Re: clockctl(4) vs settimeofday(2)

[Paul Goyette <paul%vps1.whooppee.com@localhost>]

On Mon, 7 Dec 2015, Christos Zoulas wrote:

> On Dec 7,  9:56am, rich.neswold%gmail.com@localhost (Rich Neswold) wrote:
> -- Subject: Re: clockctl(4) vs settimeofday(2)
>
> | > Using the syscall is simpler and cheaper, so I think the man page should
> | > be updated.
> |
> | Is settimeofday() called frequently enough that "cheaper" is a concern?
> | What is more correct (I don't know the answer)?
>
> If you know that you are not going to succeed (because you are not
> root), you should cache the fd, and just do the ioctl each time
> you want to sync to reduce latency.  The dumb caller is probably
> an old program that probably is used to be running as root, so
> reducing the delay/overhead for it by trying syscall first is better.

Yes, the current syscall stub checks to see if it has a cached fd, and
if so uses it.

If not, it tries the syscall first, and falls back to /dev/clockctl if
the syscall fails.

This approach makes sense, so I will treat the code as "definitive"
and I will update the man pages accordingly.


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