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List:       openjdk-hotspot-dev
Subject:    Re: RFR(XL): 8203469: Faster safepoints
From:       Robbin Ehn <robbin.ehn () oracle ! com>
Date:       2019-01-30 6:08:02
Message-ID: 3031b863-eec5-c22c-ca86-e5880bc520f8 () oracle ! com
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Thanks David!

/Robbin

On 2019-01-30 06:52, David Holmes wrote:
> Incremental looks good.
> 
> Thanks,
> David
> 
> On 28/01/2019 11:04 pm, Robbin Ehn wrote:
>> Hi all, here is v05.
>>
>> http://cr.openjdk.java.net/~rehn/8203469/v05/
>> http://cr.openjdk.java.net/~rehn/8203469/v05/inc/
>>
>> I have been asked to go on-top-of:
>> https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-January/036425.html
>> With a small grace-period.
>> There will be a v06 rebase on-top of that.
>>
>> Updated after comments and changes regarding safepoint_safe().
>> In JFR code path, thread is always current, so it should not be calling
>> safepoint_safe. It also don't control polls, so even if it returns true it is
>> not safe in that case.
>>
>> Updated to a handshake_safe() private method with a friend for handshakes.
>>
>> Test t1-3, stress testing and JFR.
>>
>> 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
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