How to Use CPT Upgrade in gem5: A Complete Beginner-Friendly Guide

1. Introduction to Checkpointing in gem5

Checkpointing in gem5 is a powerful feature that allows simulation users to save the state of the system at a given point in time. This saved state is known as a checkpoint, and it can be reloaded later to resume simulation from the exact same state, saving time and effort.

This is especially useful in computer architecture research, where full-system simulations can take hours or even days to run. By using checkpoints, researchers can skip the boot-up phase or reload after crashes, allowing them to test different configurations without restarting from scratch.

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2. What is CPT Upgrade in gem5?

The CPT Upgrade (Checkpoint Upgrade) is a tool provided by gem5 to convert checkpoints created in an older version of gem5 to be compatible with a newer version. As gem5 is constantly evolving with new features, changes in internal APIs, and updates to simulation objects, older checkpoints may become incompatible with newer versions.

The CPT upgrader helps bridge this gap, making it possible to reuse simulation work done on previous versions without needing to rerun the entire simulation from the beginning. In essence, it preserves your simulation effort when transitioning between gem5 versions.

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3. When and Why to Use CPT Upgrade

There are several situations where learning how to use CPT upgrade in gem5 becomes essential:

1. Version Migration: You created checkpoints on gem5 v21.1 but now wish to work on v23.0.
2. Architecture Testing: Testing different CPU models or configurations that need updated simulation support.
3. Bug Fixes: An upgrade solves critical bugs, but you don’t want to lose progress saved in old checkpoints.
4. Collaboration: Sharing checkpoints with a colleague using a different gem5 version.

In all these cases, using the CPT upgrader avoids the costly overhead of restarting simulations from scratch.

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4. Prerequisites and Setup

Before jumping into the upgrade process, ensure you have the following set up:

• Both old and new gem5 versions built (e.g., gem5-21.1 and gem5-23.0).
• Python 3.6+ installed, since cpt_upgrader.py is a Python script.
• Ensure the util/ directory exists in your gem5 repo. This is where cpt_upgrader.py resides.
• Know the location of your original checkpoint directory.

Use this table as a checklist:

Requirement	Description
Python	Version 3.6 or higher
gem5-old	Where the checkpoint was created
gem5-new	Where you will use the upgraded checkpoint
Checkpoint Folder	The directory containing original checkpoint
Access to Terminal	To run commands and scripts

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5. Creating a Checkpoint in Older gem5 Version

To create a checkpoint, you first run your full system simulation using the older version of gem5. At the desired simulation point (e.g., after boot), insert the following command:

bashCopyEditm5 checkpoint

This command captures the state of the simulation and writes it into a directory, typically named something like m5out/cpt.<timestamp>. Make sure to keep track of the exact folder name for use in the upgrade process.

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6. Understanding Checkpoint File Structure

A typical gem5 checkpoint folder includes several critical files:

File Name	Description
config.json	Stores simulation parameters
mem.*	Memory state files
system.*	Serialized state of various system components
cpt.*.tick	Indicates when the checkpoint was taken

Understanding this structure helps ensure you’re not missing any files before starting the upgrade.

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7. Using the CPT Upgrader Tool

Here’s the core step in learning how to use CPT upgrade in gem5.

Navigate to the directory where your new gem5 version is installed. Then, run the following command:

bashCopyEditbuild/<ISA>/util/cpt_upgrader.py --in-dir /path/to/old_checkpoint --out-dir /path/to/new_checkpoint

Replace <ISA> with your target architecture, like X86 or RISCV. For example:

bashCopyEditbuild/X86/util/cpt_upgrader.py --in-dir m5out/cpt.2025-04-12_10-30-45 --out-dir upgraded_cpt

This command reads the old checkpoint, processes it, and writes a new checkpoint directory compatible with the newer gem5 version.

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8. Command-Line Examples

Here are a few command patterns to remember:

1. Basic usage: cssCopyEditcpt_upgrader.py --in-dir old_cpt --out-dir new_cpt
2. Verbose output for debugging: cssCopyEditcpt_upgrader.py --in-dir old_cpt --out-dir new_cpt --verbose
3. Checking help menu: bashCopyEditcpt_upgrader.py --help

These commands are typically run from your terminal inside the root directory of your new gem5 installation.

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9. Validating the Upgraded Checkpoint

Before using the upgraded checkpoint, verify its integrity:

• Check that the folder contains all expected checkpoint files.
• Make sure the config.json reflects the simulation components correctly.
• If needed, you can test-load the checkpoint in gem5 using a minimal run script with the new version.

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10. Loading the Upgraded Checkpoint in New gem5 Version

Once validated, you can load the upgraded checkpoint into your new gem5 version like this:

bashCopyEdit./build/X86/gem5.opt configs/example/fs.py --checkpoint-dir=upgraded_cpt

Make sure your configuration script (fs.py or another) is compatible with the updated checkpoint.

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11. Common Issues and Troubleshooting

While using CPT upgrade in gem5, you might run into issues such as:

• Missing checkpoint files: Double-check the old checkpoint directory.
• Mismatched simulation components: Ensure the new version still supports the components used in the old checkpoint.
• Pickle or deserialization errors: Sometimes caused by incompatible Python versions or internal gem5 changes.

Rebuilding gem5 cleanly and double-checking Python environments often solves these problems.

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12. Tips for Managing Checkpoints Efficiently

1. Name wisely: Use timestamped or descriptive names (booted_kernel_cpt, post_login_cpt) for easy identification.
2. Back up: Always duplicate checkpoints before upgrading.
3. Organize by version: Store old and new checkpoints in separate directories to avoid confusion.
4. Log everything: Maintain a log file listing what each checkpoint represents.

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13. Automating CPT Upgrade in Scripts

If you have multiple checkpoints, write a simple shell script like:

bashCopyEdit#!/bin/bash
for dir in checkpoints/*; do
  out="upgraded_checkpoints/$(basename $dir)"
  python3 build/X86/util/cpt_upgrader.py --in-dir $dir --out-dir $out
done

This automates the process and saves time when working with large-scale simulations.

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14. Advanced: Modifying Upgraded Checkpoints

In some cases, you might want to edit the upgraded checkpoint manually. For instance, modifying the config.json file to change memory size or adjusting object paths.

However, such changes should be made cautiously. Always back up your checkpoint and ensure it aligns with the simulator’s expectations to avoid runtime crashes.

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15. References and Further Reading

• gem5 Official Documentation
• gem5 Tutorials on GitHub
• gem5 Community Forum
• util/cpt_upgrader.py source file inside your gem5 repo

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Final Thoughts

Knowing how to use CPT upgrade in gem5 is a crucial skill for anyone working with simulations in this powerful platform. It helps preserve time, computation, and effort when transitioning between gem5 versions. Whether you’re upgrading your experiments or collaborating across versions, the CPT upgrader ensures your previous work doesn’t go to waste.

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