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Why cgo is slow Filippo Valsorda

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cgo is a FFI (Foreign Function Interface)

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I like FFIs. • From cgo back to Go @ GopherCon 2016
 https://speakerdeck.com/filosottile/from-cgo-back-to-go-gophercon-2016 • rustgo: Building your own FFI @ GothamGo 2017
 https://speakerdeck.com/filosottile/calling-rust-from-go-without-cgo-at-gothamgo-2017 • Why cgo is slow @ CapitalGo 2018
 Hi!

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C function call Java FFI Rust FFI LuaJIT FFI Node.js FFI cgo 2.364 ns 9.01 ns 2.386 ns 1.81 ns (!) https://nullprogram.com/blog/2018/05/27/ 18.33 ns 75.95 ns https://github.com/dyu/ffi-overhead

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name old time/op new time/op delta CgoCall-4 63.1ns ± 3% 57.1ns ± 0% -9.43%

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C function call Java FFI Rust FFI LuaJIT FFI Node.js FFI cgo 2.364 ns 9.01 ns 2.386 ns 1.81 ns (!) https://nullprogram.com/blog/2018/05/27/ 18.33 ns 68.77 ns https://github.com/dyu/ffi-overhead

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C function call Java FFI Rust FFI LuaJIT FFI Node.js FFI cgo 2.364 ns 9.01 ns 2.386 ns 1.81 ns (!) https://nullprogram.com/blog/2018/05/27/ 18.33 ns 68.77 ns (29x) https://github.com/dyu/ffi-overhead

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cgo: • cmd/cgo • runtime/cgo • a sprinkle of cmd/link/internal/ld support

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cgo: • cmd/cgo • runtime/cgo • a sprinkle of cmd/link/internal/ld support • not a compiler feature!

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cgo: • cmd/cgo — a code generator • runtime/cgo • a sprinkle of cmd/link/internal/ld support • not a compiler feature!

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Reason 1: calling conventions

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C compiler Go compiler

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go build -x -work

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src/runtime/cgocall.go

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rewritten Go function calling convention trampoline arg unpacking real C function Go ASM C

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Learn more: • src/runtime/cgocall.go • rustgo: Building your own FFI @ GothamGo 2017
 https://speakerdeck.com/filosottile/calling-rust-from-go-without-cgo-at-gothamgo-2017

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Reason 2: small stacks

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Initial goroutine stack size: 2048 bytes System stack size: 1–8 megabytes

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stack

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function frame

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Function preamble

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C doesn't call morestack C code needs to run on a system stack cgocall / asmcgocall

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Learn more: • src/runtime/stack.go • src/runtime/cgocall.go • How stacks are handled in Go by Daniel Morsing
 https://blog.cloudflare.com/how-stacks-are-handled-in-go/

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Reason 3: the scheduler

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From https://morsmachine.dk/go-scheduler

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From https://morsmachine.dk/go-scheduler

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The Go scheduler is collaborative. It can't preempt running code. (ProTip: for {} is never what you want. Use select {}.)

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From https://morsmachine.dk/go-scheduler

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Learn more: • src/runtime/proc.go → reentersyscall • The Go scheduler by Daniel Morsing
 https://morsmachine.dk/go-scheduler • Performance without the event loop by Dave Cheney
 https://dave.cheney.net/2015/08/08/performance-without-the-event-loop

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Reason 4: the garbage collector

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Go memory C memory []byte

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Go memory C memory GC []byte

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Go memory C memory GC []byte

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Go memory C memory GC []byte *uint8_t C.some_func()

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The cgo rules You may pass a Go pointer … if it doesn’t point to other pointers … and C can’t keep a reference to it

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The GC must see all the Go pointers.

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panic: runtime error: cgo argument has Go pointer to Go pointer GODEBUG=cgocheck=2

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Learn more: • From cgo back to Go @ GopherCon 2016
 https://speakerdeck.com/filosottile/from-cgo-back-to-go-gophercon-2016

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Thank you! filippo@golang.org @FiloSottile Olga Shalakhina artwork under CC 3.0 license based on Renee French under Creative Commons 3.0 Attributions.