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Message-ID: <20170118142409.GA9713@amt.cnet>
Date: Wed, 18 Jan 2017 12:24:09 -0200
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krcmar <rkrcmar@...hat.com>,
Miroslav Lichvar <mlichvar@...hat.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org,
Richard Cochran <richardcochran@...il.com>
Subject: Re: [patch 3/3] PTP: add kvm PTP driver
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.
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