Date: Fri, 6 Sep 2019 22:06:32 -0600 From: Warner Losh <imp@bsdimp.com> To: Philip Paeps <philip@freebsd.org> Cc: Ian Lepore <ian@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r351918 - head/sys/kern Message-ID: <CANCZdfr6pz19NCLmsua6iGzusotrcHN=u47RH7Y1PdM4rnomZA@mail.gmail.com> In-Reply-To: <8F03EA29-0F3F-4321-9241-78F7C924FDE1@freebsd.org> References: <201909060119.x861JWrG006910@repo.freebsd.org> <4917d7507b6ea6c360dccda261f53052aa085f2b.camel@freebsd.org> <5EE266EE-E650-48D8-9B0E-E674AD026470@freebsd.org> <3cb6429acc7e520932d2c906d1cac47540156355.camel@freebsd.org> <8F03EA29-0F3F-4321-9241-78F7C924FDE1@freebsd.org>
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On Fri, Sep 6, 2019 at 9:54 PM Philip Paeps <philip@freebsd.org> wrote: > On 2019-09-06 22:18:36 (+0800), Ian Lepore wrote: > > On Fri, 2019-09-06 at 12:15 +0800, Philip Paeps wrote: > >> On 2019-09-06 11:15:12 (+0800), Ian Lepore wrote: > >>> On Fri, 2019-09-06 at 01:19 +0000, Philip Paeps wrote: > >>>> Log: > >>>> riscv: default to HZ=100 > >>> > >>> This seems like a bad idea. I've run a 90mhz armv4 chip with > >>> HZ=1000 and didn't notice any performance hit from doing so. Almost > >>> all arm kernel config files set HZ as an option, so that define > >>> doesn't do much for arm these days. It probably does still set HZ > >>> for various mips platforms. > >>> > >>> I would think 1000 is appropriate for anything modern running at > >>> 200mhz or more. > >>> > >>> Setting it to 100 has the bad side effect of making things like > >>> msleep(), tsleep(), and pause() (which show up in plenty of drivers) > >>> all have a minimum timeout of 10ms, which is a long long time on > >>> modern hardware. > >>> > >>> What benefit do you think you'll get from the lower number? > >> > >> On systems running at 10s of MHz (or slower, ick), with HZ=1000 you > >> spend an awful lot of time servicing the timer interrupt and not very > >> much time doing anything else. > >> > >> My rationale was that most RISC-V systems (including emulation and > >> FPGA prototypes) I've encountered are running slower than the tipping > >> point where HZ=1000 makes sense. With the default of HZ=100, faster > >> exceptions can still set HZ=1000 in their individual configs. > >> > >> When the RISC-V world evolves to having more actual silicon and fewer > >> slow prototypes, I definitely agree this default should be flipped > >> again for HZ=1000 by default and HZ=100 in the config files for the > >> exceptions. > > > > Wait a second... are you saying that the riscv implementation doesn't > > support event timers and uses an old-style periodic tick based on HZ? > > Depending on the hardware, there may not be an event timer (yet)... > > As I wrote: I would be more than happy to revert this change when more > silicon becomes available. Presently, there is exactly one silicon > RISC-V implementation commercially available (HiFive FU540) and even > that one is kind of difficult to source. Most people running RISC-V are > doing so in emulation or on FPGAs. > > Given how long these things take to boot to userland (where you really > notice how slow things are), HZ=100 feels like a more sensible default > than HZ=1000. > I think it show more that the defaults are bad for MIPS and ARM. All the MIPS files, except BERI/CHERI are 1000Hz. Well, Octeon is also 100Hz, due to the defaults, but it will be fine at 1000Hz, so maybe we need to attend to this as well. Arm !=v5 is also 1000Hz, so it should be changed... > I don't feel terribly strongly about this though. I've just been bitten > several times in the last week on a <15MHz FPGA forgetting to set HZ=100 > in config and figured I'd save others the trouble. ;-) > 15MHz FPGA? FreeBSD 1.0 barely ran on 25MHz i386 machines of the time.... How common are these beasts and how well does FreeBSD do on them. I assume these are early prototypes? I have no strong opinion on riscv, but do think mips and arm should change. Warner
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