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Commit message

bufio: add fast path to WriteString for strings that fit in the buffer

WriteString sets up its io.StringWriter fallback before checking
whether the string simply fits in the remaining buffer space, which
is the common case. The setup is not free: the interface value is
zeroed and state is spilled on every call, including the calls that
never use the fallback, making WriteString measurably slower than
Write for an identical payload that fits.

Handle the fits-in-buffer case first. The fallback loop then only
runs when the string does not fit, so its condition is known true on
entry: test it at the bottom of the loop instead so it is not
evaluated twice. The fallback path gets slightly faster as well.

goos: darwin
goarch: arm64
pkg: bufio
cpu: Apple M2 Pro
                        │     old     │                 new                 │
                        │   sec/op    │   sec/op     vs base                │
WriterCopyOptimal-10      51.54n ± 0%   51.67n ± 0%        ~ (p=0.233 n=25)
WriterCopyUnoptimal-10    52.42n ± 1%   52.42n ± 1%        ~ (p=0.528 n=25)
WriterCopyNoReadFrom-10   2.329µ ± 1%   2.325µ ± 1%        ~ (p=0.690 n=25)
WriterEmpty-10            483.1n ± 1%   480.6n ± 1%        ~ (p=0.567 n=25)
WriterFlush-10            5.262n ± 0%   5.025n ± 1%   -4.50% (p=0.000 n=25)
WriteString/fit-10        4.873n ± 1%   3.783n ± 0%  -22.37% (p=0.000 n=25)
WriteString/overflow-10   5.993n ± 0%   5.697n ± 0%   -4.94% (p=0.000 n=25)
geomean                   46.46n        44.17n        -4.94%

B/op and allocs/op are unchanged.

Fixes #80692

Change-Id: Iae8f1612b4aaf741ed26e20074729557085a0bdc

GitHub-Last-Rev: a645f774f6d1eb5c55a9dd6fc47b8807a11dfe02

GitHub-Pull-Request: golang/go#80693

Change diff

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index f9b762b..5f0f9bb 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -745,11 +745,22 @@
 // If the count is less than len(s), it also returns an error explaining
 // why the write is short.
 func (b *Writer) WriteString(s string) (int, error) {
+	if b.err == nil && len(s) <= b.Available() {
+		// Fast path: the whole string fits in the buffer.
+		n := copy(b.buf[b.n:], s)
+		b.n += n
+		return n, nil
+	}
+
+	if b.err != nil {
+		return 0, b.err
+	}
+
 	var sw io.StringWriter
 	tryStringWriter := true
 
 	nn := 0
-	for len(s) > b.Available() && b.err == nil {
+	for {
 		var n int
 		if b.Buffered() == 0 && sw == nil && tryStringWriter {
 			// Check at most once whether b.wr is a StringWriter.
@@ -767,6 +778,9 @@
 		}
 		nn += n
 		s = s[n:]
+		if len(s) <= b.Available() || b.err != nil {
+			break
+		}
 	}
 	if b.err != nil {
 		return nn, b.err
diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index 742e195..4fb3d9e 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -1997,3 +1997,20 @@
 		bw.Flush()
 	}
 }
+
+func BenchmarkWriteString(b *testing.B) {
+	fit := strings.Repeat("x", 50)
+	overflow := strings.Repeat("x", 8<<10)
+	for _, tc := range []struct{ name, s string }{
+		{"fit", fit},
+		{"overflow", overflow},
+	} {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ReportAllocs()
+			bw := NewWriter(io.Discard)
+			for i := 0; i < b.N; i++ {
+				bw.WriteString(tc.s)
+			}
+		})
+	}
+}

Change information

Files:

M src/bufio/bufio.go
M src/bufio/bufio_test.go

Change size: S

Delta: 2 files changed, 32 insertions(+), 1 deletion(-)

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File src/bufio/bufio.go

Line 781, Patchset 1 (Latest): if len(s) <= b.Available() || b.err != nil {

David Teather . resolved

pushed the conditional down here bc re-evaluating at the top had a benchmark regression of ~5%

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File-level comment, Patchset 1 (Latest):

Ian Lance Taylor . resolved

This change makes the function longer and harder to understand, to avoid compiler issues. I think it would be better to tweak the compiler.

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4 comments

Patchset-level comments

File-level comment, Patchset 1 (Latest):

Jorropo . resolved

Thanks

File src/bufio/bufio.go

Line 754, Patchset 1 (Latest):


	if b.err != nil {
		return 0, b.err
	}

Jorropo . unresolved

Shouldn't that come first ?

If the buffer has a pending error it's a bit weird to accept a write.

File src/bufio/bufio_test.go

Line 2001, Patchset 1 (Latest):func BenchmarkWriteString(b *testing.B) {


	fit := strings.Repeat("x", 50)

	overflow := strings.Repeat("x", 8<<10)

	for _, tc := range []struct{ name, s string }{

		{"fit", fit},
		{"overflow", overflow},

Jorropo . unresolved

```suggestion
func BenchmarkWriteString(b *testing.B) {


	for _, tc := range []struct{ name, s string }{

		{"small", strings.Repeat("x", 50)},
		{"huge", strings.Repeat("x", 8<<10)},
```
Given you don't create a new buffer on each iteration, the small string doesn't always fit.

I think it's clearer to name it small and huge.

Line 2011, Patchset 1 (Latest): for i := 0; i < b.N; i++ {

Jorropo . unresolved

```suggestion
			for range b.N {
```

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File-level comment, Patchset 2 (Latest):

David Teather . resolved

Updated. Thanks for the review!

File src/bufio/bufio.go

Line 754, Patchset 1:


	if b.err != nil {
		return 0, b.err
	}

Jorropo . resolved

Shouldn't that come first ?
If the buffer has a pending error it's a bit weird to accept a write.

David Teather

good call swapped to `b.err` comparison first

File src/bufio/bufio_test.go

Line 2001, Patchset 1:func BenchmarkWriteString(b *testing.B) {


	fit := strings.Repeat("x", 50)
	overflow := strings.Repeat("x", 8<<10)
	for _, tc := range []struct{ name, s string }{
		{"fit", fit},
		{"overflow", overflow},

Jorropo . resolved

```suggestion
func BenchmarkWriteString(b *testing.B) {
	for _, tc := range []struct{ name, s string }{
		{"small", strings.Repeat("x", 50)},
		{"huge", strings.Repeat("x", 8<<10)},
```
Given you don't create a new buffer on each iteration, the small string doesn't always fit.

I think it's clearer to name it small and huge.

David Teather

updated to small/huge

Line 2011, Patchset 1: for i := 0; i < b.N; i++ {

Jorropo . resolved

```suggestion
			for range b.N {
```

David Teather

swapped to `b.Loop()` looks like that's current best practice https://go.dev/blog/testing-b-loop

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Patchset-level comments

File-level comment, Patchset 2 (Latest):

Alan Donovan . resolved

Thanks for contributing this optimization.

File src/bufio/bufio.go

Line 753, Patchset 2 (Latest): // Fast path: the whole string fits in the buffer.


		n := copy(b.buf[b.n:], s)

		b.n += n
		return n, nil

Alan Donovan . unresolved

Once we've applied the change at L783, can we also factor this common tail with L788-... using something like this form?

```


func (b *Writer) WriteString(s string) (int, error) {

	if b.err != nil {
		return 0, b.err
	}

	nn := 0
	if len(s) > b.Available() {
		if b.Buffered() == 0 {
			if sw, ok := b.wr.(io.StringWriter); ok {
				// Large write, empty buffer, and the underlying writer supports
				// WriteString: forward the write to the underlying StringWriter.
				// This avoids an extra copy.
				var n int
				n, b.err = sw.WriteString(s)
				return n, b.err
			}
		}

		// Buffer and flush each complete chunk.
		for {


			n := copy(b.buf[b.n:], s)

			b.n += n
			b.Flush() // ignore error


			if len(s) <= b.Available() || b.err != nil {

				break


			}
			nn += n
			s = s[n:]
		}

		if b.err != nil {
			return nn, b.err
		}
	}

	// The whole string (or final chunk) fits in the buffer.


	n := copy(b.buf[b.n:], s)

	b.n += n
	nn += n
	return nn, nil
}
```

Line 763, Patchset 2 (Latest): for {

Alan Donovan . unresolved

Now that we have the precondition `len(s) > b.Available()`, we know the loop will execute at least once, so we can simplify by doing the one-time check for StringWriter eagerly, which may improve the non-fast case too.

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File-level comment, Patchset 2:

David Teather . resolved

Thanks for the review! Updated it and tried to refactor closer to the structure you provided but it had some perf regressions, but took some shapes from that which made it faster

File src/bufio/bufio.go

Line 753, Patchset 2: // Fast path: the whole string fits in the buffer.

David Teather

Happy to make the change, but held off for now as it costs the fast path.

This is what I benched, the suggested shape on top of the current logic

```go


func (b *Writer) WriteString(s string) (int, error) {
	if b.err != nil {
		return 0, b.err
	}

	nn := 0
	if len(s) > b.Available() {

		sw, ok := b.wr.(io.StringWriter)

		if ok && b.Buffered() == 0 {


			// Large write, empty buffer, and the underlying writer supports
			// WriteString: forward the write to the underlying StringWriter.
			// This avoids an extra copy.
			var n int
			n, b.err = sw.WriteString(s)

			if n == len(s) || b.err != nil {
				return n, b.err
			}
			nn = n
			s = s[n:]
		}

		// Buffer and flush each complete chunk.
		for {
			var n int
			if b.Buffered() == 0 && ok {
				n, b.err = sw.WriteString(s)
			} else {


				n = copy(b.buf[b.n:], s)
				b.n += n
				b.Flush()
			}

			nn += n
			s = s[n:]
			if len(s) <= b.Available() || b.err != nil {
				break
			}
		}
		if b.err != nil {
			return nn, b.err
		}
	}

	n := copy(b.buf[b.n:], s)


	b.n += n
	nn += n
	return nn, nil
}
```

`go test -run='^$' -bench=WriteString bufio`, n=30, that against the patchset:

```
                     │     CL      │              refactor               │
                     │   sec/op    │   sec/op     vs base                │
WriteString/small-10   3.696n ± 1%   5.129n ± 2%  +38.75% (p=0.000 n=30)
WriteString/huge-10    3.914n ± 1%   3.607n ± 0%   -7.83% (p=0.000 n=30)
geomean                3.803n        4.301n       +13.09%
```

It's mixed, but the absolute deltas are +1.43ns and -0.31ns. I'm not entirely sure why the effect is that large, I assume it's layout. Maybe the fast path stops being a straight-line return off the prologue.

Two things from your sketch I did take, and one I didn't.

I took the early return for the empty-buffer case. It needs the `n == len(s) || b.err != nil` guard. I tried it without, and hit cases where a short-writing StringWriter made WriteString return (n, nil) with n < len(s), dropping the rest of the string and breaking the doc's "if the count is less than len(s), it also returns an error". Write absorbs that by looping. Guarded as above it matches master exactly, and it's worth ~ -28% on `huge`.

I kept the StringWriter test inside the loop. Checking only on entry means a Writer with buffered data never reaches sw.WriteString at all: 242 -> 96 calls on the underlying writer across my matrix, and writing 4 bytes then 8 KiB regressed 81%. Write tests b.Buffered() == 0 inside its loop for the same reason.

Let me know what you think, or if I'm missing something on the refactor that might be hurting performance. Thanks!

Line 763, Patchset 2: for {

Alan Donovan . resolved

Now that we have the precondition `len(s) > b.Available()`, we know the loop will execute at least once, so we can simplify by doing the one-time check for StringWriter eagerly, which may improve the non-fast case too.

David Teather

Done. Hoisted the check above the loop. I also confirmed your guess about the non-fast case WriteString/huge got faster by ~5% more

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[go] bufio: add fast path to WriteString for strings that fit in the buffer

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