Date: Tue, 11 Dec 2018 09:40:45 -0800 From: Devin Teske <dteske@FreeBSD.org> To: John Baldwin <jhb@FreeBSD.org> Cc: Devin Teske <dteske@FreeBSD.org>, Conrad Meyer <cem@FreeBSD.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r341803 - head/libexec/rc Message-ID: <98481565-CDD7-4301-B86B-072D5B984AF7@FreeBSD.org> In-Reply-To: <2a76b295-b2da-3015-c201-dbe0ec63ca5a@FreeBSD.org> References: <201812110138.wBB1cp1p006660@repo.freebsd.org> <2a76b295-b2da-3015-c201-dbe0ec63ca5a@FreeBSD.org>
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> On Dec 11, 2018, at 9:23 AM, John Baldwin <jhb@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. -- Devinhelp
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