[crac] RFR: 8350845: [CRaC] Support C/R engines in form of a library

[Radim Vansa]

On Thu, 27 Feb 2025 12:19:10 GMT, Timofei Pushkin <tpushkin at openjdk.org> wrote:

> Adds support for C/R engines implemented in form of dynamic libraries.
> 
> In the proposed API the engine controls the set of the configurable options — they are mainly passed by the user via the new `CRaCEngineOptions` VM option, but some may also be passed by the VM. Other VM and/or engine implementations may extend the API via "extensions".
> 
> The JVM is supposed to know what configuration options and API extensions are supported by calling `can_configure` and `get_extension`.
> 
> The user is supposed to know which options they can pass by asking the engine with `-XX:CRaCEngineOptions=help` (support for this is engine-dependent). Example of how this looks for bundled engines:
> 
> $ java -XX:CRaCEngine=simengine -XX:CRaCEngineOptions=help -XX:CRaCCheckpointTo=cr -version
> 
> crexec — pseudo-CRaC-engine used to relay data from JVM to a "real" engine implemented as an executable (instead of a library).
> The engine executable is expected to have CRaC-CRIU-like CLI. Support of the options below also depends on the engine executable.
> 
> Configuration options:
> * keep_running=<true/false> (default: false) — keep the process running after the checkpoint or kill it.
> * direct_map=<true/false> (default: false) — on restore, map process data directly from saved files. This may speedup the restore but the resulting process will not be the same as before the checkpoint.
> * args=<string> (default: "") — free space-separated arguments passed directly to the engine executable, e.g. "--arg1 --arg2 --arg3".
> * help — print this message.
> 
> openjdk version "24-internal" 2025-03-18
> OpenJDK Runtime Environment (slowdebug build 24-internal-adhoc.timpushkin.crac)
> OpenJDK 64-Bit Server VM (slowdebug build 24-internal-adhoc.timpushkin.crac, mixed mode, sharing)
> 
> 
> Notable related changes included in the patch:
> - The ability to pass arguments to C/R engine was removed from `CRaCEngine` because it can now be implemented through `CRaCEngineOptions`. E.g. `-XX:CRaCEngine=criu,--verbose` is now `-XX:CRaCEngine=criu -XX:CRaCEngineOptions=args=--verbose`.
> - `CRaCEngine` (as well as the new `CRaCEngineOptions`) VM Option is now not updated in the restored VM. This seems more correct because engine path and options will very likely differ on restore and on checkpoint, so it is not obvious how to combine them — just replacing won't always work. Until now the updates were ignored by the CRaC implementation anyway, so this should only be visible by inspecting the options through JMX.
> - `CRaCRestoreFrom` is now co...

I like the idea of `-XX:CRaCEngineOptions=help` on the UX side, though it would be even better to have it working without `-XX:CRaCCheckpointTo=...`. However I don't think that `configure(...)` method on the API should cause any side-effects, like printing output. When this is already an *encouraged* - in other words optional but standard - way to provide output, it should be a standalone method; I don't have strong opinion about whether it would be `const char *help()` or `void print_help(int fd)` or anything else, but `configure` should only modify `conf`.

As for the extensions listing, I don't have a clear use case in mind but some way of identification in a management interface; in general it is nice if a pluggable component can self-identify (name, version, ...). `help` provides this in a non-structured way, which might be sufficient, though it is not complex at all to implement few functions returning some self-identifying strings.

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PR Comment: https://git.openjdk.org/crac/pull/207#issuecomment-2687925882