svn commit: r341803 - head/libexec/rc

[John Baldwin]

On 12/10/18 5:38 PM, Conrad Meyer wrote:
> Author: cem
> Date: Tue Dec 11 01:38:50 2018
> New Revision: 341803
> URL: https://svnweb.freebsd.org/changeset/base/341803
> 
> Log:
>   rc.subr: Implement list_vars without using 'read'
>   
>   'read' pessimistically read(2)s one byte at a time, which can be quite
>   silly for large environments in slow emulators.
>   
>   In my boring user environment, truss shows that the number of read()
>   syscalls to source rc.subr and invoke list_vars is reduced by something like
>   3400 to 60.  ministat(1) shows a significant time difference of about -71%
>   for my environment.
>   
>   Suggested by:	jilles
>   Discussed with:	dteske, jhb, jilles
>   Differential Revision:	https://reviews.freebsd.org/D18481

For some background, one my colleagues reported that it was taking hours in
(an admittedly slow) CPU simulator to get through '/etc/rc.d/netif start'.
I ended up running that script under truss in a RISC-V qemu machine.  The
entire run took 212 seconds (truss did slow it down quite a bit).  Of that
212 seconds, the read side of each list_vars invocation took ~25.5 seconds,
and with lo0 and vtnet0 there were 8 list_vars invocations, so 204 out of
the 212 seconds were spent in the single-byte read() syscalls in 'while read'.

Even on qemu without truss during bootup 'netif start' took a couple of
seconds (long enough to get 2-3 Ctrl-T's in) before this change and is now
similar to bare metal with the change.  list_vars is rarely used outside of
'netif', so it probably doesn't make a measurable difference on bare metal.

-- 
John Baldwin