| commit | 7781fed24ea79d819bc0ecfdafe8c24151a83c31 | [log] [tgz] |
|---|---|---|
| author | Russ Cox <rsc@golang.org> | Tue Nov 14 14:35:39 2017 -0500 |
| committer | Russ Cox <rsc@golang.org> | Thu Nov 16 02:24:37 2017 +0000 |
| tree | 6ffbe61b42cc7f7479c916e48f5e45e4dbb90a26 | |
| parent | 08270c3e18673f7f1319dce9d27d84c9a4c8eb49 [diff] |
io: eliminate full copy of copy loop in CopyN CL 60630 claimed to and did “improve performance of CopyN” but in doing so introduced a second copy of the I/O copying loop. This code is subtle and easy to get wrong and the last thing we need is of two copies that can drift out of sync. Even the newly introduced copy contains various subtle changes that are not obviously semantically equivalent to the original. (They probably are, but it's not obvious.) Although the CL description does not explain further what the important optimization was, it appears that the most critical one was not allocating a 32kB buffer for CopyN(w, r, 512). This CL deletes the forked copy of copy and instead applies the buffer size restriction optimization directly to copy itself. CL 60630 reported: name old time/op new time/op delta CopyNSmall-4 5.09µs ± 1% 2.25µs ±86% -55.91% (p=0.000 n=11+14) CopyNLarge-4 114µs ±73% 121µs ±72% ~ (p=0.701 n=14+14) Starting with that CL as the baseline, this CL does not change a ton: name old time/op new time/op delta CopyNSmall-8 370ns ± 1% 411ns ± 1% +11.18% (p=0.000 n=16+14) CopyNLarge-8 18.2µs ± 1% 18.3µs ± 1% +0.63% (p=0.000 n=19+20) It does give up a small amount of the win of 60630 but preserves the bulk of it, with the benefit that we will not need to debug these two copies drifting out of sync in the future. Change-Id: I05b1a5a7115390c5867847cba606b75d513eb2e2 Reviewed-on: https://go-review.googlesource.com/78122 Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
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