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Lazy Synchronization MethodsThings to watch out for

Don Stewart | Galois Dev Symposium | Feb 2010

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Control.Concurrent.*

A set of abstractions for threadcommunication and synchroniztion

Mvar synchronizing variables Chan unbounded channels

Simple, clean semantics makes

concurrency easy

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But beware ofevaluation strategies!

putMVar fills an Mvar with a value

but doesn't force evaluation

so mostly you're just putting a pointer to aclosure in there

The thread 'take'ing the message gets the

nasty surprise of having to evaluate themessage! Work gets done in the wrongplace

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Manual fixes

putMVar mv $! x + 1

ok for atomic types (one depth of

constructors)

Harder for normal forms:

let x' = x + 1 in rnf x `seq` putMVar mv x

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Same risks hold for...

IORef

Mvar

Chans (lists of Mvars)

Semaphores...

Data.Map.insertWith...

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More insidious...

modifyMVar considered harmful!

ex1.hs

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Seen in the wild...

Seen in a production system

newUnique (US ref) = atomicModifyIORef ref $ \u ->let !u' = u+1 in (u', Unique u)

This catches even the best programmers!(And worse with the atomic* functions)

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One way to fix it...

ex2.hs

mean 68.3ms, 451us std dev.

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A big hammer

RNF-by-default for synchronizationvariables!

cabal install strict-concurrency First library I wrote at Galois. :-)

ex3.hs

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Timings

mean: 61.7ms, 2ms std dev!

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importControl.Concurrent.MVar.Strict

ex4.hs

Raises the issue: rnf can be very heavy (atraversal on every commit!)

Often wasting work. Need an 'rnf' bit...

(see Andy Gill's proposal)

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Lessons

Be careful with modify* functions

Be careful with lazy synchronization

work gets migrated around

In general,

always use putMVar mv $!

always use strict channels, or manuallyevaluate before putting into the channel

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Addtionally...

block/unblock on exceptions can be reallysubtle.

Be careful! And threadDelay (or other thread stopping

methods) complicates block.

Be careful with async messages +synchronization!