Date: Mon, 28 Mar 2016 05:09:57 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: Konstantin Belousov <kostikbel@gmail.com>, Bruce Evans <brde@optusnet.com.au>, John Baldwin <jhb@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org, "'rstone@freebsd.org'" <rstone@freebsd.org> Subject: Re: svn commit: r297039 - head/sys/x86/x86 Message-ID: <20160328042313.N850@besplex.bde.org> In-Reply-To: <87078.1459088476@critter.freebsd.dk> References: <20160323075842.GX1741@kib.kiev.ua> <2922763.uITxoCVqGR@ralph.baldwin.cx> <20160324090917.GC1741@kib.kiev.ua> <20160325010649.H898@besplex.bde.org> <20160324162447.GD1741@kib.kiev.ua> <20160325060901.N2059@besplex.bde.org> <20160325084902.GH1741@kib.kiev.ua> <20160326021219.X911@besplex.bde.org> <20160326174210.GU1741@kib.kiev.ua> <20160327051601.S2318@besplex.bde.org> <20160327130706.GA1741@kib.kiev.ua> <87078.1459088476@critter.freebsd.dk>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sun, 27 Mar 2016, Poul-Henning Kamp wrote: > -------- > In message <20160327130706.GA1741@kib.kiev.ua>, Konstantin Belousov writes: > >>> I haven't seen a single (hardware) system since I started measuring >>> this 15-20 yeas ago on which the i8524 is not fed from the same clock >>> as the TSC. > > A very important and relevant detail here is that the *only* > clock/counter which has a standardized frequency *is* the i8254 > counter. Not even the ACPI timer? >> RTC needs its own dedicated crystal. > > The RTC *crystal* may not even exist, only the RTC itself which may > be driven by gremlins on a threadmill for all we care. > > Besides, getting hold of *precise* timing from the RTC is a nightmare, > it's not meant to be used for that. It can give a precision of 1/32K seconds fairly easily (but with lots of overhead) by interrupting at 32 KHz. 1KHz is sometimes used for profiling. >> I was not able to find any specifications for allowed jitter in PCH PLLs, >> but I would expect that above IDT chips have much better stability than >> something in overheating PCH. > > Jitter requirements are pretty tight for anything you're going to > PLL into the GHz range, but any quartz crystal is going to have much > better phase-noise spec than anything we can measure in the digital > noise of a modern computer. > > (Often the PLL chips modulate the quartz to spread out EMI spurs, > look for "spread-spectrum" settings.) Jitter is what I'm most worried about for virtual systems. Jitter is quite good for a hardware RTC. I use the RTC for PPS mainly to determine its quality. Its hardware jitter is insignificant compared with the jitter from interrupt latency. >>>> some hypervisors start offering modes where old ISA peripherals >>>> are not emulated, > > How do they expect people to run MS Flight Simulator then ? :-) > >> I think we have no choice but do something for ISA/LPC-less configurations. >> We could even trust CPU report about its frequency as the last resort. > > Usually the errors will be magnificient, so a trivial sanity-check will > catch them. Don't leave home without it. But then you have the problem of handling them. There might be just 1 timecounter source that is emulated correctly, but you don't know which it is. There might be none. Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20160328042313.N850>