Why cgo is slow @ CapitalGo 2018

https://capitalgolang.com/program#filippo_valsorda

This talk uses cgo and its below-average performance as an excuse to look into Go internals and what makes Go different from C.

We learn about calling conventions and the code-generated cgo trampolines; about the small goroutines stacks and how C doesn't know how to grow them; about the Go scheduler and how C doesn't yield to it; and about the garbage collector and how pointers in C memory can't be tracked.

Transcript

1. Why cgo is slow
   Filippo Valsorda
   

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3. cgo is a FFI
   (Foreign Function Interface)
   

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4. 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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5. 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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6. name old time/op new time/op delta
   CgoCall-4 63.1ns ± 3% 57.1ns ± 0% -9.43%
   

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7. 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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8. 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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9. cgo:
   • cmd/cgo
   • runtime/cgo
   • a sprinkle of cmd/link/internal/ld support
   

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

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

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12. Reason 1:
    calling conventions
    

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13. C compiler
    Go compiler
    

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14. go build -x -work
    

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19. src/runtime/cgocall.go
    

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

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24. 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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25. Reason 2:
    small stacks
    

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

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27. stack
    

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28. function frame
    

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34. Function preamble
    

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

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36. 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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37. Reason 3:
    the scheduler
    

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

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

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

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

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43. 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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44. Reason 4:
    the garbage collector
    

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45. Go memory C memory
    []byte
    

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46. Go memory C memory
    GC
    []byte
    

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47. Go memory C memory
    GC
    []byte
    

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

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49. 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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50. The GC must see all the Go pointers.
    

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

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52. Learn more:
    • From cgo back to Go @ GopherCon 2016

    https://speakerdeck.com/filosottile/from-cgo-back-to-go-gophercon-2016
    

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

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