• Understanding RSS Performance
  • RSS and virtualization
  • LBFO and RSS
  • Performance Tuning Guidelines for Windows Server 2012 April 12, 2013 Abstract

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    * NumberofReceiveQueues: If your logical processors seem to be underutilized for receive traffic (for example, as viewed in Task Manager), you can try increasing the number of RSS queues from the default of 2 to the maximum that is supported by your network adapter. Your network adapter may have options to change the number of RSS queues as part of the driver.

    set-netadapterRSS –Name “Ethernet” –NumberOfReceiveQueues

    For more information, see Scalable Networking: Eliminating the Receive Processing Bottleneck—Introducing RSS.

    Understanding RSS Performance

    Tuning RSS requires understanding the configuration and the load-balancing logic. To verify that the RSS settings have taken effect, the Get-NetAdapterRss Windows PowerShell cmdlet gives better insight.

    PS C:\Users\Administrator> get-netadapterrss

    Name : testnic 2

    InterfaceDescription : Broadcom BCM5708C NetXtreme II GigE (NDIS VBD Client) #66

    Enabled : True

    NumberOfReceiveQueues : 2

    Profile : NUMAStatic

    BaseProcessor: [Group:Number] : 0:0

    MaxProcessor: [Group:Number] : 0:15

    MaxProcessors : 8

    IndirectionTable: [Group:Number] :

    0:0 0:4 0:0 0:4 0:0 0:4 0:0 0:4

    (# indirection table entries are a power of 2 and based on # of processors)

    0:0 0:4 0:0 0:4 0:0 0:4 0:0 0:4

    In addition to echoing parameters that were set, the key aspect of the output is to understand indirection table output. The indirection table displays the hash table buckets that are used to distribute incoming traffic. In this example, the n:c notation designates the Numa K-Group:CPU index pair that is used to direct incoming traffic. We see exactly 2 unique entries (0:0 and 0:4), which represent k-group 0/cpu0 and k-group 0/cpu 4, respectively.
    We further see only one k-group for this system (k-group 0) and a n (where n <= 128) indirection table entry. Because the number of receive queues is set to 2, only 2 processors (0:0, 0:4) are chosen even though maximum processors is set to 8. In effect, the indirection table is hashing incoming traffic to only use 2 CPUs out of the 8 that are available.
    To fully utilize the CPUs, the number of RSS Receive Queues should be equal to or greater than Max Processors. For the previous example, the Receive Queue should be set to 8 or greater.

    RSS and virtualization

    RSS provides hashing and scalability to host interface only. RSS does not provide any interaction with virtual machines, instead users can configure VMQ in those scenarios.

    RSS can be enabled for guest virtual machines in the case of SR-IOV because the virtual function driver supports RSS capability. In this case, the guest and the host will have the benefit of RSS. Note that the host does not get RSS capability because the virtual switch is enabled with SR-IOV.

    LBFO and RSS

    RSS can be enabled on a network adapter that is teamed. In this scenario, only the underlying physical network adapter can be configured to use RSS. A user cannot set RSS cmdlets on the teamed network adapter.

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    Performance Tuning Guidelines for Windows Server 2012 April 12, 2013 Abstract

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