Re: Problem with shared L2 cache

[RW <kvm@tauceti.net>]

You can add the option "-cpu host" to KVM. Then you get
the same cache size information as your host has.

Robert


On 01/22/10 15:31, Hristo Tomov wrote:
> Hello!
> 
>         I have following problem with KVM. I have quad-core processor
> Xeon X7350 ( 8MB L2 cache ). The processor shares 4 MB L2 cache on two
> cores. By XEN emulation the guest system have 4 MB cache at disposal.
> 
> xen guest: cat /proc/cpuinfo
> 
> processor    : 3
> vendor_id    : GenuineIntel
> cpu family    : 6
> model        : 15
> model name    : Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
> stepping    : 11
> cpu MHz        : 2932.026
> cache size    : 4096 KB
> fpu        : yes
> fpu_exception    : yes
> cpuid level    : 10
> wp        : yes
> flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat clflush dts mmx fxsr sse sse2 ss syscall nx lm
> constant_tsc pni ds_cpl cx16 xtpr lahf_lm
> bogomips    : 5865.67
> clflush size    : 64
> cache_alignment    : 64
> address sizes    : 40 bits physical, 48 bits virtual
> power management:
> 
>  By KVM in the guest system have 2 MB cache at disposal.
> 
> kvm guest: cat /proc/cpuinfo
> 
> processor       : 3
> vendor_id       : GenuineIntel
> cpu family      : 6
> model           : 6
> model name      : QEMU Virtual CPU version 0.9.1
> stepping        : 3
> cpu MHz         : 2932.026
> cache size      : 2048 KB
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 2
> wp              : yes
> flags           : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca
> cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm pni
> bogomips        : 5861.69
> clflush size    : 64
> cache_alignment : 64
> address sizes   : 40 bits physical, 48 bits virtual
> power management:
> 
> On specific loads with the same number of processors, this difference
> in the cache size leads to negative results. The productivity of the
> KVM virtual machine shows 2 - 5 times worse results in comparison with
> XEN virtual machine.
> 
> 
> Host:
> 
> cpu:                     Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
> os:                       Centos 5.4 x86_64
> kvm:                    kvm-83-105.el5_4.13
> kernel:                kernel-2.6.18-164.10.1.el5
> kernel arch:        x86_64
> 
> Guest:
> 
> os:                      Centos 5.4 x86_64
> qemu command: /usr/libexec/qemu-kvm -S -M pc -m 4096 -smp 4 -name
> centos -uuid 17136bf5-a7a3-0323-7e9b-90ec5aa06cdb
> -no-kvm-pit-reinjection -monitor pty -pidfile
> /var/run/libvirt/qemu//centos.pid -boot c -drive
> file=/kvm/centos.img,if=virtio,index=0,boot=on,cache=none -net
> nic,macaddr=52:54:00:43:81:4f,vlan=0,model=virtio -net
> tap,fd=14,script=,vlan=0,ifname=vnet0 -serial pty -parallel none -usb
> -vnc 127.0.0.1:0 -k en-us
> 
> Host CPU:
> processor    : 0
> vendor_id    : GenuineIntel
> cpu family    : 6
> model        : 15
> model name    : Intel(R) Xeon(R) CPU X7350  @ 2.93GHz
> stepping    : 11
> cpu MHz        : 2931.962
> cache size    : 4096 KB
> physical id    : 3
> siblings    : 4
> core id        : 0
> cpu cores    : 4
> apicid        : 12
> fpu        : yes
> fpu_exception    : yes
> cpuid level    : 10
> wp        : yes
> flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall
> nx lm constant_tsc pni monitor ds_cpl vmx est tm2 cx16 xtpr lahf_lm
> bogomips    : 5863.92
> clflush size    : 64
> cache_alignment    : 64
> address sizes    : 40 bits physical, 48 bits virtual
> power management:
> 
> processor    : 1
> vendor_id    : GenuineIntel
> cpu family    : 6
> model        : 15
> model name    : Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
> stepping    : 11
> cpu MHz        : 2931.962
> cache size    : 4096 KB
> physical id    : 8
> siblings    : 4
> core id        : 0
> cpu cores    : 4
> apicid        : 32
> fpu        : yes
> fpu_exception    : yes
> cpuid level    : 10
> wp        : yes
> flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall
> nx lm constant_tsc pni monitor ds_cpl vmx est tm2 cx16 xtpr lahf_lm
> bogomips    : 5863.11
> clflush size    : 64
> cache_alignment    : 64
> address sizes    : 40 bits physical, 48 bits virtual
> power management:
> 
> processor       : 2
> vendor_id       : GenuineIntel
> cpu family      : 6
> cpu family      : 6
> model           : 15
> model name      : Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
> stepping        : 11
> cpu MHz         : 2931.962
> cache size      : 4096 KB
> physical id     : 4
> siblings        : 4
> core id         : 0
> cpu cores       : 4
> apicid          : 16
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 10
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall
> nx lm constant_tsc pni monitor ds_cpl vm
> x est tm2 cx16 xtpr lahf_lm
> bogomips        : 5863.10
> clflush size    : 64
> clflush size    : 64
> cache_alignment : 64
> address sizes   : 40 bits physical, 48 bits virtual
> power management:
> 
> processor       : 3
> vendor_id       : GenuineIntel
> cpu family      : 6
> model           : 15
> model name      : Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
> stepping        : 11
> cpu MHz         : 2931.962
> cache size      : 4096 KB
> physical id     : 15
> siblings        : 4
> core id         : 0
> apicid          : 60
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 10
> wp              : yes
> x est tm2 cx16 xtpr lahf_lm
> bogomips        : 5863.19
> clflush size    : 64
> cache_alignment : 64
> address sizes   : 40 bits physical, 48 bits virtual
> power management:
> --
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