Date: Tue, 11 Dec 2018 09:23:56 -0800 From: John Baldwin <jhb@FreeBSD.org> To: 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: <2a76b295-b2da-3015-c201-dbe0ec63ca5a@FreeBSD.org> In-Reply-To: <201812110138.wBB1cp1p006660@repo.freebsd.org> References: <201812110138.wBB1cp1p006660@repo.freebsd.org>
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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
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