2017-01-18 13:46+0100, Paolo Bonzini: > > > On 18/01/2017 13:24, Marcelo Tosatti wrote: > > On Wed, Jan 18, 2017 at 10:17:38AM -0200, Marcelo Tosatti wrote: > >> On Tue, Jan 17, 2017 at 04:36:21PM +0100, Radim Krcmar wrote: > >>> 2017-01-17 09:30-0200, Marcelo Tosatti: > >>>> On Tue, Jan 17, 2017 at 09:03:27AM +0100, Miroslav Lichvar wrote: > >>>>> Users of the PTP_SYS_OFFSET ioctl assume that (ts[0]+ts[2])/2 > >>>>> corresponds to ts[1], (ts[2]+ts[4])/2 corresponds to ts[3], and so on. > >>>>> > >>>>> ts[1] ts[3] > >>>>> Host time ---------+---------+........ > >>>>> | | > >>>>> | | > >>>>> Guest time ----+---------+---------+...... > >>>>> ts[0] ts[2] ts[4] > >>> > >>> KVM PTP delay moves host ts[i] to be close to guest ts[i+1] and makes > >>> the offset very consistent, so the graph would look like: > >>> > >>> ts[1] ts[3] > >>> Host time -------------+---------+........ > >>> | | > >>> | | > >>> Guest time ----+---------+---------+...... > >>> ts[0] ts[2] ts[4] > >>> > >>> which doesn't sound good if users assume that the host reading is in the > >>> middle -- the guest time would be ahead of the host time. > >> > >> Testcase: run a guest and a loop sending SIGUSR1 to vcpu0 (emulating > >> intense interrupts). Follows results: > >> > >> Without TSC delta calculation: > >> ============================= > >> > >> #* PHC0 0 3 377 2 -99ns[ +206ns] +/- 116ns > >> #* PHC0 0 3 377 8 +202ns[ +249ns] +/- 111ns > >> #* PHC0 0 3 377 8 -213ns[ +683ns] +/- 88ns > >> #* PHC0 0 3 377 6 +77ns[ +319ns] +/- 56ns > >> #* PHC0 0 3 377 4 -771ns[-1029ns] +/- 93ns > >> #* PHC0 0 3 377 10 -49ns[ -58ns] +/- 121ns > >> #* PHC0 0 3 377 9 +562ns[ +703ns] +/- 107ns > >> #* PHC0 0 3 377 6 -2ns[ -3ns] +/- 94ns > >> #* PHC0 0 3 377 4 +451ns[ +494ns] +/- 138ns > >> #* PHC0 0 3 377 11 -67ns[ -74ns] +/- 113ns > >> #* PHC0 0 3 377 8 +244ns[ +264ns] +/- 119ns > >> #* PHC0 0 3 377 7 -696ns[ -890ns] +/- 89ns > >> #* PHC0 0 3 377 4 +468ns[ +560ns] +/- 110ns > >> #* PHC0 0 3 377 11 -310ns[ -430ns] +/- 72ns > >> #* PHC0 0 3 377 9 +189ns[ +298ns] +/- 54ns > >> #* PHC0 0 3 377 7 +594ns[ +473ns] +/- 96ns > >> #* PHC0 0 3 377 5 +151ns[ +280ns] +/- 71ns > >> #* PHC0 0 3 377 10 -590ns[ -696ns] +/- 94ns > >> #* PHC0 0 3 377 8 +415ns[ +526ns] +/- 74ns > >> #* PHC0 0 3 377 6 +1381ns[+1469ns] +/- 101ns > >> #* PHC0 0 3 377 4 +571ns[+1304ns] +/- 54ns > >> #* PHC0 0 3 377 8 -5ns[ +71ns] +/- 139ns > >> #* PHC0 0 3 377 7 -247ns[ -502ns] +/- 69ns > >> #* PHC0 0 3 377 5 -283ns[ +879ns] +/- 73ns > >> #* PHC0 0 3 377 3 +148ns[ -109ns] +/- 61ns > >> > >> With TSC delta calculation: > >> ============================ > >> > >> #* PHC0 0 3 377 7 +379ns[ +432ns] +/- 53ns > >> #* PHC0 0 3 377 9 +106ns[ +420ns] +/- 42ns > >> #* PHC0 0 3 377 7 -58ns[ -136ns] +/- 62ns > >> #* PHC0 0 3 377 12 +93ns[ -38ns] +/- 64ns > >> #* PHC0 0 3 377 8 +84ns[ +107ns] +/- 69ns > >> #* PHC0 0 3 377 3 -76ns[ -103ns] +/- 52ns > >> #* PHC0 0 3 377 7 +52ns[ +63ns] +/- 50ns > >> #* PHC0 0 3 377 11 +29ns[ +31ns] +/- 70ns > >> #* PHC0 0 3 377 7 -47ns[ -56ns] +/- 42ns > >> #* PHC0 0 3 377 10 -35ns[ -42ns] +/- 33ns > >> #* PHC0 0 3 377 7 -32ns[ -34ns] +/- 42ns > >> #* PHC0 0 3 377 11 -172ns[ -173ns] +/- 118ns > >> #* PHC0 0 3 377 6 +65ns[ +76ns] +/- 23ns > >> #* PHC0 0 3 377 9 +18ns[ +23ns] +/- 37ns > >> #* PHC0 0 3 377 6 +41ns[ -60ns] +/- 30ns > >> #* PHC0 0 3 377 10 +39ns[ +183ns] +/- 42ns > >> #* PHC0 0 3 377 6 +50ns[ +102ns] +/- 86ns > >> #* PHC0 0 3 377 11 +50ns[ +75ns] +/- 52ns > >> #* PHC0 0 3 377 6 +50ns[ +116ns] +/- 100ns > >> #* PHC0 0 3 377 10 +46ns[ +65ns] +/- 79ns > >> #* PHC0 0 3 377 7 -38ns[ -51ns] +/- 29ns > >> #* PHC0 0 3 377 10 -11ns[ -12ns] +/- 32ns > >> #* PHC0 0 3 377 7 -31ns[ -32ns] +/- 99ns > >> #* PHC0 0 3 377 10 +222ns[ +238ns] +/- 58ns > >> #* PHC0 0 3 377 6 +185ns[ +207ns] +/- 39ns > >> #* PHC0 0 3 377 10 -392ns[ -394ns] +/- 118ns > >> #* PHC0 0 3 377 6 -9ns[ -50ns] +/- 35ns > >> #* PHC0 0 3 377 10 -346ns[ -355ns] +/- 111ns > >> > >> > >> Do you still want to drop it in favour of simplicity? > > > > This is the output of "chronyc sources". See section "Time sources" > > of https://chrony.tuxfamily.org/doc/2.4/chronyc.html. > > It's just that it's not obvious why you get better results with biased > host timestamps. What makes the biased host timestamp more precise? I think that TSC offsetting is more precise (the difference between two samples smaller), but less accurate (the difference between guest and host clock is bigger). And chronyc can't tell how much off from the host clock our guest clock is (that would be measured how I wrote above). The main reason why it is more precise is because the TSC offset erases the effect of many latency sources that can happen before or after the host time read. The precision with TSC offsetting is almost only affected by the duration between two guest time measurements, because the host time is shifted to be very close to the latter guest time measurement. Without TSC offsetting, the host time will be closer to the middle between two guest reads (= better accuracy), but the moment of the tsc read on the host has more variables -- host interrupts and whatnot, which will result in lower precision. > I'd rather use PTP_SYS_OFFSET_PRECISE instead, but unfortunately chrony > does not support it---but I would still prefer you to support > PTP_SYS_OFFSET_PRECISE as well. Oh, I didn't expect that userspace can't use PTP_SYS_OFFSET_PRECISE ... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
