RFR(XL): 8203469: Faster safepoints

Thu Feb 7 16:05:11 UTC 2019

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