Date: Fri, 22 Jun 2012 08:27:17 +1000 From: Peter Jeremy <peter@rulingia.com> To: Doug Barton <dougb@FreeBSD.org> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@FreeBSD.org> Subject: Re: Replacing rc(8) (Was: FreeBSD Boot Times) Message-ID: <20120621222717.GA91640@server.rulingia.com> In-Reply-To: <4FE3552D.9090202@FreeBSD.org> References: <20120615124849.GI96212@ass.kameli.org> <CAKYr3zxSOFOUghqao=3U_8sBNHwqce8AGK6mSFd6is11qzjS8w@mail.gmail.com> <20120618081140.GK96212@ass.kameli.org> <4FDF6177.5050608@unsane.co.uk> <4FDF6586.9060501@gentoo.org> <4FDFB166.2040709@FreeBSD.org> <4FDFB761.60406@brandonfa.lk> <20120621122802.GA71656@server.rulingia.com> <4FE3552D.9090202@FreeBSD.org>
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On 2012-Jun-21 10:09:01 -0700, Doug Barton <dougb@FreeBSD.org> wrote:
>On 06/21/2012 05:28 AM, Peter Jeremy wrote:
>> 32.0s - rc scripts ("mounting root" through VTY login prompt)
>
>I think that there is some confusion about what I wrote originally, so
>let me clarify. From the time that /etc/rc starts through the time that
>the prompt appears almost all of the time is spent waiting for the
>services to start. There is very little time spent IN the rc scripts
>themselves (barring something that is poorly written of course).
Agreed - I (and I expect everyone else) am using "rc script" to cover
the total wall time between exec()ing the script and it returning - in
most cases, this is almost entirely synchronous service startup time.
The end-user experience is governed by "how long does it take between
rebooting or turning the power on and when I can login or interact
with my service". Reducing this total time is going to require a
combination of changes in multiple areas.
One point I'd make is that the rc scripts run with cold caches so
reads cause physical I/O. There are somewhat over 150 scripts in
/etc/rc.d and a variable number in /usr/local/etc/rc.d (I have between
6 & 33 on different systems). rcorder(8) needs to read each script
so, on a system using spinning rust, this amounts to 2-3 seconds
overhead.
>So the only way to improve the time from /etc/rc to "usable system"
>(whatever that means for the user) is to see what we can parallelize.
>The problem is that this is a really hard problem. :) And as someone
>pointed out, changing from a serial to a parallel process is going to be
>disruptive because it will uncover the inadequately specified
>dependencies that we have now which are hidden by the serial process ...
(I mentioned this problem). One (though intrusive) way would be to
use the approach the ports system used when it enabled parallelism
within port builds: Add new keyword(s) within each script to control
parallelism for that script. Initially, the infrastructure would
assume "serial" unless scripts were explicitly marked "parallel" or
"background" but once the situation was sufficiently under control, it
could be flipped to assume "parallel" unless a script specifically
specified "serial". (Note that I haven't looked at the detail of
actually implementing this).
As an aside, "usable system" is a relevant point. My Netbook
originally came with Linpus and it took about 30s from poweron until
the Linpus GUI was displayed and allowed user interaction. This
looked quite impressive but it took another 30-60s before the system
was actually usable because the GUI was started quite early before
(eg) the network was up.
--=20
Peter Jeremy
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