How to "hibernate" a process in Linux by storing its memory to disk and restoring it later?

Is it possible to 'hibernate' a process in linux? Just like 'hibernate' in laptop, I would to write all the memory used by a process to disk, free up the RAM. And then later on, I can 'resume the process', i.e, reading all the data from memory and put it back to RAM and I can continue with my process?

I used to maintain CryoPID, which is a program that does exactly what you are talking about. It writes the contents of a program's address space, VDSO, file descriptor references and states to a file that can later be reconstructed. CryoPID started when there were no usable hooks in Linux itself and worked entirely from userspace (actually, it still does work, depending on your distro / kernel / security settings).

Problems were (indeed) sockets, pending RT signals, numerous X11 issues, the glibc caching getpid() implementation amongst many others. Randomization (especially VDSO) turned out to be insurmountable for the few of us working on it after Bernard walked away from it. However, it was fun and became the topic of several masters thesis.

If you are just contemplating a program that can save its running state and re-start directly into that state, its far .. far .. easier to just save that information from within the program itself, perhaps when servicing a signal.

I'd like to put a status update here, as of 2014.

The accepted answer suggests CryoPID as a tool to perform Checkpoint/Restore, but I found the project to be unmantained and impossible to compile with recent kernels. Now, I found two actively mantained projects providing the application checkpointing feature.

The first, the one I suggest 'cause I have better luck running it, is CRIU that performs checkpoint/restore mainly in userspace, and requires the kernel option CONFIG_CHECKPOINT_RESTORE enabled to work.

> Checkpoint/Restore In Userspace, or CRIU (pronounced kree-oo, IPA: /krɪʊ/, Russian: криу), is a software tool for Linux operating system. Using this tool, you can freeze a running application (or part of it) and checkpoint it to a hard drive as a collection of files. You can then use the files to restore and run the application from the point it was frozen at. The distinctive feature of the CRIU project is that it is mainly implemented in user space.

The latter is DMTCP; quoting from their main page:

> DMTCP (Distributed MultiThreaded Checkpointing) is a tool to transparently checkpoint the state of multiple simultaneous applications, including multi-threaded and distributed applications. It operates directly on the user binary executable, without any Linux kernel modules or other kernel modifications.

There is also a nice Wikipedia page on the argument: Application_checkpointing

The answers mentioning ctrl-z are really talking about stopping the process with a signal, in this case SIGTSTP. You can issue a stop signal with kill:

kill -STOP <pid>

That will suspend execution of the process. It won't immediately free the memory used by it, but as memory is required for other processes the memory used by the stopped process will be gradually swapped out.

When you want to wake it up again, use

kill -CONT <pid>

The more complicated solutions, like CryoPID, are really only needed if you want the stopped process to be able to survive a system shutdown/restart - it doesn't sound like you need that.

Linux Kernel has now partially implemented the checkpoint/restart futures:https://ckpt.wiki.kernel.org/, the status is here.

Some useful information are in the lwn(linux weekly net): http://lwn.net/Articles/375855/ http://lwn.net/Articles/412749/ ......

So the answer is "YES"

The issue is restoring the streams - files and sockets - that the program has open.

When your whole OS hibernates, the local files and such can obviously be restored. Network connections don't, but then the code that accesses the internet is typically more error checking and such and survives the error conditions (or ought to).

If you did per-program hibernation (without application support), how would you handle open files? What if another process accesses those files in the interim? etc?

Maintaining state when the program is not loaded is going to be difficult.

Simply suspending the threads and letting it get swapped to disk would have much the same effect?

Or run the program in a virtual machine and let the VM handle suspension.

Short answer is "yes, but not always reliably". Check out CryoPID:

http://cryopid.berlios.de/

Open files will indeed be the most common problem. CryoPID states explicitly:

> Open files and offsets are restored. > Temporary files that have been > unlinked and are not accessible on the > filesystem are always saved in the > image. Other files that do not exist > on resume are not yet restored. > Support for saving file contents for > such situations is planned.

The same issues will also affect TCP connections, though CryoPID supports tcpcp for connection resuming.

I extended Cryopid producing a package called Cryopid2 available from SourceForge. This can migrate a process as well as hibernating it (along with any open files and sockets - data in sockets/pipes is sucked into the process on hibernation and spat back into these when process is restarted).

The reason I have not been active with this project is I am not a kernel developer - both this (and/or the original cryopid) need to get someone on board who can get them running with the lastest kernels (e.g. Linux 3.x).

The Cryopid method does work - and is probably the best solution to general purpose process hibernation/migration in Linux I have come across.

The short answer is "yes." You might start by looking at this for some ideas: [ELF executable reconstruction from a core image][1] (http://vx.netlux.org/lib/vsc03.html)

[1]: http://vx.netlux.org/lib/vsc03.html "ELF executable reconstruction from a core image"

As others have noted, it's difficult for the OS to provide this functionality, because the application needs to have some error checking builtin to handle broken streams.

However, on a side note, some programming languages and tools that use virtual machines explicitly support this functionality, such as the Self programming language.

adding another workaround: you can use virtualbox. run your applications in a regular virtual machine and simply "save the machine state" whenever you want. I know this is not an answer, but I thought it could be useful when there are no real options.

if for any reason you don't like virtualbox, vmware and Qemu are as good.

Ctrl-Z increases the chances the process's pages will be swapped, but it doesn't free the process's resources completely. The problem with freeing a process's resources completely is that things like file handles, sockets are kernel resources the process gets to use, but doesn't know how to persist on its own. So Ctrl-Z is as good as it gets.

There was some research on checkpoint/restore for Linux back in 2.2 and 2.4 days, but it never made it past prototype. It is possible (with the caveats described in the other answers) for certain values of possible - I you can write a kernel module to do it, it is possible. But for the common value of possible (can I do it from the shell on a commercial Linux distribution), it is not yet possible.

This is sort of the ultimate goal of clustered operating system. Mathew Dillon puts a lot of effort to implement something like this in his http://www.dragonflybsd.org/">Dragonfly BSD project.

There's ctrl+z in linux, but i'm not sure it offers the features you specified. I suspect you asked this question since it doesn't