On Wed, Jan 18, 2017 at 01:46:58PM +0100, Paolo Bonzini wrote: > 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?
The issue is that, without it you have a larger window for interruptions to take place and therefore the read value when ->gettime64 return is from longer time in the past. > 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. Sure, I'll check if it makes sense to implement PTP_SYS_OFFSET_PRECISE for KVM case.
