The Linux "copy problem"

Posted May 30, 2019 13:22 UTC (Thu) by ecree (guest, #95790)
In reply to: The Linux "copy problem" by desbma
Parent article: The Linux "copy problem"

I was wondering about splice(). Wouldn't that be a more efficient way to do copies?

The Linux "copy problem"

Posted May 30, 2019 13:38 UTC (Thu) by desbma (guest, #118820) [Link] (2 responses)

According to `man splice`:
> splice() moves data between two file descriptors [...] where one of the file descriptors must refer to a pipe

sendfile does not have this restriction (it can also work if the destination fd is a socket), so it is the ideal candidate for copying files.

Apparently, someone proposed to update coreutil's cp to use sendfile in 2012, but it was rejected: https://lists.gnu.org/archive/html/coreutils/2012-10/msg0...

A while ago I did some benchmarks in Python to compare "read/write chunk" vs "sendfile" based copy, and it let to a 30-50% speedup : https://github.com/desbma/pyfastcopy#performance

The Linux "copy problem"

Posted May 30, 2019 13:54 UTC (Thu) by ecree (guest, #95790) [Link] (1 responses)

> > one of the file descriptors must refer to a pipe
Yes, so you make two splice calls:

int p[2];
pipe(p);
splice(fd_in, NULL, p[1], NULL, len, flags);
splice(p[0], NULL, fd_out, NULL, len, flags);

I haven't actually tried this, but in theory it should enable the kernel to do a zero-copy copy where the underlying files support that. The pipe is really no more than a way to associate a userspace handle with a kernel buffer; see https://yarchive.net/comp/linux/splice.html for details.

The Linux "copy problem"

Posted May 30, 2019 14:00 UTC (Thu) by desbma (guest, #118820) [Link]

> I haven't actually tried this, but in theory it should enable the kernel to do a zero-copy copy where the underlying files support that.

This is exactly what sendfile does, with a single system call, instead of 3 for your example.