GoのGC (garbage collector)について理解する

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スライドトップと  してご利用ください  マネーフォワード事業本部   山田太郎  © Money Forward, Inc. GoのGC  について理解する  Understanding Go's GC  Money Forward, Inc.  uji  © Money Forward, Inc.

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@uji_rb  ● Software engineer  @Money Forward, Inc.(Osaka)    ● A three-year-old Gopher.    ● Kyoto.go  © Money Forward, Inc. uji 

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Today’s goal  ● GCの目的を理解してもらう  Help them understand the purpose of GC.    ● GoのGCの仕組み、設計思想をざっくり理解してもらう  Get a rough understanding of Go's GC structure and design philosophy.    ● Goのランタイムに興味を持ってもらう  Get people interested in Go's runtime.  © Money Forward, Inc.

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煩わしい手動メモリ管理からの解放される  You can get rid of manual memory management.    より本質的なプログラミングに集中できる  You can focus on   more essential programming.  © Money Forward, Inc. With GC 

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  使われなくなったメモリ領域を  手動で解放しなければならない  We must release   unused memory space manually.    メモリリークや脆弱性などが発生しないように  プログラムを書く必要がある  It could lead to memory leaks and vulnerabilities.  © Money Forward, Inc. The world without GC 

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  → No    GC動作中にアプリの完全停止が発生  During GC operation, Complete stoppage of the application occurs.  © Money Forward, Inc. GCがあると良い事ばかり？  Is it all good to have a GC? 

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  → No    GC動作中にアプリの完全停止が発生  During GC operation, Complete stoppage of the application occurs.      Stop The World (STW)  © Money Forward, Inc. GCがあると良い事ばかり？  Is it all good to have a GC? 

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サービスが常に低レイテンシを求める場合STW の影響を考慮する必要がある    If the service always requires low latency, we need to consider the impact of STW.  © Money Forward, Inc. Stop The World 

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Goはプロダクト開発をより生産的に  するために生まれた言語  Go was designed and developed to make product development more productive.    本質的なプログラミングに集中させる思想がGoの哲学と一致した  The idea of focusing on essential programming coincided with Go's philosophy.  © Money Forward, Inc. Why did Go choose GC?  https://talks.golang.org/2012/splash.article 

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GCにおいて、  スループットとレイテンシはトレードオフ  In GC,   the Throughput and latency are a trade-off.      Goはレイテンシに重きをおいている  Go is latency oriented.  © Money Forward, Inc.

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スループットは  ハードウェアの進化によって向上していく  Throughput increases with hardware evolution.    ハードウェアで解決しづらいレイテンシを重視  Focus on latency, which is difficult to solve with hardware  © Money Forward, Inc. The Go Blog "Go GC: Prioritizing low latency and simplicity"  Why does Go focus on latency? 

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● チューニングパラメータは”GOGC”のみ  Tuning parameter is "GOGC" only.    ● 非同期処理はGoユーザーと同様にGoroutineを利用  Asynchronous processing uses Goroutine just like Go users.    ● アルゴリズムがシンプル  Algorithm is simple.  © Money Forward, Inc. 簡潔性を重視  Emphasize simplicity. 

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“Concurrent Mark Sweep” (CMS)    最古のGCアルゴリズム   “Mark Sweep” を改良したもの    It is an improved version of the oldest  GC algorithm "Mark Sweep".    © Money Forward, Inc. Go's GC algorithm 

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Mark Sweep  アプリケーションで使われている  メモリオブジェクトをMark  Mark memory objects used in the application    ↓     MarkされていないオブジェクトをSweep  Sweep unmarked objects  © Money Forward, Inc.

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Mark phase  © Money Forward, Inc.

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Sweep phase  © Money Forward, Inc.

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Sweep phase  © Money Forward, Inc.

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Mark Sweepの問題  Mark Sweep's problems.  GCが動き続けている間  ずっとアプリケーションが止まり  レイテンシが高くなる    While the GC keeps moving.  The application stops all the time and latency is high.  © Money Forward, Inc.

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Mark Sweep とアプリケーションを  同時に動作させる    Run Mark Sweep and application in the same time.  © Money Forward, Inc. Concurrent Mark Sweep (CMS)  White  未探索 unsearched  Gray  探索中 sarching  Black  探索済み sharched  Use tri-color marking  

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1. Sweep termination  2. Mark  3. Mark termination  4. Sweep  © Money Forward, Inc. Phases  https://github.com/golang/go/blob/master/src/runtime/mgc.go

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Sweep termination phase  © Money Forward, Inc. Application STW 前回のGCループを止めて  消し残りを消しきる  Erase the remainder  of the previous loop.  ↓  オブジェクトを全て白にする  Mark all objects white. 

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Mark phase  © Money Forward, Inc. Application

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Mark phase  © Money Forward, Inc. Application Queuing

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Mark terminate phase  © Money Forward, Inc. Application STW

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Sweep phase  © Money Forward, Inc. Application

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オブジェクトの参照が切り替わったときに  色の塗り忘れが発生する場合がある    When object references are switched Color forgetting occurs.  © Money Forward, Inc. 同時実行の問題  Concurrency problems. 

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スループットの低下があるが  アプリケーションを同時に動かせる  Throughput is reduced, but applications  can run concurrency.  © Money Forward, Inc. 新しく確保されたオブジェクトは黒に   Mark the newly allocated object black.   親からの参照が切り替わったオブジェクトは灰色に Mark objects that have switched references from their parents gray.  Write Barrier 

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GoのGCはCMS + Write Barrierで  低レイテンシを実現している  Go's GC with CMS + Write Barrier Low latency is achieved.      コンパクションや世代別GCなどは  採用されていない  No compaction or generational GC is employed.  © Money Forward, Inc.

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なぜこのようなシンプルなアルゴリズムで低レイテンシを実現できているのか?  Why is it possible to achieve low latency with such a simple algorithm?    ● 値型の存在や高性能なエスケープ解析により、スタックが効率よく使われるのでGCの負担が少ない  The presence of value types and high-performance escape analysis make efficient use of the stack and reduce the burden on the GC.    ● TCMallocベースのメモリアロケーターが断片化を起こりづらい  TCMalloc-based memory allocator is less prone to fragmentation.  © Money Forward, Inc. ・Why golang garbage-collector not implement Generational and Compact gc? ・ Go Does Not Need a Java Style GC

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Go 1.8  スタック再スキャンによるSTWを改善  Eliminate STW due to stack rescanning.(proposal)  Go 1.10 アロケーションのレイテンシと全体的なオーバヘッド削減  Reduce allocation latency and overall overhead.  Go 1.12 Go1.8から対応していたスタック再スキャンによるSTW解消  STW resolution by stack rescanning, supported since Go 1.8  Go 1.13  Scavengingをよりスマートに  Smarter Scavenging.(issue)  Go 1.14 goroutineがプリエンプティブになりGCの遅延削減  Goroutine becomes preemptive and reduces GC delay  Go 1.15  GC動作中にReadMemStatsがブロックされる問題解消  Fixed ReadMemStats blocking during GC operation.  © Money Forward, Inc. Release History 

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GC Pacerが改善された  GC Pacer has been improved.    GCの頻度を決める際、  ヒープ以外(スタック等)も見るようになった  When deciding how often to GC,  added to look at other than heap (stack, etc.).    GCの処理時間が減るかも  Might reduce GC processing time.  © Money Forward, Inc. Go 1.18 (released 2022/03)  proposal "GC Pacer Redesign" 

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改善の余地はまだまだある  There is still room for improvement.    どこまで進化するのかとても楽しみ  I'm very excited to see how far it will evolve.  © Money Forward, Inc. Future of Go's GC 

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Summary  ● GCはプログラミングの難易度を下げ、  プロダクト開発の本質に集中させてくれる  GC makes programming easy and helps us focus on the essence of product development.    ● GoのGCは低レイテンシを重視しており、  それをシンプルなアルゴリズムで実現している  Go's GC emphasizes low latency and it is achieved with a simple   algorithm.    ● GoのGCはこれからも進化する  Go's GC will continue to evolve.  © Money Forward, Inc.

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今回のセッションをきっかけに  GoやGC、ランタイムについて  興味を持ってもらえると嬉しいです  I hope this session will spark your interest in   Go, GC, and runtime.      ご清聴ありがとうございました  Thank you for your attention.  © Money Forward, Inc.

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Bibliography  ● ガベージコレクションのアルゴリズムと実装中村成洋, 相川光, 竹内郁雄(監修) https://tatsu-zine.com/books/gcbook  ● The Go Blog  ○ Getting to Go: The Journey of Go's Garbage Collector https://go.dev/blog/ismmkeynote  ○ Go GC: Prioritizing low latency and simplicity https://go.dev/blog/go15gc  ○ Go's New Brand https://go.dev/blog/go-brand  ● Go GC: Latency Problem Solved https://talks.golang.org/2015/go-gc.pdf  ● go gc algorithm 101https://speakerdeck.com/taxio/go-gc-algorithm-101  ● Go言語のGCについてhttps://engineering.linecorp.com/ja/blog/go-gc  ● GolangのGCを追う https://deeeet.com/writing/2016/05/08/gogc-2016  ● GoはいつGCするのか? https://zenn.dev/koron/articles/b96cccfa82c0c1  ● Dumpster diving the Go garbage collector https://blog.px.dev/go-garbage-collector/  ● Go言語のリアルタイムGC　理論と実践 https://postd.cc/golangs-real-time-gc-in-theory-and-practice    Illustrations  ©tottie / Renée French https://github.com/tottie000/GopherIllustrations   © Money Forward, Inc.