quick into to software transactional memory in frege

STMSoftware Transactional Memory

in Frege - a purely functional JVM language Quickie

OOP 2016

Dierk König Canoo

mittie

Dierk König Canoo

mittie

Silly Clock

10010

Silly Clock

10010 every ms

Silly Clock

10010 every ms

on overflow

Silly Clock

10010 every ms

on overflow

every so often

Transactional Variables

10010

CounterCounter

millissecs

TVar Int

Create new TVartype Counter = TVar Int

newCounter :: STM CounternewCounter = TVar.new 0

STM action type

Read & Write TVartick :: Counter -> STM ()tick counter = do value <- counter.read counter.write (value + 1)

STM action (no IO)

Check transaction invariantmaxTick :: Counter -> Int -> STM ()maxTick counter max = do tick counter value <- counter.read check (value <= max)

Check transaction invariantmaxTick :: Counter -> Int -> STM ()maxTick counter max = do tick counter value <- counter.read check (value <= max)

Composition !

Consistent updateonOverflow :: Counter->Counter->Int->STM ()onOverflow counter overflowCounter max = do value <- counter.read check (value == max) tick overflowCounter reset counter

Composition !

Type of reset enforcedreset :: Counter -> STM ()reset counter = counter.write 0

must be STM action

Atomicallyreport :: 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

Starting the threadsmain _ = 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)

STMWorks like compare and swap but for all TVars inside an atomically functionplus invariants (check)plus alternatives (orElse)plus proper exception handling

No locks -> No deadlocks

First one wins

isolated work commit

failretry

STMReplaces 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).

Classic: Account transferdeposit :: 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

Classic: Ant colony

AntsFood Pheromones Evaporation Reporter

STM relies upon

STM relies uponNo access to TVars outside transactionsNo side effects inside transactions

STM relies uponNo access to TVars outside transactionsNo side effects inside transactions

TYPE SYSTEM (pure FP)Developer discipline (everybody else)

STM relies uponNo access to TVars outside transactionsNo side effects inside transactions

TYPE SYSTEM (pure FP)Developer discipline (everybody else)

www.frege-lang.org is the only option on the JVM

Dierk König canoo

mittie

Credits Volker Steiss master thesis Simon Peyton-Jones Beautiful concurrency

Dierk König canoo

mittie

Please give feedback!

Credits Volker Steiss master thesis Simon Peyton-Jones Beautiful concurrency