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Reserving Processors for Applications

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The receive side scaling (RSS) interface enables an administrator to reserve a set of processors for applications to use. The administrator can reserve a set of processors starting at logical CPU number 0 and ending at a specified CPU number. The RSS base CPU number is the CPU number of the first CPU that RSS can use. RSS cannot use the CPUs that are numbered below the base CPU number. For example, on a quad-core system with hyper-threading turned off, if base CPU number is set to 1, processors 1, 2, and 3 can be used for RSS.

NDIS uses the default value of 0 for base CPU number, but an administrator can change this value. The RSS interface does not permit the administrator to reserve a non-contiguous, arbitrary subset of CPUs for applications to use.

In Microsoft Windows Server 2003 with the Scalable Networking Pack, administrators can set the base CPU number with the RssBaseCpu registry keyword in HKEY_LOCAL_MACHINE\\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters. The RssBaseCpu value is a DWORD type and, if it is not present, NDIS uses the default value of 0.

In Windows Server 2008, administrators can set the base CPU number with the RssBaseCpu registry keyword in HKEY_LOCAL_MACHINE\\SYSTEM\CurrentControlSet\Services\NDIS\Parameters. The RssBaseCpu value is a DWORD type and, if it is not present, NDIS uses the default value of 0. This registry keyword also applies to later versions of Windows Server.

Note Starting in Windows 8 and Windows Server 2012, administrators can control many aspects of Network Adapters by using PowerShell cmdlets. Directly editing the registry is now discouraged.

The PowerShell cmdlet for reserving RSS CPUs is Set-NetAdapterRss. The primary difference between using Set-NetAdapterRss and using RssBaseCpu is that PowerShell cmdlets operate on a per-Network Adapter basis while RssBaseCpu is global, meaning it applies to all Network Adapters. Generally, working with each Network Adapter separately is recommended because it offers more flexibility, granularity, and understandability in giving each Network Adapter its own configuration. However, administrators might still use the global RssBaseCpu key if they would like to apply a configuration to all current and all future Network Adapters at the same time.

For a complete list of Network Adapter cmdlets, see Network Adapter Cmdlets in Windows PowerShell.