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Gmane
From: Ingo Molnar <mingo-X9Un+BFzKDI <at> public.gmane.org>
Subject: Re: [RFT] mmu optimizations branch
Newsgroups: gmane.comp.emulators.kvm.devel
Date: Tuesday 2nd January 2007 16:11:17 UTC (over 10 years ago)
* Avi Kivity  wrote:

> This is a request for testing of the mmu optimizations branch.
> 
> Currently the shadow page tables are discarded every time the guest 
> performs a context switch.  The mmu branch allows shadow page tables 
> to be cached across context switches, greatly reducing the cpu 
> utilization on multi process workloads.  It is now stable enough for 
> testing (though perhaps not for general use).

i have tested it with Fedora Core 6 guest (32-bit, nopae), under a FC6 
host (32-bit CoreDuo2, nopae, enough RAM), and it's working great!

Here are some quick numbers. Context-switch overhead with lmbench 
lat_ctx -s 0 [zero memory footprint]:
              
  -------------------------------------------------
    #tasks    native    kvm-r4204    kvm-r4232(mmu) 
  -------------------------------------------------
        2:      2.02       180.91         9.19
       20:      4.04       183.21        10.01
       50:      4.30       185.95        11.27

so here it's a /massive/, almost 20 times speedup!

Context-switch overhead with -s 1000 (1MB memory footprint):

  -------------------------------------------------
    #tasks    native    kvm-r4204    kvm-r4232(mmu)
  -------------------------------------------------
        2:     150.5      1032.97       295.16
       20:     216.6      1020.34       393.01
       50:     218.1      1015.58      2335.99[*]

the speedup is nice here too. Note the outlier at 50 tasks: it's 
consistently reproducable. Could KVM be trashing the pagetable cache due 
to some sort of internal limit? It's not due to guest size

The -mmu FC6 guest is visibly faster, so it's not just microbenchmarks 
that benefit from this change. KVM got /massively/ faster in every 
aspect, kudos Avi!  (Note that r4204 already included the interactivity 
IRQ fixes so the improvements are i think purely due to pagetable 
caching speedups.)

on a related note, i also got:

 vmwrite error: reg 6802 value cfd3c4a4 (err 17408)

and:

 kvm: unhandled wrmsr: 0xc1
 inject_general_protection: rip 0xc011f7f3
 kvm: unhandled wrmsr: 0x186
 inject_general_protection: rip 0xc011f7f3
 kvm: unhandled wrmsr: 0xc1
 inject_general_protection: rip 0xc011f7f3
 kvm: unhandled wrmsr: 0x186
 inject_general_protection: rip 0xc011f7f3

unfortunately 0xc011f7f3 is in native_write_msr(), which isnt very 
helpful. (i have CONFIG_PARAVIRT enabled in the -rt guest and host 
kernels) But the MSR values suggest that this is the NMI watchdog thing 
again, trying to program MSR_ARCH_PERFMON_EVENTSEL0 and 
MSR_ARCH_PERFMON_PERFCTR0, but this time Linux recovered due to a more 
robust MSR handling. The guest disabled the NMI watchdog with:

  Testing NMI watchdog ... CPU#0: NMI appears to be stuck (0->0)!

the FC6 installer hang that i saw with earlier MMU-branch snapshots is 
fixed.

	Ingo

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