How is GNU `yes` so fast?

Re:How is GNU `yes` so fast? by mkj on Tuesday June 13, 2017 @03:08AM (#14543304)

It's 2x times faster for a quick test here. Only copying on the read side?

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Re:How is GNU `yes` so fast? by readittwice on Tuesday June 13, 2017 @06:10AM (#14543823)

Out of interest: Could you please post the code?

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Re:How is GNU `yes` so fast? by mkj on Tuesday June 13, 2017 @09:13AM (#14544451)

I've realised straight tee() is actually wrong - it works fine for piping to something but "zcyes > /dev/null" won't work, it needs vmsplice() like mrb said.

  core2duo e8400, 4.8 kernel, coreutils 8.25
  > yes | dd bs=1M count=10000 of=/dev/null iflag=fullblock
  10485760000 bytes (10 GB, 9.8 GiB) copied, 4.1901 s, 2.5 GB/s
  > ./zcyes | dd bs=1M count=10000 of=/dev/null iflag=fullblock
  10485760000 bytes (10 GB, 9.8 GiB) copied, 1.8542 s, 5.7 GB/s

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