[Xen-devel] Re: [PATCH] xen: core dom0 support

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Ingo Molnar wrote:
> * H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
>   
> > Ingo Molnar wrote:
> >     
> > > Since it's the same kernel image i think the only truly reliable 
> > > method would be to reboot between _different_ kernel images: 
> > > same instructions but randomly re-align variables both in terms 
> > > of absolute address and in terms of relative position to each 
> > > other. Plus randomize bootmem allocs and never-gets-freed-really 
> > > boot-time allocations.
> > > Really hard to do i think ...
> > >
> > >       
> > Ouch, yeah.
> >
> > On the other hand, the numbers made sense to me, so I don't 
> > see why there is any reason to distrust them.  They show a 5% 
> > overhead with pv_ops enabled, reduced to a 2% overhead with 
> > the changed.  That is more or less what would match my 
> > intuition from seeing the code.
> >     
> Yeah - it was Jeremy expressed doubt in the numbers, not me.
>   
Mainly because I was seeing the instruction and cycle counts completely 
unchanged from run to run, which is implausible.  They're not zero, so 
they're clearly measurements of *something*, but not cycles and 
instructions, since we know that they're changing.  So what are they 
measurements of?  And if they're not what they claim, are the other 
numbers more meaningful?
It's easy to read the numbers as confirmations of preconceived 
expectations of the outcomes, but that's - as I said - unsatisfying.
> And we need to eliminate that 2% as well - 2% is still an awful 
> lot of native kernel overhead from a kernel feature that 95%+ of 
> users do not make any use of.
>   
Well, I think there's a few points here:

  1. the test in question is a bit vague about kernel and user
     measurements.  I assume the stuff coming from perfcounters is
     kernel-only state, but the elapsed time includes the usermode
     component, and so will be affected by the usermode page placement
     and cache effects.  If I change the test to copy the test
     executable (statically linked, to avoid libraries), then that
     should at least fuzz out user page placement.
  2. Its true that the cache effects could be due to the precise layout
     of the kernel executable; but if those effects are swamping
     effects of the changes to improve pvops then its unclear what the
     point of the exercise is.  Especially since:
  3. It is a config option, so if someone is sensitive to the
     performance hit and it gives them no useful functionality to
     offset it, then it can be disabled.  Distros tend to enable it
     because they tend to value function and flexibility over raw
     performance; they tend to enable things like audit, selinux,
     modules which all have performance hits of a similar scale (of
     course, you could argue that more people get benefit from those
     features to offset their costs).  But,
  4. I think you're underestimating the number of people who get
     benefit from pvops; the Xen userbase is actually pretty large, and
     KVM will use pvops hooks when available to improve Linux-as-guest.
  5. Also, we're looking at a single benchmark with no obvious
     relevance to a real workload.  Perhaps there are workloads which
     continuously mash mmap/munmap/mremap(!), but I think they're
     fairly rare.  Such a benchmark is useful for tuning specific
     areas, but if we're going to evaluate pvops overhead, it would be
     nice to use something a bit broader to base our measurements on. 
     Also, what weighting are we going to put on 32 vs 64 bit?  Equally
     important?  One more than the other?

All that said, I would like to get the pvops overhead down to 
unmeasureable - the ideal would be to be able to justify removing the 
config option altogether and leave it always enabled.
The tradeoff, as always, is how much other complexity are we willing to 
stand to get there?  The addition of a new calling convention is already 
fairly esoteric, but so far it has got us a 60% reduction in overhead 
(in this test).  But going further is going to get more complex.
For example, the next step would be to attack set_pte (including 
set_pte_*, pte_clear, etc), to make them use the new calling convention, 
and possibly make them inlineable (ie, to get it as close as possible to 
the non-pvops case).  But that will require them to be implemented in 
asm (to guarantee that they only use the registers they're allowed to 
use), and we already have 3 variants of each for the different pagetable 
modes.  All completely doable, and not even very hard, but it will be 
just one more thing to maintain - we just need to be sure the payoff is 
worth it.
   J

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