On restore the "main" thread is started before the Resource's afterRestore has completed

Wed Apr 5 16:00:11 UTC 2023

Lot of interesting ideas, comments inline....

On 05. 04. 23 16:28, Dan Heidinga wrote:
> Hi Radim,
>
> Thanks for the write up of the various options in this space.
>
> On Tue, Apr 4, 2023 at 2:49 AM Radim Vansa <rvansa at azul.com<mailto:rvansa at azul.com>> wrote:
> Hi Christian,
>
> I believe this is a common problem when porting existing architecture
> under CRaC; the obvious solution is to guard access to the resource
> (ProcessorContext in this case) with a RW lock that'd be read-acquired
> by 'regular' access and acquired for write in beforeCheckpoint/released
> in afterRestore. However this introduces extra synchronization (at least
> in form of volatile writes) even in case that C/R is not used at all,
> especially if the support is added into libraries.
>
> I've seen variations of this approach go by in code reviews but have we written up a good example of how to do this well?  Having a canonical pattern would help to highlight the best way to do it today and make the tradeoffs explicit.

TBH I am rather shy to demo a solution that's quite imperfect unless we 
find that there's no way around that.

>
>
> Anton Kozlov proposed techniques like RCU [1] but at this point there's
> no support for this in Java. Even the Linux implementation might require
> some additional properties from the code in critical (read) section like
> not calling any blocking code; this might be too limiting.
>
> The situation is simpler if the application uses a single threaded
> event-loop; beforeCheckpoint can enqueue a task that would, upon its
> execution, block on a primitive and notify the C/R notification thread
> that it may now deinit the resource; in afterRestore the resource is
> initialized and the eventloop is unblocked. This way we don't impose any
> extra overhead when C/R is happening.
>
> That's a nice idea!
>
>
> To avoid extra synchronization it could be technically possible to
> modify CRaC implementation to keep all other threads frozen during
> restore. There's a risk of some form of deadlock if the thread
> performing C/R would require other threads to progress, though, so any
> such solution would require extra thoughts. Besides, this does not
> guarantee exclusivity so the afterRestore would need to restore the
> resource to the *exactly* same state (as some of its before-checkpoint
> state might have leaked to the thread in Processor). In my opinion this
> is not the best way.
>
> This is the approach that OpenJ9 took to solve the consistency problems introduced by updating resources before / after checkpoints.  OpenJ9 enters "single threaded mode" when creating the checkpoint and executing the before checkkpoint fixups.  On restore, it continues in single-threaded mode while executing the after checkpoint fixups.  This makes it easier to avoid additional runtime costs related to per-resource locking for checkpoints, but complicates locking and wait/notify in general.
>
> This means a checkpoint hook operation can't wait on another thread (would block indefinitely as other threads are paused), can't wait on a lock being held by another thread (again, would deadlock), and sending notify may result in inconsistent behaviour (wrong number of notifies received by other threads).  See "The checkpointJVM() API" section of their blog post on CRIU for more details [0].

Great post, I should probably go through the whole blog. I think that 
the single-threaded mode is conceptually simple to think about and with 
the @NotCheckpointSafe annotation deals well with the issue. Have you 
run into any edge cases where this doesn't work well? For example I've 
seen a deadlock because in beforeCheckpoint something was supposed to 
run in the reference cleaner thread.

Also, did you run any tests to see the performance impact of your 
changes to wait/notify? Do you also have to tweak higher-level 
synchronization such as java.util.concurrent.*?

>
>
> The problem with RCU is tracking which threads are in the critical
> section. I've found RCU-like implementations for Java that avoid
> excessive overhead using a spread out array - each thread marks
> entering/leaving the critical section by writes to its own counter,
> preventing cache ping-pong (assuming no false sharing). Synchronizer
> thread uses another flag to request synchronization; reading this by
> each thread is not totally without cost but reasonably cheap, and in
> that case worker threads can enter a blocking slow path. The simple
> implementation assumes a fixed number of threads; if the list of threads
> is dynamic the solution would be probably more complicated. It might
> also make sense to implement this in native code with a per-CPU
> counters, rather than per-thread. A downside, besides some overhead in
> terms of both cycles and memory usage, is that we'd need to modify the
> code and explicitly mark the critical sections.
>
> Another solution could try to leverage existing JVM mechanics for code
> deoptimization, replacing the critical sections with a slower, blocking
> stub, and reverting back after restore. Or even independently requesting
> a safe-point and inspecting stack of threads until the synchronization
> is possible.
>
> This will have a high risk of livelock.  The OpenJ9 experience implementing single-threaded mode for CRIU indicates there are a lot of strange locking patterns in the world.

There are weird patterns but a more fine-grained exclusivity should be 
more permissive than single-threaded mode which works as an implicit Big 
Fat Lock. Yes, any problems are probably much better reproducible with 
that one.

>
>
> So I probably can't offer a ready-to-use performant solution; pick your
> poison. The future, though, offers a few possibilities and I'd love to
> hear others' opinions about which one would look the most feasible.
> Because unless we offer something that does not harm a no-CRaC use-case
> I am afraid that the adoption will be quite limited.
>
> Successful solutions will push the costs into the checkpoint / restore paths as much as possible.  Going back to the explicit lock mechanism you first mentioned, I wonder if there's a role for java.lang.invoke.Switchpoint [1] here?  Switchpoint was added as a tool for language implementers that wanted to be able speculate on a particular condition (ie: CHA assumptions) and get the same kind of low cost state change that existing JITTED code gets.  I'm not sure how well that vision worked in practice or how well Hotspot optimizes it yet, but this might be a reason to push on its performance.
>
> Roughly the idea would be to add a couple of Switchpoints to jdk.crac.Core:
>
>     public SwitchPoint getBeforeSwitchpoint();
>     public SwitchPoint getAfterSwitchpoint();
>
> and users could then write their code using MethodHandles to implementing the branching logic:
>
>      MethodHandle normalPath = ...... // existing code
>      MethodHandle fallbackPath = ..... // before Checkpoint extra work
>      MethodHandle guardWithTest = getBeforeSwitchPoint.guardWithTest(normalPath, fallbackPath);
>
> and the jdk.crac.Core class would invalidate the "before" SwitchPoint prior to the checkpoint and "after" one after the restore.  Aside from the painful programming model, this might give us the tools we need to make it performant.
>
> Needs more exploration and prototyping but would provide a potential path to reasonable performance by burying the extra locking in the fallback paths.  And it would be a single pattern to optimize, rather than all the variations users could produce.

Well noted, I admit that I haven't heard about SwitchPoint before, I'll 
need some time to ingest it and maybe write a JMH tests to see. However 
from the first look it is not something that would be too convenient for 
users, I would consider some form of interception introduced through 
annotations (yes, I come from the EE world).

Thanks!

Radim

>
> --Dan
>
>
> [0] https://blog.openj9.org/2022/10/14/openj9-criu-support-a-look-under-the-hood/
> [1] https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/invoke/SwitchPoint.html
>
> Cheers,
>
> Radim
>
> [1] https://en.wikipedia.org/wiki/Read-copy-update
>
> On 03. 04. 23 22:30, Christian Tzolov wrote:
>> Hi, I'm testing CRaC in the context of long-running applications (e.g. streaming, continuous processing ...) and I've stumbled on an issue related to the coordination of the resolved threads.
>>
>> For example, let's have a Processor that performs continuous computations. This processor depends on a ProcessorContext and later must be fully initialized before the processor can process any data.
>>
>> When the application is first started (e.g. not from checkpoints) it ensures that the ProcessorContext is initialized before starting the Processor loop.
>>
>> To leverage CRaC I've implemented a ProcessorContextResource gracefully stops the context on beforeCheckpoint and then re-initialized it on afterRestore.
>>
>> When the checkpoint is performed, CRaC calls the ProcessorContextResource.beforeCheckpoint and also preserves the current Processor call stack. On Restore processor's call stack is expectedly restored at the point it was stopped but unfortunately it doesn't wait for the ProcessorContextResource.afterRestore complete. This expectedly crashes the processor.
>>
>> The https://github.com/tzolov/crac-demo illustreates this issue. The README explains how to reproduce the issue. The OUTPUT.md (https://github.com/tzolov/crac-demo/blob/main/OUTPUT.md ) offers terminal snapshots of the observed behavior.
>>
>> I've used latest JDK CRaC release:
>>     openjdk 17-crac 2021-09-14
>>     OpenJDK Runtime Environment (build 17-crac+5-19)
>>     OpenJDK 64-Bit Server VM (build 17-crac+5-19, mixed mode, sharing)
>>
>> As I'm new to CRaC, I'd appreciate your thoughts on this issue.
>>
>> Cheers,
>> Christian
>>
>>
>>
>>
>