Extending OpenJDK To Support Hybrid STM/HTM
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This document discusses extending OpenJDK to support hybrid software transactional memory (STM) and hardware transactional memory (HTM). It describes the current XJ system architecture, which uses bytecode rewriting and a runtime library to support transactions in Java. The document proposes direct extensions to OpenJDK to support transactions more efficiently, including modifications to the locking protocol and compiler optimizations to better leverage Intel TSX hardware transactional memory.
Extending OpenJDK To Support Hybrid STM/HTM
- 1. Extending OpenJDK To Support Hybrid STM/HTM Keith Chapman Purdue U Tony Hosking ANU/Data61, Purdue U Eliot Moss UMass
- 2. Transactions are Good They deal with concurrency • Atomic transactions avoid problems with locks • Deadlock, wrong lock, priority inversion, etc.
- 3. Transactions are Good They deal with concurrency • Atomic transactions avoid problems with locks • Deadlock, wrong lock, priority inversion, etc. They handle recovery • Retry in case of conflict • Cleanup in face of exceptions/errors
- 4. Transactions are Good They deal with concurrency • Atomic transactions avoid problems with locks • Deadlock, wrong lock, priority inversion, etc. They handle recovery • Retry in case of conflict • Cleanup in face of exceptions/errors More practical for ordinary programmers than locks for robust concurrent systems
- 5. Semantics of Transactions They offer A,C,I of database ACID properties: • Atomicity: all or nothing • Consistency: single global order • Isolation: intermediate states invisible
- 6. Semantics of Transactions They offer A,C,I of database ACID properties: • Atomicity: all or nothing • Consistency: single global order • Isolation: intermediate states invisible In sum, serializability, in face of concurrent execution and transaction failures
- 7. Semantics of Transactions They offer A,C,I of database ACID properties: • Atomicity: all or nothing • Consistency: single global order • Isolation: intermediate states invisible In sum, serializability, in face of concurrent execution and transaction failures Can be provided by Transactional Memory • Hardware, software, or hybrid
- 8. XJ: Transactional Java XJ Language • Dialect of Java • Supports flat and closed/open/boosted nested transactions
- 9. XJ: Transactional Java XJ Language • Dialect of Java • Supports flat and closed/open/boosted nested transactions XJ framework supports hybrid transactions • HTM support via Intel TSX
- 10. Current XJ System Architecture
- 11. Current XJ System Architecture XJ source code
- 12. Current XJ System Architecture XJ source code XJ Compiler
- 13. Current XJ System Architecture XJ source code XJ Compiler compile
- 14. Current XJ System Architecture XJ source code XJ Compiler standard Java bytecode compile
- 15. Current XJ System Architecture XJ source code XJ Compiler standard Java bytecode XJ Rewriter compile
- 16. Current XJ System Architecture XJ source code XJ Compiler standard Java bytecode XJ Rewriter compile load
- 17. Current XJ System Architecture XJ source code XJ Compiler standard Java bytecode XJ Rewriter bytecode + run-time calls compile load
- 18. Current XJ System Architecture XJ source code XJ run-time library XJ Compiler standard Java bytecode XJ Rewriter bytecode + run-time calls compile load
- 19. Current XJ System Architecture XJ source code XJ run-time library XJ Compiler standard Java bytecode XJ Rewriter bytecode + run-time calls HTM-enabled JVM compile load
- 20. Current XJ System Architecture XJ source code XJ run-time library XJ Compiler standard Java bytecode XJ Rewriter bytecode + run-time calls HTM-enabled JVM compile load run
- 21. Current XJ System Architecture HTM 5-10 times faster than STM XJ source code XJ run-time library XJ Compiler standard Java bytecode XJ Rewriter bytecode + run-time calls HTM-enabled JVM compile load run
- 22. Drawbacks of Outside-VM Approach Requires bytecode rewriting for transaction mechanisms & metadata • Extra level of indirection when accessing object lock/version
- 23. Drawbacks of Outside-VM Approach Requires bytecode rewriting for transaction mechanisms & metadata • Extra level of indirection when accessing object lock/version Interpreter and compiler challenges • Jump through hoops to get compiled code to work under HTM
- 24. Drawbacks of Outside-VM Approach Requires bytecode rewriting for transaction mechanisms & metadata • Extra level of indirection when accessing object lock/version Interpreter and compiler challenges • Jump through hoops to get compiled code to work under HTM Less opportunity for optimization
- 25. Proposed Extensions to OpenJDK 1. Locking protocol 2. Compiler enhancements for TSX 3. Optimizations
- 26. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 hash code age 0 01 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) if currently unlocked if currently locked revoke biasrebiasinitial lock thin lock inflate lock recursive lock lock / unlock If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 00 pointer to heavyweight monitor 10 (heavyweight locked object)
- 27. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (heavyweight hashed/locked object) if currently unlocked if currently locked revoke biasrebiasinitial lock thin lock inflate lock recursive lock lock / unlock If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 28. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock thin lock inflate lock recursive lock lock / unlock txnread If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 29. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock thin lock inflate lock recursive lock lock / unlock txnread txnwrite If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 30. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock txn write thin lock inflate lock recursive lock lock / unlock txnread txnwrite If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 31. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock txn write txn commit thin lock inflate lock recursive lock lock / unlock txnread txnwrite If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 32. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock txn write txn commit thin lock inflate lock recursive lock lock / unlock txn read/write txnread txnwrite If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 33. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock txn write txn commit thin lock inflate lock recursive lock lock / unlock lock / hash lock / hash txn read/write txnread txnwrite If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 34. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock txn write txn commit thin lock inflate lock recursive lock lock / unlock lock / hash unlock (unhashed) lock / hash unlock (unhashed) txn read/write txnread txnwrite If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 35. allocate object 0 epoch age 1 01 thread ID epoch age 1 01 Txn ID epoch age 1 000 version epoch age 1 001 hash code age 0 01 pointer to heavyweight monitor 100 (unlocked and unbiased but biasable object) (unlocked non-biasable object) (biased object, locked or unlocked) (lightweight locked object) (transactionally write locked object) (heavyweight hashed/locked object) (transactionally read object) if currently unlocked if currently locked revoke biasrebiasinitial lock txn write txn commit thin lock inflate lock recursive lock lock / unlock lock / hash unlock (unhashed) lock / hash unlock (unhashed) txn read/write txnread txnwrite unlock(non-txnal) txnread/write If biased locking is disabled for the classIf biased locking is enabled for the class pointer to lock record 000
- 36. Locking Protocol (continued) What about statics? • Distinct field to hold lock/version for statics • Can extend to have distinct locks for disjoint subsets of statics
- 37. Locking Protocol (continued) What about statics? • Distinct field to hold lock/version for statics • Can extend to have distinct locks for disjoint subsets of statics How about arrays? • Use arraylets (add a lock field for each arraylet)
- 38. Compiler Challenges HTM does not work under the interpreter • Will fail because of buffer overflow
- 39. Compiler Challenges HTM does not work under the interpreter • Will fail because of buffer overflow HTM does not work under compilation either :-(
- 40. Compiler Challenges HTM does not work under the interpreter • Will fail because of buffer overflow HTM does not work under compilation either :-( • Txn-illegal instructions in HotSpot’s dynamic code patching stubs
- 41. Compiler Challenges HTM does not work under the interpreter • Will fail because of buffer overflow HTM does not work under compilation either :-( • Txn-illegal instructions in HotSpot’s dynamic code patching stubs • “Short-term memory loss” (so stubs can’t be updated)
- 42. Compiler Challenges (continued) TSX classifies certain instructions as illegal inside of a transaction • e.g., FXRSTOR and FXSAVE • Extensively used in HotSpot’s dynamic code patching stubs • These never work inside a hardware transaction
- 43. TSX XBEGIN — Begin a transaction XEND — End a transaction XABORT — Abort a transaction explicitly • Takes an 8-bit value as argument • Returned to the user in bits 31:24 of EAX
- 44. Addressing Compiler Challenges Using HTM failure codes • “Return” 8 bits of information, via the EAX register • Simple if less than 256 guards • Can use a multi-step encoding technique to handle more guards
- 45. Addressing Compiler Challenges Using HTM failure codes • “Return” 8 bits of information, via the EAX register • Simple if less than 256 guards • Can use a multi-step encoding technique to handle more guards Maintain correlated STM and HTM code versions
- 46. More Integrated Optimizations • Additional data-flow based optimization for STM/HTM methods • Elide redundant locks • Hoist certain locks
- 47. Future TM Hardware Suggestions Return additional full-word error code
- 48. Future TM Hardware Suggestions Return additional full-word error code Some kind of extra-transactional writes
- 49. Questions?