RFR: 8291555: Implement alternative fast-locking scheme [v78]

Mon May 8 16:03:17 UTC 2023

On Fri, 5 May 2023 16:49:38 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 fixed size, currently with 8 elements. According to my experiments with various workloads, this covers the vast majority of workloads (in-fact, most workloads seem to never exceed 5 active locks per thread at a time). We check for overflow in the fast-paths and when the lock-stack is full, we take the slow-path, which would inflate the lock to a monitor. That case should be very rare.
>> 
>> 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
>> 
>> Also, and I might be mistaken here, this new lightweight locking would make synchronized work better with Loom: Because the lock-records are no longer scattered across the stack, but instead are densely packed into the lock-stack, it should be easy for a vthread to save its lock-stack upon unmounting and restore it when re-mounting. However, I am not sure about this, and this PR does not attempt to implement that support.
>> 
>> 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
>> 
>> #### Simple Microbenchmark
>> 
>> The microbenchmark exercises only the locking primitives for monitorenter and monitorexit, without contention. The benchmark can be found (here)[https://github.com/rkennke/fastlockbench]. Numbers are in ns/ops.
>> 
>> |  | x86_64 | aarch64 |
>> | -- | -- | -- |
>> | -UseFastLocking | 20.651 | 20.764 |
>> | +UseFastLocking | 18.896 | 18.908 |
>> 
>> 
>> #### 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 | 11041.427 | 11181.451 | -1.25% |   | 11381.751 | 11521.318 | -1.21%
>> 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% |   | 7569.219 | 7646.828 | -1.01%%
>> 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 | 2434.551 | 2468.763 | -1.39% |   | 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:
> 
>   Only allow lock-stack verification for owning Java threads or at safepoints

I discussed the perf testing results with Eric Caspole and here's our summary:

Promotion performance testing was done on the jdk-21+21-1704 baseline, the v66 patch with
default-stack-locking and the v66 patch with forced-fast-locking. Comparing the baseline with
default-stack-locking: 62 improvements and 54 regressions, none of the improvements or
regressions are statistically significant.

Comparing the baseline with forced-fast-locking: 68 improvements and 177 regressions, none
of the improvements or regressions are statistically significant.

Comparing default-stack-locking with forced-fast-locking: 45 improvements, 173 regressions,
none of the improvements or regressions are statistically significant.

Eric Caspole and I are both "go" from a performance testing POV! (This is for the stack-locking
as the default configuration.)

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

PR Comment: https://git.openjdk.org/jdk/pull/10907#issuecomment-1538649719