Quick into to Software Transactional Memory in Frege
1 like1,340 views
The document discusses the implementation of Software Transactional Memory (STM) in the Frege programming language, which is a purely functional JVM language. It highlights several STM actions such as creating transactional variables, managing counters, and handling transactions with checks and invariants to avoid locks and deadlocks. Additionally, it emphasizes the benefits of STM in writing compositional and modular code while acknowledging its limitations.
Quick into to Software Transactional Memory in Frege
- 1. STM Software Transactional Memory in Frege - a purely functional JVM language Quickie OOP 2016
- 5. Silly Clock 10010 every ms
- 6. Silly Clock 10010 every ms on overflow
- 7. Silly Clock 10010 every ms on overflow every so often
- 9. Create new TVar type Counter = TVar Int newCounter :: STM Counter newCounter = TVar.new 0 STM action type
- 10. Read & Write TVar tick :: Counter -> STM () tick counter = do value <- counter.read counter.write (value + 1) STM action (no IO)
- 11. Check transaction invariant maxTick :: Counter -> Int -> STM () maxTick counter max = do tick counter value <- counter.read check (value <= max)
- 12. Check transaction invariant maxTick :: Counter -> Int -> STM () maxTick counter max = do tick counter value <- counter.read check (value <= max) Composition !
- 13. Consistent update onOverflow :: Counter->Counter->Int->STM () onOverflow counter overflowCounter max = do value <- counter.read check (value == max) tick overflowCounter reset counter Composition !
- 14. Type of reset enforced reset :: Counter -> STM () reset counter = counter.write 0 must be STM action
- 15. Atomically report :: Counter -> Counter -> IO () report millis secs = do (millisValue, secsValue) <- atomically $ do a <- millis.read b <- secs.read return (a, b) println $ show secsValue ++ " " ++ show millisValue Transaction demarcation calls STM action inside IO action
- 16. Starting the threads main _ = do millis <- atomically newCounter secs <- atomically newCounter milliOverflow = 1000 runTicker = maxTick millis milliOverflow runSec = onOverflow millis secs milliOverflow forkOS $ forever (atomically runTicker >> Thread.sleep 1) forkOS $ forever (atomically runSec ) forever (report millis secs >> Thread.sleep 100)
- 17. STM Works like compare and swap but for all TVars inside an atomically function plus invariants (check) plus alternatives (orElse) plus proper exception handling No locks -> No deadlocks
- 18. First one wins isolated work commit fail retry
- 19. STM Replaces all your locks just like GC replaces manual memory management Makes compositional and modular code Is optimistic and thus works best in low contention scenarios Is not a panacea (sorry).
- 20. Classic: Account transfer deposit :: Account -> Int -> STM () deposit account amount = do balance <- account.read account.write (balance + amount) withdraw :: Account -> Int -> STM () withdraw account amount = deposit account (- amount) -- composed limitedWithdraw :: Account -> Int -> STM () limitedWithdraw account amount = do withdraw account amount -- composed balance <- account.read check (balance >= 0) transfer :: Account -> Account -> Int -> STM () transfer from to amount = do limitedWithdraw from amount -- composed deposit to amount -- composed
- 23. STM relies upon
- 24. STM relies upon No access to TVars outside transactions No side effects inside transactions
- 25. STM relies upon No access to TVars outside transactions No side effects inside transactions TYPE SYSTEM (pure FP) Developer discipline (everybody else)
- 26. STM relies upon No access to TVars outside transactions No side effects inside transactions TYPE SYSTEM (pure FP) Developer discipline (everybody else) www.frege-lang.org is the only option on the JVM
- 27. Dierk König canoo mittie Credits Volker Steiss master thesis Simon Peyton-Jones Beautiful concurrency
- 28. Dierk König canoo mittie Please give feedback! Credits Volker Steiss master thesis Simon Peyton-Jones Beautiful concurrency