svn commit: r341803 - head/libexec/rc

[Devin Teske]

> On Dec 11, 2018, at 9:23 AM, John Baldwin <jhb at FreeBSD.org> wrote:
> 
> 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.
> 

Thank you for the background which was lost by the time I got to the phab.

I can't help but ask though,...

If it was noticed that read(2) processes the stream one byte at a time,
why not just optimize read(2)?

I'm afraid of the prospect of having to hunt down every instance of while-read,
but if we can fix the underlying read(2) inefficiency then we make while-read OK.
-- 
Devin