GHC heap internals Nikita Frolov < frolov@chalmers.se > - PowerPoint PPT Presentation

Feb 06, 2024 •299 likes •448 views

GHC heap internals Nikita Frolov < frolov@chalmers.se > (courtesy of Bob Ippolito, http://bob.ippoli.to/haskell-for-erlangers-2014/) GHC RTS https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts scheduler garbage collector

GHC heap internals Nikita Frolov < frolov@chalmers.se >
(courtesy of Bob Ippolito, http://bob.ippoli.to/haskell-for-erlangers-2014/)
GHC RTS https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts • scheduler • garbage collector • I/O manager
Scheduler • OS threads vs. Haskell Threads • Thread State Object + stack ( +RTS -ki ) • Tasks (one per OS thread) • can hold a Capability • and hand over too • Capabilities (one per CPU) • +RTS -N
Scheduler • Run queue (one per Capability) • keeps Threads ( forkIO or …) • Sparks • spark pool (one per Capability) • points to a thunk ⇒ can become a Thread
Sparks • can be work-stolen by other Capabilities • can fizzle if thunk already in WHNF • machine busy ⇒ many fizzled sparks • if thunks are never used, sparks will be collected (almost) immediately!
Heap Everything is a closure!
Heap • Roots • NOT Threads (linked to by Run Queues) • Run queue • Spark pool • Generations ( +RTS -G ) • Nursery ( +RTS -A )
Garbage collector • Traverse from the root, copy, scrap the rest • Oldest generations are collected least often • Eager promotion: if pointed to by an old object • Aging: don’t promote to quickly though
Garbage collector • “Allocation wall” • per-thread nurseries fitting into L2 cache • but frequent collections will stop the world often • running mutator and collector concurrently hurts cache • Private heaps to every CPU!
holds/releases OS thread Task Capability run queue empty thread preempted Spark pool Run queue scheduler selects converted par Thunk Spark Haskell thread fizzled not referenced finished execution GC
Remember • Play with heap and nursery sizes • Too big nursery: bad locality, less promotions • Too small nursery: unnecessary promotions • Too small starting heap: takes time to expand • Distribute work evenly between sparks!
To read • https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts • http://www.haskell.org/ghc/docs/7.10.3 /html/users_guide/runtime-control.html • GHC Illustrated • Runtime Support for Multicore Haskell • Multicore Garbage Collection with Local Heaps • Mio: A High-Performance Multicore IO Manager for GHC • The Implementation of Functional Programming Languages
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