RFR(XL): 8203469: Faster safepoints

[Robbin Ehn]

Hi all,

Updated of v2:
Full:
http://cr.openjdk.java.net/~rehn/8203469/v06_2_u1/full/
(open.changeset still two patches, e.g. if you look at interfaceSupport.inline.hpp it's patched twice)
Inc:
http://cr.openjdk.java.net/~rehn/8203469/v06_2_u1/inc/

Passes several hours more stress testing and t1-5, KS 24H stress still running.

I did update alternative one also with Dan's feedback, and it also still passes stress tests and t1-5.
I'll leave that unpublished since we are focusing on this version where we can get some simplifications.

Thanks, Robbin

On 2019-02-07 17:05, Robbin Ehn wrote:
> Hi all, here is the promised re-base (v06) on
> 8210832: Remove sneaky locking in class Monitor.
> 
> v06_1 is just a straight re-base.
> 
> Full:
> http://cr.openjdk.java.net/~rehn/8203469/v06_1/full/
> Inc:
> http://cr.openjdk.java.net/~rehn/8203469/v06_1/inc/
> 
> Passes stress test and t1-5.
> 
> But there is a 'better' way.
> Before I added the more graceful "_vm_wait->wait();" semaphore in the while
> (_waiting_to_block > 0) { loop, it was a just a busy spin using the same
> back-off as the rolling forward loop. It turns out that we mostly never spin
> here at all, when all java threads are stop the callbacks is often already done.
> So the addition of the semaphore have no impact on our benchmarks and is mostly
> unused. This is because most threads are in java which we need to spin-wait
> since they can elide into native without doing a callback. My proposed re-base
> removes the the callbacks completely and let the vm thread do all thread
> accounting. All that the stopping threads needs to do is write state and
> safepoint id, everything else is handle by the vm thread. We trade 2 atomics +
> a local store per thread against doing 2 stores per thread by the vm thread.
> This makes it possible for a thread in vm to transition into blocked WITHOUT
> safepoint poll. Just set thread_blocked and promise to do safepoint poll when
> leaving that state.
> 
> v06_2
> Full:
> http://cr.openjdk.java.net/~rehn/8203469/v06_2/full/
> Inc against v05:
> http://cr.openjdk.java.net/~rehn/8203469/v06_2/inc/
> Inc against v06_1:
> http://cr.openjdk.java.net/~rehn/8203469/v06_2/rebase_inc/
> 
> v06_2 simplifies and removes ~200 LOC with same performance.
> If there is a case with a thread in vm taking long time, it will already
> screw-up latency and thus should be fixed regardless of v06_1 vs v06_2. So I
> see no reason why we should not push v06_2.
> 
> Passes stress test and t1-5.
> 
> Thanks, Robbin
> 
> 
> On 1/15/19 11:39 AM, Robbin Ehn wrote:
>> Hi all, please review.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8203469
>> Code: http://cr.openjdk.java.net/~rehn/8203469/v00/webrev/
>>
>> Thanks to Dan for pre-reviewing a lot!
>>
>> Background:
>> ZGC often does very short safepoint operations. For a perspective, in a
>> specJBB2015 run, G1 can have young collection stops lasting about 170 ms. While
>> in the same setup ZGC does 0.2ms to 1.5 ms operations depending on which
>> operation it is. The time it takes to stop and start the JavaThreads is relative
>> very large to a ZGC safepoint. With an operation that just takes 0.2ms the
>> overhead of stopping and starting JavaThreads is several times the operation.
>>
>> High-level functionality change:
>> Serializing the starting over Threads_lock takes time.
>> - Don't wait on Threads_lock use the WaitBarrier.
>> Serializing the stopping over Safepoint_lock takes time.
>> - Let threads stop in parallel, remove Safepoint_lock.
>>
>> Details:
>> JavaThreads have 2 abstract logical states: unsafe or safe.
>> - Safe means the JavaThread will not touch Java heap or VM internal structures
>>    without doing a transition and block before doing so.
>>          - The safe states are:
>>                  - When polls armed: _thread_in_native and _thread_blocked.
>>                  - When Threads_lock is held: externally suspended flag is set.
>>          - VM Thread have polls armed and holds the Threads_lock during a
>>            safepoint.
>> - Unsafe means that either Java heap or VM internal structures can be accessed
>>    by the JavaThread, e.g., _thread_in_Java, _thread_in_vm.
>>          - All combination that are not safe are unsafe.
>>
>> We cannot start a safepoint until all unsafe threads have transitioned to a safe
>> state. To make them safe, we arm polls in compiled code and make sure any
>> transition to another unsafe state will be blocked. JavaThreads which are unsafe
>> with state _thread_in_Java may transition to _thread_in_native without being
>> blocked, since it just became a safe thread and we can proceed. Any safe thread
>> may try to transition at any time to an unsafe state, thus coming into the
>> safepoint blocking code at any moment, e.g., after the safepoint is over, or
>> even at the beginning of next safepoint.
>>
>> The VMThread cannot tolerate false positives from the JavaThread thread state
>> because that would mean starting the safepoint without all JavaThreads being
>> safe. The two locks (Threads_lock and Safepoint_lock) make sure we never observe
>> false positives from the safepoint blocking code, if we remove them, how do we
>> handle false positives?
>>
>> By first publishing which barrier tag (safepoint counter) we will call
>> WaitBarrier.wait() with as the threads safepoint id and then change the state to
>> _thread_blocked, the VMThread can ignore JavaThreads by doing a stable load of
>> the state. A stable load of the thread state is successful if the thread
>> safepoint id is the same both before and after the load of the state and
>> safepoint id is current or InactiveSafepointCounter. If the stable load fails,
>> the thread is considered safepoint unsafe. It's no longer enough that thread is
>> have state _thread_blocked it must also have correct safepoint id before and
>> after we read the state.
>>
>> Performance:
>> The result of faster safepoints is that the average CPU time for JavaThreads
>> between safepoints is higher, thus increasing the allocation rate. The thread
>> that stops first waits shorter time until it gets started. Even the thread that
>> stops last also have shorter stop since we start them faster. If your
>> application is using a concurrent GC it may need re-tunning since each java
>> worker thread have an increased CPU time/allocation rate. Often this means max
>> performance is achieved using slightly less java worker threads than before.
>> Also the increase allocation rate means shorter time between GC safepoints.
>> - If you are using a non-concurrent GC, you should see improved latency and
>>    throughput.
>> - After re-tunning with a concurrent GC throughput should be equal or better but
>>    with better latency. But bear in mind this is a latency patch, not a
>>    throughput one.
>> With current code a java thread is not to guarantee to run between safepoint (in
>> theory a java thread can be starved indefinitely), since the VM thread may
>> re-grab the Threads_locks before it woke up from previous safepoint. If the
>> GC/VM don't respect MMU (minimum mutator utilization) or if your machine is very
>> over-provisioned this can happen.
>> The current schema thus re-safepoint quickly if the java threads have not
>> started yet at the cost of latency. Since the new code uses the WaitBarrier with
>> the safepoint counter, all threads must roll forward to next safepoint by
>> getting at least some CPU time between two safepoints. Meaning MMU violations
>> are more obvious.
>>
>> Some examples on numbers:
>> - On a 16 strand machine synchronization and un-synchronization/starting is at
>>    least 3x faster (in non-trivial test). Synchronization ~600 -> ~100us and
>>    starting ~400->~100us.
>>    (Semaphore path is a bit slower than futex in the WaitBarrier on Linux).
>> - SPECjvm2008 serial (untuned G1) gives 10x (1 ms vs 100 us) faster
>>    synchronization time on 16 strands and ~5% score increase. In this case the GC
>>    op is 1ms, so we reduce the overhead of synchronization from 100% to 10%.
>> - specJBB2015 ParGC ~9% increase in critical-jops.
>>
>> Thanks, Robbin