apd: Improve SetFloat64 efficiency #129
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nvb
reviewed
Jul 14, 2023
| func (d *Decimal) SetFloat64(f float64) (*Decimal, error) { | ||
| _, _, err := d.SetString(strconv.FormatFloat(f, 'E', -1, 64)) | ||
| var buf [32]byte // Avoid most of the allocations in strconv. | ||
| _, _, err := d.SetString(string(strconv.AppendFloat(buf[:0], f, 'E', -1, 64))) |
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The []byte to string allocation is unfortunate. Do you want to throw the following into an unsafe.go file and use it here?
// Copyright 2023 The Cockroach Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.
package apd
import "unsafe"
func unsafeConvertBytesToString(b []byte) string {
return *(*string)(unsafe.Pointer(&b))
}There was a problem hiding this comment.
I think you looked at it before I backed out of this approach.
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What do you mean by that? You had the unsafe cast and then removed it?
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It's kinda strange... I think if you look at the string converstion that's done inside bytes.HasPrefix(...) the comment there says that string([]byte) conversion does not incur copy when using various compilers. Perhaps that's what's at play here and might be the reason why the unsafeGetString approach is both slower and allocates more.
|
Sorry, I misunderstood. I actually had that, and it appears the the
performance is worse when using this unsafe hack:
pkg: github.com/cockroachdb/apd/v3
BenchmarkSetFloat-10 4642030 242.8 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4793172 246.3 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4975635 243.0 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4903842 244.0 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4940126 240.5 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4915200 243.6 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4970551 241.4 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4978546 242.4 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4885584 241.9 ns/op
75 B/op 3 allocs/op
BenchmarkSetFloat-10 4995925 240.9 ns/op
75 B/op 3 allocs/op
PASS
ok github.com/cockroachdb/apd/v3 14.862s
~/go/src/github.com/cockroachdb/apd | decimal !1 go test
-run="nothingmatches" -bench=BenchmarkSetFloat -count=10 2>&1 | tee
/tmp/old.txt ok | 16s
goos: darwin
goarch: arm64
pkg: github.com/cockroachdb/apd/v3
BenchmarkSetFloat-10 4610048 246.9 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4849850 246.2 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4850758 245.9 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4890546 249.4 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4724228 249.1 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4828840 246.6 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4843838 250.1 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4808347 250.0 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4612788 251.4 ns/op
67 B/op 3 allocs/op
BenchmarkSetFloat-10 4814672 248.9 ns/op
67 B/op 3 allocs/op
PASS
ok github.com/cockroachdb/apd/v3 14.824s
~/go/src/github.com/cockroachdb/apd | decimal !1 benchstat /tmp/old.txt
/tmp/new.txt
ok | 15s
name old time/op new time/op delta
SetFloat-10 248ns ± 1% 243ns ± 1% -2.32% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
SetFloat-10 67.0B ± 0% 75.0B ± 0% +11.94% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
SetFloat-10 3.00 ± 0% 3.00 ± 0% ~ (all equal)
…On Fri, Jul 14, 2023 at 1:14 PM Yevgeniy Miretskiy ***@***.***> wrote:
***@***.**** commented on this pull request.
------------------------------
In decimal.go
<#129 (comment)>:
> @@ -241,7 +242,8 @@ func (d *Decimal) setCoefficient(x int64) {
// SetFloat64 sets d's Coefficient and Exponent to x and returns d. d will
// hold the exact value of f.
func (d *Decimal) SetFloat64(f float64) (*Decimal, error) {
- _, _, err := d.SetString(strconv.FormatFloat(f, 'E', -1, 64))
+ var buf [32]byte // Avoid most of the allocations in strconv.
+ _, _, err := d.SetString(string(strconv.AppendFloat(buf[:0], f, 'E', -1, 64)))
I think you looked at it before I backed out of this approach.
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nvb
approved these changes
Jul 14, 2023
bench_test.go
Outdated
| } | ||
| } | ||
|
|
||
| func BenchmarkSetFloat(b *testing.B) { |
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nit: BenchmarkDecimalSetFloat because this is a method on Decimal.
Prefer to use append and allocate scratch space to improve allocation efficiency of SetFloat64. ``` name old time/op new time/op delta SetFloat-10 261ns ± 0% 243ns ± 1% -6.66% (p=0.000 n=9+9) name old alloc/op new alloc/op delta SetFloat-10 91.0B ± 0% 67.0B ± 0% -26.37% (p=0.000 n=10+10) name old allocs/op new allocs/op delta SetFloat-10 4.00 ± 0% 3.00 ± 0% -25.00% (p=0.000 n=10+10) ```
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Prefer to use append and allocate scratch space to improve allocation efficiency of SetFloat64.