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CRIU (Checkpoint/Restore In User-space)

CRIU is a tool for saving the state of a running application to a set of files (checkpointing) and restoring it back to a live state. It is primarily used for live migration of containers, in-place updates, and fast application startup.

It is implemented as a command-line tool called criu. The two primary commands are dump and restore.

  • dump: Saves a process tree and all its related resources (file descriptors, IPC, sockets, namespaces, etc.) into a collection of image files.
  • restore: Restores processes from image files to the same state they were in before the dump.

Quick Start

To get a feel for criu, you can try checkpointing and restoring a simple process.

  1. Run a simple process: Open a terminal and run a command that will run for a while. Find its PID.

    sleep 1000 &
[1] 12345

  2. Dump the process: As root, use criu dump with the process ID (-t) and a directory for the image files (-D).

    sudo criu dump -t 12345 -D /tmp/sleep_images -v4 --shell-job

    The sleep process will no longer be running.

  3. Restore the process: Use criu restore to bring the process back to life from the images.

    sudo criu restore -D /tmp/sleep_images -v4 --shell-job

    The sleep process will be running again as if nothing happened.

For Developers and Contributors

This section contains more technical details about CRIU's internals and development process.

Dump Process

On dump, CRIU uses available kernel interfaces to collect information about processes. For properties that can only be retrieved from within the process itself, CRIU injects a binary blob (called a "parasite") into the process's address space and executes it in the context of one of the process's threads. This injection is handled by a subproject called Compel.

Restore Process

On restore, CRIU reads the image files to reconstruct the processes. The goal is to restore them to the exact state they were in before the dump. The restore process is divided into several stages (defined as CR_STATE_* in ./criu/include/restorer.h).

The main criu process acts as a coordinator. It first restores resources with inter-process dependencies (file descriptors, sockets, shared memory, namespaces, etc.). It then forks the process tree and sets up namespaces. Finally, it restores process-specific resources like file descriptors and memory mappings.

A key step involves a small, self-contained binary called the "restorer". All restored processes switch to executing this code, which unmaps the CRIU-specific memory and restores the application's original memory mappings. On the final step, the restorer calls sigreturn on a prepared signal frame to resume the process with the state it had at the moment of the dump.

Compel

Compel is a subproject responsible for generating the binary blobs used for the parasite code (for dumping) and the restorer code (for restoring). It provides a library for injecting and executing this code within the target process's address space. It is a separate project because the logic for generating and injecting Position-Independent Executable (PIE) code is complex and self-contained.

Coding Style

The C code in the CRIU project follows the Linux Kernel Coding Style. Here are some of the main points:

  • Indentation: Use tabs, which are set to 8 characters.
  • Line Length: The preferred line limit is 80 characters, but it can be extended to 120 if it improves code readability.
  • Braces:
    • The opening brace for a function goes on a new line.
    • The opening brace for a block (like if, for, while, switch) goes on the same line.
  • Spaces: Use spaces around operators (+, -, *, /, %, <, >, =, etc.).
  • Naming: Use descriptive names for functions and variables.
  • Comments: Use C-style comments (/* ... */). For multi-line comments, the preferred format is: 
    /*
 * This is a multi-line
 * comment.
 */

Code Layout

The code is organized into the following directories:

  • ./compel: The Compel sub-project.
  • ./criu: The main criu tool source code.
  • ./images: Protobuf descriptions for the image files.
  • ./test: All tests.
  • ./test/zdtm: The Zero-Downtime Migration (ZDTM) test suite.
  • ./test/zdtm.py: The executor script for ZDTM tests.
  • ./scripts: Helper scripts.
  • ./scripts/build: Docker image files used for CI and cross-compilation checks.
  • ./crit: A tool to inspect and manipulate CRIU image files.
  • ./soccr: A library for TCP socket checkpoint/restore.

Tests

The main test suite is ZDTM. Here is an example of how to run a single test:

sudo ./test/zdtm.py run -t zdtm/static/env00

Each ZDTM test has three stages: preparation, C/R, and results checks. During the test, a process calls test_daemon() to signal it is ready for C/R, then calls test_waitsig() to wait for the C/R stage to complete. After being restored, the test checks that all its resources are still in a valid state.