Go &The garbage collection

“Joris BonnefoyDevOps @ OVH

@devatoria

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IntroductionReminder of basic algorithmic structures

» Objects have a pointer to the next element of the list

» The last element have a null pointer

Linked list 4

» Objects have pointers to the next and the previous element of the list

» The first element have a null previous pointer» The last element have a null next pointer

Doubly linked list 5

» Objects have pointers to the next and the previous element of the list

» The first element have a previous pointer to the last element of the list

» The last element have a next pointer to the first element of the list

Circular doubly linked list 6

A simple problemHow to free a dereferenced object?

How to free a dereferenced object? 8

How to free a dereferenced object? 9

How to free a dereferenced object? 10

A solutionThe garbage collection

The garbage collection

» John McCarthy - 1959 - Lisp» Opposite of manual memory management» Used to free unused allocated memory» Can handle more types of resources (network

sockets, database handlers, …) using finalization and region-based allocation

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The garbage collection

» No dangling pointers» No memory leaks» No double free

» Can reduce performances» Stop-the-world issues» Can be unpredictable

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Two types of garbage collector

Reference counting

How reference counting is implemented?

» Each object has a count of other objects referencing it

» An object is considered as garbage when its counter reach 0

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How reference counting is implemented? 17

How reference counting is implemented? 18

Disadvantages of reference counting

» Sensitive to cycle referencing

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Disadvantages of reference counting

» Increased space overhead» Must be atomic» Not real-time

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Tracing garbage collection

Concept of tracing garbage collection

» Determine which objects are reachable from a certain “root” object

» Unreachable objects are considered as garbage» Can use a lot of algorithms

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Two ways of reachability

» The object is reachable from a certain root» Local variables» Parameters» Global variables

» The object is reachable via another reachable object

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Tracing garbage collectionThe naïve mark-and-sweep algorithm

The naïve mark-and-sweep algorithm

» Two stages algorithm» First, run through the root set tree and mark

reachable objects» Second, run through the whole memory, sweep

unmarked objects and clear mark of marked objects

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The naïve mark-and-sweep algorithm disadvantages

» Must run through the memory twice» Must freeze the execution of the program to avoid

mutation of the working set

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Go and the garbage collectionThe tri-color marking

The tri-color marking

» Proposed by Dijkstra in 1978» Tri-color» Concurrent» Mark-and-sweep

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The tri-color marking concept

» Heap is a graph of connected objects» Uses 3 colored sets

» White: contains objects to collect» Gray: contains objects reachable from the root

but not yet scanned» Black: contains reachable and scanned objects

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The tri-color marking steps

» At the beginning, everyone is in the white set» Root objects are moved in the gray set» A root object is scanned

» Its references are colored gray, while the root object itself is colored black

» Its references are scanned too and moved to the black set» Once the gray set is empty, the remaining objects in the white set

are freed and sets are recolored

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The tri-color marking by example 31

The tri-color marking by example 32

The tri-color marking by example 33

The tri-color marking by example 34

The tri-color marking by example 35

The tri-color marking by example 36

The tri-color marking by example 37

The algorithm invariant

“The tri-color marking invariant

No black object may contain a pointer to a white object

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How to always satisfy the invariant?

» Stop the world until GC is complete» Use a write barrier

» Function executed by the mutator to tell the GC that a pointer has been changed

» Enforce an object to be gray when it is created during the garbage collector cycle

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How to always satisfy the invariant? 41

Go back to lower versions of GoThe garbage collector review by Richard Hudson

Go 1.4 and lower

» Stop-the-world from the beginning of the cycle to the end

» High GC latency» Not concurrent

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Go 1.5 (and actual) garbage collector

» Made by Richard Hudson and his team» Concurrent» Low latency» Decrease program performances but increase

throughput

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Go 1.5 new garbage collector 45

Benchmarking 46

Go 1.6 and further

» Improvements on when to start a GC cycle, used metrics, etc.

» Predictive

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What’s new in Go 1.8?The garbage collector review

Actual garbage collector pain point 49

New type of write barrier

» Hybrid write barrier» Dijkstra (1978)» Yuasa (1990)

» New algorithm invariant (weak invariant)» No need to rescan the whole memory» Reduces GC cycle

» 100 microseconds for the worst case» 50 microseconds for 95 percentile

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“Any white object pointed to by a black object is reachable from a grey object via a chain of

white pointers (it is grey-protected).

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Go 1.8 hybrid write barrier 52

Languages without garbage collector

Rust

» Based on ownership and borrowing» Every value has a scope» Passing or returning a value means transferring its ownership» When a scope ends, owned values are destroyed» Everything is checked at compile time

» Everything is allocated on the stack by default» Using stack frames» Stack frames are freed on function return

» Can be allocated on the heap using boxes

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Rust - Ownership 55

Rust - Ownership 56

Rust - Ownership 57

Rust - Borrowing 58

“Thank you for listening

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References

» Pusher https://blog.pusher.com/golangs-real-time-gc-in-theory-and-practice/» Golang new GC proposal

https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md» Go GC 1.5 blog post https://blog.golang.org/go15gc» GopherCon 2015 GC Latency talk https://www.youtube.com/watch?v=aiv1JOfMjm0» Dijkstra 1978 GC paper http://dl.acm.org/citation.cfm?id=359655» Rust lang https://blog.rust-lang.org/2015/04/10/Fearless-Concurrency.html

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