Inner Workings of Safepoints

A Java thread in the JVM regularly checks whether it should do extra work besides the execution of the bytecode. This work is done during so-called safepoints. There are two types of safepoints: local and global. This allows the JVM to do activities like method deoptimizations or stop-the-world garbage collections, where the amount of concurrency should be limited. But how do they actually work?

This talk covers the implementation of global and local safepoints in the OpenJDK.

Transcript

1. Inner Workings of Safepoints Johannes Bechberger mostlynerdless.de
2. Kinds of Safepoints Local Safepoint/Handshake since 2017
3. Kinds of Safepoints Global Safepoint/Handshake
4. And they get ever more important
5. int mul(int a, int b) { int res = 0; while (b > 0) { res += a; b--; } return res; } Beware of inlining
6. int mul(int a, int b) { int res = 0; for (int i = 0; i < b; i++) { res += a; } return res; }
7. int mul(int a, int b) { int res = 0; for (int j = 0; j < b; j += 1000) { for (int i = j; i < j + 1000; i++) { res += i; } } return res; } Loop Strip Mining
10. Safepoint biased profilers are: • Tricksy • Sneaksy • Filthy • All of the above — Nitsan Wakart “
11. Implementation
12. We could just... if (thread->at_safepoint()) { SafepointMechanism::process(); } else { // do nothing } Slow, only in interpreted mode
13. Emit in MacroAssembler testb(Address(thread_reg, JavaThread::polling_word_offset()), SafepointMechanism::poll_bit()); // handshake bit set implies poll jcc(Assembler::notZero, slow_path); https://github.com/openjdk/jdk/blob/a18b03b86fdd0eef773badbced46607a8e5a068a/src/hotspot/cpu/x8 6/macroAssembler_x86.cpp#L3073
14. We could just... if (thread->at_safepoint()) { SafepointMechanism::process(); } else { // do nothing } rare often
15. We could just... if (thread->at_safepoint()) { SafepointMechanism::process(); } else { // do nothing } slow path fast path
16. Read from pointer data Segmentation fault or
17. void method() { ... // check } name ... polling_page ... good page bad page Current Thread Safepoint disabled enabled ↯ Segfault Safepoint
19. Initialize per thread char* bad_page = polling_page; char* good_page = polling_page + page_size; os::protect_memory(bad_page, page_size, os::MEM_PROT_NONE); os::protect_memory(good_page, page_size, os::MEM_PROT_READ); //... _poll_page_armed_value = reinterpret_cast<uintptr_t>(bad_page); _poll_page_disarmed_value = reinterpret_cast<uintptr_t>(good_page); https://github.com/openjdk/jdk/blob/70e7cdcb9ecedd455cf701b5c7ad05286ac0067d/src/hotspot/share/r untime/safepointMechanism.cpp#L65
20. Emit e.g. in C1 int LIR_Assembler::safepoint_poll( LIR_Opr tmp, CodeEmitInfo* info) { int offset = __ offset(); const Register poll_addr = rscratch1; __ movptr(poll_addr, Address(r15_thread, JavaThread::polling_page_offset())); // ... } https://github.com/openjdk/jdk/blob/70e7cdcb9ecedd455cf701b5c7ad05286ac0067d/src/hotspot/cpu/x8 6/c1_LIRAssembler_x86.cpp#L543
21. Arming local safepoints void SafepointMechanism::arm_local_poll( JavaThread* thread) { thread->poll_data() ->set_polling_word(_poll_word_armed_value); thread->poll_data() ->set_polling_page(_poll_page_armed_value); }
22. Arming global safepoints _state = _synchronizing; for (JavaThreadIteratorWithHandle jtiwh; JavaThread *cur = jtiwh.next();) { SafepointMechanism::arm_local_poll(cur); }
23. Tracking Safepoints
25. -Xlog:safepoint
26. G1PauseRemar k 15% G1TryInitiateConcMark 15% G1CollectFull 2% ICBufferFull 1% G1PauseCleanup 15% G1CollectForAllocation 49% Cleanup 3%
27. @parttimen3rd on Twitter parttimenerd on GitHub mostlynerdless.de @SweetSapMachine sapmachine.io