RFR: 8291555: Implement alternative fast-locking scheme [v4]

Fri Jan 27 23:31:26 UTC 2023

On Wed, 18 Jan 2023 20:10:21 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This change adds a fast-locking scheme as an alternative to the current stack-locking implementation. It retains the advantages of stack-locking (namely fast locking in uncontended code-paths), while avoiding the overload of the mark word. That overloading causes massive problems with Lilliput, because it means we have to check and deal with this situation when trying to access the mark-word. And because of the very racy nature, this turns out to be very complex and would involve a variant of the inflation protocol to ensure that the object header is stable. (The current implementation of setting/fetching the i-hash provides a glimpse into the complexity).
>> 
>> What the original stack-locking does is basically to push a stack-lock onto the stack which consists only of the displaced header, and CAS a pointer to this stack location into the object header (the lowest two header bits being 00 indicate 'stack-locked'). The pointer into the stack can then be used to identify which thread currently owns the lock.
>> 
>> This change basically reverses stack-locking: It still CASes the lowest two header bits to 00 to indicate 'fast-locked' but does *not* overload the upper bits with a stack-pointer. Instead, it pushes the object-reference to a thread-local lock-stack. This is a new structure which is basically a small array of oops that is associated with each thread. Experience shows that this array typcially remains very small (3-5 elements). Using this lock stack, it is possible to query which threads own which locks. Most importantly, the most common question 'does the current thread own me?' is very quickly answered by doing a quick scan of the array. More complex queries like 'which thread owns X?' are not performed in very performance-critical paths (usually in code like JVMTI or deadlock detection) where it is ok to do more complex operations (and we already do). The lock-stack is also a new set of GC roots, and would be scanned during thread scanning, possibly concurrently, via the normal 
 protocols.
>> 
>> The lock-stack is grown when needed. This means that we need to check for potential overflow before attempting locking. When that is the case, locking fast-paths would call into the runtime to grow the stack and handle the locking. Compiled fast-paths (C1 and C2 on x86_64 and aarch64) do this check on method entry to avoid (possibly lots) of such checks at locking sites.
>> 
>> In contrast to stack-locking, fast-locking does *not* support recursive locking (yet). When that happens, the fast-lock gets inflated to a full monitor. It is not clear if it is worth to add support for recursive fast-locking.
>> 
>> One trouble is that when a contending thread arrives at a fast-locked object, it must inflate the fast-lock to a full monitor. Normally, we need to know the current owning thread, and record that in the monitor, so that the contending thread can wait for the current owner to properly exit the monitor. However, fast-locking doesn't have this information. What we do instead is to record a special marker ANONYMOUS_OWNER. When the thread that currently holds the lock arrives at monitorexit, and observes ANONYMOUS_OWNER, it knows it must be itself, fixes the owner to be itself, and then properly exits the monitor, and thus handing over to the contending thread.
>> 
>> As an alternative, I considered to remove stack-locking altogether, and only use heavy monitors. In most workloads this did not show measurable regressions. However, in a few workloads, I have observed severe regressions. All of them have been using old synchronized Java collections (Vector, Stack), StringBuffer or similar code. The combination of two conditions leads to regressions without stack- or fast-locking: 1. The workload synchronizes on uncontended locks (e.g. single-threaded use of Vector or StringBuffer) and 2. The workload churns such locks. IOW, uncontended use of Vector, StringBuffer, etc as such is ok, but creating lots of such single-use, single-threaded-locked objects leads to massive ObjectMonitor churn, which can lead to a significant performance impact. But alas, such code exists, and we probably don't want to punish it if we can avoid it.
>> 
>> This change enables to simplify (and speed-up!) a lot of code:
>> 
>> - The inflation protocol is no longer necessary: we can directly CAS the (tagged) ObjectMonitor pointer to the object header.
>> - Accessing the hashcode could now be done in the fastpath always, if the hashcode has been installed. Fast-locked headers can be used directly, for monitor-locked objects we can easily reach-through to the displaced header. This is safe because Java threads participate in monitor deflation protocol. This would be implemented in a separate PR
>> 
>> 
>> Testing:
>>  - [x] tier1 x86_64 x aarch64 x +UseFastLocking
>>  - [x] tier2 x86_64 x aarch64 x +UseFastLocking
>>  - [x] tier3 x86_64 x aarch64 x +UseFastLocking
>>  - [x] tier4 x86_64 x aarch64 x +UseFastLocking
>>  - [x] tier1 x86_64 x aarch64 x -UseFastLocking
>>  - [x] tier2 x86_64 x aarch64 x -UseFastLocking
>>  - [x] tier3 x86_64 x aarch64 x -UseFastLocking
>>  - [x] tier4 x86_64 x aarch64 x -UseFastLocking
>>  - [x] Several real-world applications have been tested with this change in tandem with Lilliput without any problems, yet
>> 
>> ### Performance
>> 
>> #### Renaissance
>> 
>> 
>> 
>>   | x86_64 |   |   |   | aarch64 |   |  
>> -- | -- | -- | -- | -- | -- | -- | --
>>   | stack-locking | fast-locking |   |   | stack-locking | fast-locking |  
>> AkkaUct | 841.884 | 836.948 | 0.59% |   | 1475.774 | 1465.647 | 0.69%
>> Reactors | 11444.511 | 11606.66 | -1.40% |   | 11382.594 | 11638.036 | -2.19%
>> Als | 1367.183 | 1359.358 | 0.58% |   | 1678.103 | 1688.067 | -0.59%
>> ChiSquare | 577.021 | 577.398 | -0.07% |   | 986.619 | 988.063 | -0.15%
>> GaussMix | 817.459 | 819.073 | -0.20% |   | 1154.293 | 1155.522 | -0.11%
>> LogRegression | 598.343 | 603.371 | -0.83% |   | 638.052 | 644.306 | -0.97%
>> MovieLens | 8248.116 | 8314.576 | -0.80% |   | 9898.1 | 10097.867 | -1.98%
>> NaiveBayes | 587.607 | 581.608 | 1.03% |   | 541.583 | 550.059 | -1.54%
>> PageRank | 3260.553 | 3263.472 | -0.09% |   | 4376.405 | 4381.101 | -0.11%
>> FjKmeans | 979.978 | 976.122 | 0.40% |   | 774.312 | 771.235 | 0.40%
>> FutureGenetic | 2187.369 | 2183.271 | 0.19% |   | 2685.722 | 2689.056 | -0.12%
>> ParMnemonics | 2527.228 | 2564.667 | -1.46% |   | 4278.225 | 4263.863 | 0.34%
>> Scrabble | 111.882 | 111.768 | 0.10% |   | 151.796 | 153.959 | -1.40%
>> RxScrabble | 210.252 | 211.38 | -0.53% |   | 310.116 | 315.594 | -1.74%
>> Dotty | 750.415 | 752.658 | -0.30% |   | 1033.636 | 1036.168 | -0.24%
>> ScalaDoku | 3072.05 | 3051.2 | 0.68% |   | 3711.506 | 3690.04 | 0.58%
>> ScalaKmeans | 211.427 | 209.957 | 0.70% |   | 264.38 | 265.788 | -0.53%
>> ScalaStmBench7 | 1017.795 | 1018.869 | -0.11% |   | 1088.182 | 1092.266 | -0.37%
>> Philosophers | 6450.124 | 6565.705 | -1.76% |   | 12017.964 | 11902.559 | 0.97%
>> FinagleChirper | 3953.623 | 3972.647 | -0.48% |   | 4750.751 | 4769.274 | -0.39%
>> FinagleHttp | 3970.526 | 4005.341 | -0.87% |   | 5294.125 | 5296.224 | -0.04%
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert UseFastLocking to default to off

src/hotspot/share/oops/markWord.hpp line 183:

> 181:   }
> 182:   markWord set_fast_locked() const {
> 183:     return markWord(value() & ~lock_mask_in_place);

Perhaps add a comment above L183:

    // Clear the lock_mask_in_place bits to set locked_value:

src/hotspot/share/runtime/lockStack.cpp line 34:

> 32: 
> 33: LockStack::LockStack() :
> 34:   _base(UseFastLocking && !UseHeavyMonitors ? NEW_C_HEAP_ARRAY(oop, INITIAL_CAPACITY, mtSynchronizer) : NULL),

Okay so `UseFastLocking && UseHeavyMonitors`, then we don't need the lock stack.

src/hotspot/share/runtime/lockStack.inline.hpp line 81:

> 79:     }
> 80:   }
> 81:   validate("post-contains");

You only do the `validate("post-contains")` call when `false` is
returned. Why not also validate for the `true` branch?

src/hotspot/share/runtime/objectMonitor.cpp line 1141:

> 1139:       assert(_recursions == 0, "invariant");
> 1140:       set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
> 1141:       _recursions = 0;

Hmmm... fast-locking also has two different ways that a thread can own an
ObjectMonitor: 1) owner == thread* and 2) owner == anonymous and lock on
the owning thread's lock stack.

If `UseFastLocking` is enabled and `ObjectMonitor::exit()` is called by a thread that
owns the ObjectMonitor in the secondary way, then this code will fail the assert on
L1158 in non-product bits. There needs to be check to see if the current thread owns
the fast-lock and then update the owner in the ObjectMonitor after asserting about
recursions and resetting recursions to 0.

src/hotspot/share/runtime/synchronizer.cpp line 568:

> 566:           monitor->set_owner_from_anonymous(current);
> 567:           monitor->exit(current);
> 568:         }

Hmmm... We're in `ObjectSynchronizer::exit()` so we should be the owner of
the ObjectMonitor so I'm not yet sure what "Another thread beat us" means.

XXX

-------------

PR: https://git.openjdk.org/jdk/pull/10907