3fb3ca083c
The preallocated memory size is comparable to bytes.Buffer bootstraping array length (bytes.Buffer was used before rewrite to strings.Builder). Without preallocation, encodeWord does more than one allocation for almost any possible input. The regression happens because bytes.Buffer did a 80-bytes allocation at the beginning of encodeWord while strings.Builder did several smaller allocations (started with cap=0). Comparison with reported regression: name old time/op new time/op delta QEncodeWord-4 781ns ± 1% 593ns ± 1% -24.08% (p=0.008 n=5+5) name old alloc/op new alloc/op delta QEncodeWord-4 152B ± 0% 80B ± 0% -47.37% (p=0.008 n=5+5) name old allocs/op new allocs/op delta QEncodeWord-4 5.00 ± 0% 2.00 ± 0% -60.00% (p=0.008 n=5+5) Comparison with buffer solution (like before strings.Builder, but without sync pool for buffer re-using): name old time/op new time/op delta QEncodeWord-4 595ns ± 1% 593ns ± 1% ~ (p=0.460 n=5+5) name old alloc/op new alloc/op delta QEncodeWord-4 160B ± 0% 80B ± 0% -50.00% (p=0.008 n=5+5) name old allocs/op new allocs/op delta QEncodeWord-4 2.00 ± 0% 2.00 ± 0% ~ (all equal) We avoid allocation in buf.String(), as expected. Fixes #25379 Change-Id: I19763f0e593a27390c1a549b86ce6507b489046b Reviewed-on: https://go-review.googlesource.com/113235 Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> |
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