JIT deoptimization, reason="constraint". Why JIT deoptimizes method?

Question

could someone point me in direction which might lead me to get why JIT deoptimizes my loop? (OSR). It looks like it gets compiled once by C1 and then its deoptimized multiple times (I can see tens or maybe hundred logs which starts with <deoptimized...>)

This is the class which contains that important loop:

@SynchronizationRequired
public class Worker implements Runnable
{
    private static final byte NOT_RUNNING = 0, RUNNING = 1, SHUTDOWN = 2, FORCE_SHUTDOWN = 3;
    private static final AtomicIntegerFieldUpdater<Worker> isRunningFieldUpdater =
            AtomicIntegerFieldUpdater.newUpdater(Worker.class, "isRunning");

    private volatile int isRunning = NOT_RUNNING;

    private final Queue<FunkovConnection> tasks = new SpscUnboundedArrayQueue<>(512);

    /**
     * Executing tasks from queue until closed.
     */
    @Override
    public void run()
    {
        if (isRunning())
        {
            return;
        }

        while (notClosed())
        {
            FunkovConnection connection = tasks.poll();
            if (null != connection)
            {
                connection.run();
            }
        }

        if (forceShutdown())
        {
            setNonRunning();
            return;
        }

        FunkovConnection connection;
        while ((connection = tasks.poll()) != null)
        {
            connection.run();
        }

        setNonRunning();
    }

    public void submit(FunkovConnection connection)
    {
        tasks.add(connection);
    }


    /**
     * Shutdowns worker after it finish processing all pending tasks on its queue
     */
    public void shutdown()
    {
        isRunningFieldUpdater.compareAndSet(this, RUNNING, SHUTDOWN);
    }

    /**
     * Shutdowns worker after it finish currently processing task. Pending tasks on queue are not handled
     */
    public void shutdownForce()
    {
        isRunningFieldUpdater.compareAndSet(this, RUNNING, FORCE_SHUTDOWN);
    }

    private void setNonRunning()
    {
        isRunningFieldUpdater.set(this, NOT_RUNNING);
    }

    private boolean forceShutdown()
    {
        return isRunningFieldUpdater.get(this) == FORCE_SHUTDOWN;
    }

    private boolean isRunning()
    {
        return isRunningFieldUpdater.getAndSet(this, RUNNING) == RUNNING;
    }

    public boolean notClosed()
    {
        return isRunningFieldUpdater.get(this) == RUNNING;
    }
}

JIT logs:

 1. <task_queued compile_id='535' compile_kind='osr' method='Worker run ()V' bytes='81' count='1' backedge_count='60416' iicount='1' osr_bci='8' level='3' stamp='0,145' comment='tiered' hot_count='60416'/>
 2. <nmethod compile_id='535' compile_kind='osr' compiler='c1' level='3' entry='0x00007fabf5514ee0' size='5592' address='0x00007fabf5514c10' relocation_offset='344' insts_offset='720' stub_offset='4432' scopes_data_offset='4704' scopes_pcs_offset='5040' dependencies_offset='5552' nul_chk_table_offset='5560' oops_offset='4624' metadata_offset='4640' method='Worker run ()V' bytes='81' count='1' backedge_count='65742' iicount='1' stamp='0,146'/>
 3. <deoptimized thread='132773' reason='constraint' pc='0x00007fabf5515c24' compile_id='535' compile_kind='osr' compiler='c1' level='3'>
<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='68801' iicount='1'/>
</deoptimized>
4. <deoptimized thread='132773' reason='constraint' pc='0x00007fabf5515c24' compile_id='535' compile_kind='osr' compiler='c1' level='3'>
<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='76993' iicount='1'/>
</deoptimized>
5.<deoptimized thread='132773' reason='constraint' pc='0x00007fabf5515c24' compile_id='535' compile_kind='osr' compiler='c1' level='3'>
<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='85185' iicount='1'/>
</deoptimized>
6. <deoptimized thread='132773' reason='constraint' pc='0x00007fabf5515c24' compile_id='535' compile_kind='osr' compiler='c1' level='3'>
<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='93377' iicount='1'/>
</deoptimized>

Two questions here:

1. What could be the reason for deoptimization? "constraint" seems not that meaningful to me
2. Why there is so many logs about deoptimization, not just one? It look likes it's compiled once but decompiled multipile times

Answer 1

I'm glad that @aran's suggestion helped in your case, however, it's just a lucky coincidence. After all, JIT inlining options affect many things, including the compilation order, timings, and so on. In fact, deoptimization has nothing to do with inlining.

I was able to reproduce your problem, and here is my analysis.

We see in HotSpot sources that <deoptimized> message is printed by Deoptimization::deoptimize_single_frame function. Let's engage async-profiler to find where this function is called from. To do so, add the following JVM option:

-agentlib:asyncProfiler=start,event=Deoptimization::deoptimize_single_frame,file=deopt.html

Here is the relevant part of the output:

So, the deoptimization reason is Runtime1::counter_overflow function. A method compiled by C1 at tier 3, counts invocations and backward branches (loop iterations). Every 2^{Tier3BackedgeNotifyFreqLog} iterations a method calls Runtime1::counter_overflow to decide whether it should be recompiled at a higher tier.

In your logs we see that backedge_count increments exactly by 8192 (2¹³), and the bytecode at index 37 is goto corresponding to while (notClosed()) loop.

<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='76993' iicount='1'/>
<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='85185' iicount='1'/>
<jvms bci='37' method='Worker run ()V' bytes='81' count='1' backedge_count='93377' iicount='1'/>

When the counter overflows (every 8192 iterations), the JVM checks if an OSR compiled method for the given bytecode index is ready (it might not be ready yet, since JIT compilation runs in background). But if the JVM finds such method, it performs OSR transition by deoptimizing the current frame and replacing it with the corresponding OSR method.

It turns out that in your example the JVM finds an existing OSR method compiled at tier 3. Basically, it deoptimizes Worker.run frame compiled at tier 3, and replaces it by exactly the same method! This repeats again and again, until C2 finishes its background job. Then Worker.run is replaced by tier 4 compilation, and everything becomes fine.

Of course, this should not normally happen. This is actually a JVM bug JDK-8253118. It has been fixed in JDK 16 and will be likely backported to JDK 11u. I have verified that excessive deoptimization does not happen with JDK 16 Early-Access builds.

Answer 2

SPOILER

This was a lucky shot, as @apangin comments. If you want to know what really happens, do not waste time reading this answer.

-- pic of me while answering

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Although the JIT compiler may inline aggressively in some cases, it still has its own time constraints, and won't inline if that's going to take significant time to do. As a result, a method is eligible for inlining only if its bytecode size is less than 35 bytes (by default).

In your case, your methods are 81 bytes of size, hence not eligible:

<jvms bci='37' method='Worker run ()V' bytes='81' ...

Java Performance: The Definitive Guide by Scott Oaks

The basic decision about whether to inline a method depends on how hot it is and its size. The JVM determines if a method is hot (i.e., called frequently) based on an internal calculation; it is not directly subject to any tunable parameters. If a method is eligible for inlining because it is called frequently, then it will be inlined only if its bytecode size is less than 325 bytes (or whatever is specified as the -XX:MaxFreqInlineSize=N flag). Otherwise, it is eligible for inlining only if it is small: less than 35 bytes (or whatever is specified as the -XX:MaxInlineSize=N flag).

In order to make your methods be inlined, you could change the inline size limit by command line, specifying -XX:MaxInlineSize=N.

As a test, you could try specifying something like -XX:MaxInlineSize=90 in order to check if those methods are now inlined.

To the reader - the suggestion above "fixed" the issue(not really fixed). How? No idea. I was even given the correct answer tick lol

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Appendix

(I'll just let this here as seems cool)

-- Workload characterization of JVM languages

Aibek S. Lukas S. Lubomír B. Andreas S. Andrej P. Yudi Z. Walter B.