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IIS 10.0 processor affinity feature doesn't work on NUMA hardware

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This article provides information about resolving the problem where Internet Information Services (IIS) 10.0 processor affinity does not work on non-uniform memory access (NUMA) hardware if IIS thread pool ideal CPU optimization is enabled.

Original product version:   Internet Information Services 10.0
Original KB number:   4041818

Symptoms

The processor affinity feature does not work on out-of-box NUMA hardware in IIS 10.0.

You can specify whether a particular worker process that's assigned to an application pool should be assigned to a particular CPU. To do this, you can use the smpAffinitized application pool setting together with the smpProcessorAffinityMask and smpProcessorAffinityMask2 specified affinity masks.

However, on IIS 10.0 that is running on a NUMA server, a worker process is still assigned to all available processors even if you enable processor affinity and set the affinity mask to indicate only a subset of CPUs.

Note

  • The issue does not apply to non-NUMA hardware.
  • You can check the processor affinity of each worker process from the Task Manager. To do this, follow these steps:
    1. Select Details.
    2. Select w3wp.exe.
    3. Set affinity.

Cause

The issue occurs because a new feature, IIS Thread Pool Ideal CPU Optimization for NUMA hardware, is enabled in IIS 10.0.

By default, this feature is enabled. The feature optimizes IIS performance by evenly distributing loads across all CPUs of all NUMA nodes. However, this setting conflict with the specified processor affinity.

Workaround

To work around this issue, disable IIS Thread Pool Ideal CPU Optimization on a NUMA server if you want to specify processor affinity. To do this, update the value of the following registry from 1 to 0:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\InetInfo\Parameters\ThreadPoolUseIdealCpu

Resolution

To fix this issue, upgrade your IIS to a later version, such as IIS 10.0 RS3, when it is available.

Reference