RFR(XL): 8203469: Faster safepoints

Fri Jan 25 16:36:56 UTC 2019

On 1/24/19 10:51 AM, Robbin Ehn wrote:
> Hi here is v04, updated after the comments.
>
> http://cr.openjdk.java.net/~rehn/8203469/v04/inc

src/hotspot/share/code/dependencyContext.hpp
     L110: 
assert(SafepointSynchronize::is_same_safepoint(_safepoint_counter), 
"safepoint happened");
         Perhaps: "must be the same safepoint" for the mesg.

src/hotspot/share/runtime/safepoint.cpp
     L749:   // previous safepoint and reading the reset 
(0/InactiveSafepointCounter) we
         Not quite grammatically correct. Perhaps:

             // previous safepoint and reading the reset value 
(0/InactiveSafepointCounter) we

src/hotspot/share/runtime/safepoint.hpp
     L59:                                                // VM thread 
and any non-Java thread may be running.
         Perhaps:
                                                         // VM thread 
and any NonJavaThread may be running.

     L102:   // If VM thread only waits for callback threads, we wait 
for them on this semaphore.
         Perhaps:
             // If VM thread has to wait for callback threads, it will 
wait for them on this semaphore.

src/hotspot/share/runtime/safepointMechanism.inline.hpp
     Nice catch here!

Thumbs up!

Dan

>
> http://cr.openjdk.java.net/~rehn/8203469/v04/
>
> Still running some tests.
>
> 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
>