x/sys/unix: Pwritev and Preadv always allocate

[elagergren-spideroak]

What version of Go are you using (go version)?

$ go version
go version go1.19.3 darwin/amd64

But for sys/unix:

golang.org/x/sys v0.3.0 h1:w8ZOecv6NaNa/zC8944JTU3vz4u6Lagfk4RPQxv92NQ=
golang.org/x/sys v0.3.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

Does this issue reproduce with the latest release?

n/a

What operating system and processor architecture are you using (go env)?

n/a

What did you do?

Wrote a program that regularly calls unix.Pwritev with a similar number of discontinuous buffers.

What did you expect to see?

A way to call Pwritev without allocating a slice of Iovecs each time.

What did you see instead?

No way to do that.

I agree that using [][]byte is nicer, but unless the Preadv, Pwritev, etc. maintain a cache of Iovecs, each call necessarily allocates.

[randall77]

It is possible to stack-allocate a reasonably-sized iovec that would cover a lot of cases.

func Pwritev(fd int, iovs [][]byte, offset int64) (n int, err error) {
	iovecs := make([]Iovec, 0, 8)
	iovecs = appendBytes(iovecs, iovs)

Where appendBytes appends an Iovec for each []byte argument. As long as there are at most 8 entries in iovs, it would not allocate.

(This is assuming pwritev doesn't escape its argument. I'm not sure if that is encoded somewhere or not?)

[ericlagergren]

@randall77 that seems to work, sending a CL

[gopherbot]

Change https://go.dev/cl/457815 mentions this issue: unix: avoid allocations for common uses of Readv, Writev, etc.