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如何在 kvm 客户机上启用 L3 缓存？

tag-icon • tag-icon kvm-virtualization libvirt virsh
如何在 kvm 客户机上启用 L3 缓存？

我在一台Intel机器(centos6.5)上启动了一个kvm客户端(centos6.5)，使用libvirt，客户端的xml如下

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>test-1</name>
  <uuid>9377bce1-ae83-e356-ed15-919c8625fb4b</uuid>
  <memory unit='KiB'>8388608</memory>
  <currentMemory unit='KiB'>4194304</currentMemory>
  <vcpu placement='static' current='2'>8</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.5.0'>hvm</type>
    <boot dev='hd'/>
    <boot dev='cdrom'/>
    <bootmenu enable='yes'/>
    <bios useserial='yes' rebootTimeout='0'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <cpu mode='host-passthrough'>
  </cpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/data/vhosts//test-1.disk'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
    <controller type='ide' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <controller type='virtio-serial' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </controller>
    <controller type='usb' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <interface type='bridge'>
      <mac address='52:54:00:ea:12:d9'/>
      <source bridge='br-ex'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' listen='0.0.0.0'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </memballoon>
  </devices>
  <qemu:commandline>
    <qemu:env name='SPICE_DEBUG_ALLOW_MC' value='1'/>
  </qemu:commandline>
</domain>

现在我很困惑，尽管使用了“host-passthrough”，我仍然无法在客户机中看到 L3 缓存，只有 L1、L2 缓存，如下所示

[root@vm-kvm-115 results]# lscpu 
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    1
Core(s) per socket:    1
Socket(s):             2
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 26
Stepping:              5
CPU MHz:               2266.746
BogoMIPS:              4533.49
Hypervisor vendor:     KVM
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              4096K
NUMA node0 CPU(s):     0,1

以下是我的物理机器信息

[root@host-kvm-22 linux]# rpm -qa | grep libvirt
libvirt-client-0.10.2-54.el6_7.2.x86_64
libvirt-0.10.2-54.el6_7.2.x86_64
libvirt-devel-0.10.2-54.el6_7.2.x86_64
libvirt-python-0.10.2-54.el6_7.2.x86_64

[root@host-kvm-22 linux]# rpm -qa | grep qemu
qemu-img-0.12.1.2-2.479.el6_7.2.x86_64
gpxe-roms-qemu-0.9.7-6.14.el6.noarch
qemu-kvm-0.12.1.2-2.479.el6_7.2.x86_64

[root@host-kvm-22 linux]# uname -r
2.6.32-573.8.1.el6.x86_64

[root@host-kvm-22 linux]# lscpu 
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                16
On-line CPU(s) list:   0-15
Thread(s) per core:    2
Core(s) per socket:    4
Socket(s):             2
NUMA node(s):          2
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 26
Stepping:              5
CPU MHz:               2266.743
BogoMIPS:              4532.68
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              8192K
NUMA node0 CPU(s):     0-3,8-11
NUMA node1 CPU(s):     4-7,12-15

有人能告诉我如何在客户机上启用 L3 缓存吗？

答案1

无论如何，您误解了主机直通 CPU 模型的范围。它仅控制 CPU 及其功能标志的识别 - CPU 的某些方面仍未向客户机公开。例如，使用您那里的 XML，所有 8 个 CPU 都作为同一 NUMA 节点中的单独插槽向客户机公开。您的主机反而有 2 个 NUMA 节点，每个节点有两个插槽，每个节点有四个核心。仅这一点就足以使主机中的 L3 缓存概念无法很好地映射到客户机中。

您可以在 XML 中设置虚拟 CPU 拓扑，但我仍然认为它不会使 L3 缓存出现。这也并不重要，因为您允许 8 个虚拟 CPU 在所有 16 个主机 CPU 上浮动。由于您的主机 CPU 分布在 2 个 NUMA 节点上，您将在大部分时间获得跨 NUMA 节点内存访问，这会带来很高的延迟损失，从而抵消缓存的任​​何好处。换句话说，您最好专注于更高效的 VM 放置，通过使用 CPU 固定将客户机限制在单个主机 NUMA 节点上。

答案2

从 2.8.0 版开始，QEMU 添加了 L3 缓存支持。请参阅Bugzilla。

将 L3 缓存暴露给客户机将提高性能，因为客户机 CPU 可以避免大量 IPI（处理器间中断）。请阅读此内容文章了解更多信息。

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