• Frank Somers's avatar
    runtime: use vDSO on linux/386 to improve time.Now performance · af40cbe8
    Frank Somers authored
    This change adds support for accelerating time.Now by using
    the __vdso_clock_gettime fast-path via the vDSO on linux/386
    if it is available.
    
    When the vDSO path to the clocks is available, it is typically
    5x-10x faster than the syscall path (see benchmark extract
    below).  Two such calls are made for each time.Now() call
    on most platforms as of go 1.9.
    
    - Add vdso_linux_386.go, containing the ELF32 definitions
      for use by vdso_linux.go, the maximum array size, and
      the symbols to be located in the vDSO.
    
    - Modify runtime.walltime and runtime.nanotime to check for
      and use the vDSO fast-path if available, or fall back to
      the existing syscall path.
    
    - Reduce the stack reservations for runtime.walltime and
      runtime.monotime from 32 to 16 bytes. It appears the syscall
      path actually only needed 8 bytes, but 16 is now needed to
      cover the syscall and vDSO paths.
    
    - Remove clearing DX from the syscall paths as clock_gettime
      only takes 2 args (BX, CX in syscall calling convention),
      so there should be no need to clear DX.
    
    The included BenchmarkTimeNow was run with -cpu=1 -count=20
    on an "Intel(R) Celeron(R) CPU J1900 @ 1.99GHz", comparing
    released go 1.9.1 vs this change. This shows a gain in
    performance on linux/386 (6.89x), and that no regression
    occurred on linux/amd64 due to this change.
    
    Kernel: linux/i686, GOOS=linux GOARCH=386
       name      old time/op  new time/op  delta
       TimeNow   978ns ± 0%   142ns ± 0%  -85.48%  (p=0.000 n=16+20)
    
    Kernel: linux/x86_64, GOOS=linux GOARCH=amd64
       name      old time/op  new time/op  delta
       TimeNow   125ns ± 0%   125ns ± 0%   ~       (all equal)
    
    Gains are more dramatic in virtualized environments,
    presumably due to the overhead of virtualizing the syscall.
    
    Fixes #22190
    
    Change-Id: I2f83ce60cb1b8b310c9ced0706bb463c1b3aedf8
    Reviewed-on: https://go-review.googlesource.com/69390
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarIan Lance Taylor <iant@golang.org>
    af40cbe8
vdso_linux.go 7.3 KB