actions

Some actions combine restart with execution:
  • Restart and run all: Fresh kernel, then run everything
  • Restart and run up to current: Fresh kernel, then run to current cell
  • Restart and clear: Fresh kernel, clear all outputs

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Execution indicator

When cells are running:
  • A spinner appears in the toolbar
  • Click the spinner to scroll to the executing cell
  • The spinner persists until execution completes
This helps you track long-running computations — click to jump back to where the action is.

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Default kernel

Set your preferred default kernel in Settings:
  1. Go to Settings > Environments > Interpreters
  2. Under Default Python Environment, select your preferred Python
  3. New notebooks use this kernel automatically
Qualia validates that the selected kernel exists. If it’s missing (e.g., a virtual environment was deleted), you’ll see a warning.

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Responsive layout

When the window is narrow, toolbar actions collapse into a More menu (⋮). All the same actions are available — just fewer visible buttons.

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Troubleshooting

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Kernel not starting

  • Check that the Python/R/Julia installation exists
  • Verify virtual environments are activated correctly
  • Look at the error message in the status area
On desktop, if a notebook’s saved environment was deleted or its interpreter is gone, Qualia starts the notebook on your default kernel when it runs the same language, or otherwise on another installed kernel of that language. The kernel picker shows which kernel is running. The notebook still records its original kernel, so rebuilding that environment brings it back. Qualia never switches a notebook to a different language, and choosing a missing kernel from the picker still shows an error.

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Kernel keeps dying

  • Memory issues can crash kernels — check your system resources
  • Some packages have installation problems — try in a fresh environment
  • The kernel log may have more details
  • In Qualia Cloud, if the kernel dies while a cell runs, Qualia stops waiting and reports “Kernel died during execution”

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Wrong environment

  • Verify the kernel picker shows the expected path
  • Check that packages are installed in that environment
  • The environment shown in the picker should match where you installed dependencies