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Working with Expensive Notebooks

marimo’s reactive execution automatically runs cells when their dependencies change. For notebooks with expensive computations, you can configure marimo to use lazy execution mode, which marks cells as stale instead of automatically running them.

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Lazy Execution Mode

Lazy execution gives you the benefits of reactive programming (tracking dependencies and detecting stale cells) while preventing accidental execution of expensive operations.

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Configuring Lazy Mode

import marimo as mo

# Configure at notebook level
app = mo.App(
    runtime={
        "on_cell_change": "lazy"
    }
)

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Execution Modes

marimo supports two execution modes:
ModeBehaviorUse Case
"autorun"Cells automatically run when dependencies changeDefault, interactive exploration
"lazy"Cells are marked as stale, require manual executionExpensive computations, ETL pipelines

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Runtime Configuration Options

From marimo/_config/config.py: 
class RuntimeConfig(TypedDict):
    # If False, cells won't run on startup (edit mode only)
    auto_instantiate: bool
    
    # Module reload behavior: "off", "lazy", or "autorun"
    auto_reload: Literal["off", "lazy", "autorun"]
    
    # Automatically run pytest on test cells
    reactive_tests: bool
    
    # Cell execution mode: "lazy" or "autorun"
    on_cell_change: OnCellChangeType
    
    # File watcher behavior: "lazy" or "autorun"
    watcher_on_save: Literal["lazy", "autorun"]
    
    # Maximum size of cell outputs
    output_max_bytes: int
    std_stream_max_bytes: int

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Key Settings for Expensive Notebooks

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auto_instantiate

Control whether cells run automatically when opening a notebook: 
app = mo.App(
    runtime={
        "auto_instantiate": False  # Don't run on startup
    }
)
This only applies in edit mode. Apps always run automatically.

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on_cell_change

Control how cells react to changes: 
app = mo.App(
    runtime={
        "on_cell_change": "lazy"  # Mark stale instead of auto-run
    }
)

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auto_reload

Control behavior when imported modules change: 
app = mo.App(
    runtime={
        "auto_reload": "lazy"  # Mark cells using modified modules as stale
    }
)

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Controlling Cell Execution

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Stale Cell Indicators

In lazy mode, marimo marks cells as stale with visual indicators:
  • Yellow dot: Cell’s dependencies have changed
  • Run button: Click to execute stale cells
  • Run All Stale: Batch execute all stale cells

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Manual Execution Strategies

Click the run button on specific stale cells when you need their results:
# Expensive data loading
df = load_large_dataset()  # Won't auto-run in lazy mode
Use the “Run All Stale” button to execute all stale cells in dependency order:
  • Keyboard shortcut: Cmd/Ctrl + Shift + Enter
  • Menu: Runtime → Run stale cells
Disable expensive cells to prevent them from running:
# Right-click cell → Disable cell
# Or use keyboard shortcut: Cmd/Ctrl + D

# Disabled cells won't run and their outputs are hidden
expensive_computation()

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Performance Optimization Strategies

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1. Caching Expensive Computations

import functools

@functools.lru_cache(maxsize=128)
def expensive_function(param):
    # Heavy computation
    return result

# Or use marimo's caching
import marimo as mo

with mo.cache:
    result = expensive_computation()

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2. Lazy Data Loading

# Load data only when needed
def get_data():
    if not hasattr(get_data, '_cache'):
        get_data._cache = load_large_dataset()
    return get_data._cache

df = get_data()

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3. Incremental Processing

# Process data in chunks
import duckdb

# Use lazy evaluation with DuckDB or Polars
query = duckdb.sql("""
    SELECT * FROM large_table
    WHERE condition
""")

# Only materialized when needed
result = query.df()  # Convert to pandas when ready

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4. Conditional Execution

import marimo as mo

# Use mo.stop to halt execution conditionally
if not run_expensive_cell:
    mo.stop()

result = expensive_operation()

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Output Size Management

Limit output sizes to prevent frontend performance issues: 
app = mo.App(
    runtime={
        "output_max_bytes": 5_000_000,  # 5MB limit for cell outputs
        "std_stream_max_bytes": 1_000_000  # 1MB for console output
    }
)
Large outputs are automatically truncated with a download link.

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Execution Type: Strict vs Relaxed

marimo offers two execution types for different memory management strategies: 
app = mo.App(
    runtime={
        "execution_type": "strict"  # or "relaxed"
    }
)
  • relaxed: Faster, shares objects between cells (default)
  • strict: Clones cell outputs to prevent hidden state accumulation
Use "strict" mode when working with mutable objects that could create unexpected side effects between cells.

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Working with Module Reloading

When developing modules imported by your notebook: 
app = mo.App(
    runtime={
        "auto_reload": "lazy"
    }
)
Behavior:
  • "off": Never reload modules
  • "lazy": Mark cells importing modified modules as stale
  • "autorun": Automatically re-run cells when modules change
marimo uses intelligent code analysis to track module dependencies, similar to IPython’s %autoreload but with better integration.

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Example: ETL Pipeline

Complete example of configuring a notebook with expensive operations: 
import marimo as mo

app = mo.App(
    runtime={
        "auto_instantiate": False,  # Don't run on open
        "on_cell_change": "lazy",   # Manual execution
        "auto_reload": "lazy",      # Track module changes
        "output_max_bytes": 10_000_000
    }
)

@app.cell
def load_data():
    # Won't run automatically
    import duckdb
    conn = duckdb.connect('large_database.db')
    return conn,

@app.cell
def transform(conn):
    # Only runs when explicitly executed
    result = conn.sql("""
        SELECT * FROM large_table
        WHERE date >= '2024-01-01'
    """).df()
    return result,

@app.cell
def export(result):
    # Stale until previous cell completes
    result.to_parquet('output.parquet')
    return mo.md("✓ Export complete")

if __name__ == "__main__":
    app.run()

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Best Practices

ETL notebooks benefit from manual control over execution flow:
# Extract → Transform → Load
# Run each stage manually after validation
Add runtime controls for conditional execution:
import marimo as mo

run_experiment = mo.ui.checkbox(label="Run expensive experiment")

if not run_experiment.value:
    mo.stop()

result = run_expensive_experiment()
Use marimo’s cell timing to identify bottlenecks:
  • Cell execution times shown in editor
  • Focus optimization on slowest cells
  • Consider caching for repeated computations

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