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• Disable Windows Auto Tuning Level
• Tcp Receive Window Auto-tuning Level 5
• Tcp Receive Window Auto-tuning Level 4
• Tcp Receive Window Auto-tuning Level Guide

Oct 05, 2012 The Receive Window Auto-Tuning feature lets the operating system continually monitor routing conditions such as bandwidth, network delay, and application delay. Therefore, the operating system can configure connections by scaling the TCP receive window to maximize the network performance. To determine the optimal receive window size, the Receive Window Auto-Tuning feature measures the products. Jun 27, 2017  TCP Receive Window. In the above diagram, the client and server are advertising their window size values as they communicate. Each TCP header will display the most recent window value, which can grow or shrink as the connection progresses. In this example, the client has a TCP receive window of 65,535 bytes, and the server has 5,840. However, the TCP auto tuning feature may get things wrong sometimes. Instead of optimal true receive window size, incompatible and out of range RWIN size may be used. By default, Windows in normal auto tuning level will use RWIN size of 256 bytes with a scale factor of 8. Mar 29, 2010 TCP Auto-Tuning To turn off the default RWIN auto tuning behavior, (in elevated command prompt) type: netsh int tcp set global autotuninglevel=disabled The default auto-tuning level is 'normal', and the possible settings for the above command are: disabled: uses a fixed value for the tcp receive window. Limits it to 64KB (limited at 65535).

Applies to: Windows Server 2019, Windows Server 2016, Windows Server (Semi-Annual Channel)

Therefore, the operating system can configure connections by scaling the TCP receive window to maximize the network performance. To determine the optimal receive window size, the Receive Window Auto-Tuning feature measures the products that delay bandwidth and the application retrieve rates. Windows (Vista/7/8/etc) will automatically set - and more importantly, increase - the size of the TCP receive window for you, as needed, to maximize throughput. Receive Window Auto-Tuning: Microsoft calls this automatic management of the receive window size 'auto-tuning'. To see the settings associated with this, go into a DOS prompt and run.

Use the information in this topic to tune the performance network adapters for computers that are running Windows Server 2016 and later versions. If your network adapters provide tuning options, you can use these options to optimize network throughput and resource usage.

The correct tuning settings for your network adapters depend on the following variables:

Disable Windows Auto Tuning Level

• The network adapter and its feature set
• The type of workload that the server performs
• The server hardware and software resources
• Your performance goals for the server

The following sections describe some of your performance tuning options.

Enabling offload features

Turning on network adapter offload features is usually beneficial. However, the network adapter might not be powerful enough to handle the offload capabilities with high throughput.

Important

Do not use the offload features IPsec Task Offload or TCP Chimney Offload. These technologies are deprecated in Windows Server 2016, and might adversely affect server and networking performance. In addition, these technologies might not be supported by Microsoft in the future.

For example, consider a network adapter that has limited hardware resources.In that case, enabling segmentation offload features might reduce the maximum sustainable throughput of the adapter. However, if the reduced throughput is acceptable, you should go ahead an enable the segmentation offload features.

Note

Some network adapters require you to enable offload features independently for the send and receive paths.

Enabling receive-side scaling (RSS) for web servers

RSS can improve web scalability and performance when there are fewer network adapters than logical processors on the server. When all the web traffic is going through the RSS-capable network adapters, the server can process incoming web requests from different connections simultaneously across different CPUs.

Important

Avoid using both non-RSS network adapters and RSS-capable network adapters on the same server. Because of the load distribution logic in RSS and Hypertext Transfer Protocol (HTTP), performance might be severely degraded if a non-RSS-capable network adapter accepts web traffic on a server that has one or more RSS-capable network adapters. In this circumstance, you should use RSS-capable network adapters or disable RSS on the network adapter properties Advanced Properties tab.

To determine whether a network adapter is RSS-capable, you can view the RSS information on the network adapter properties Advanced Properties tab.

RSS Profiles and RSS Queues

The default RSS predefined profile is NUMAStatic, which differs from the default that the previous versions of Windows used. Before you start using RSS profiles, review the available profiles to understand when they are beneficial and how they apply to your network environment and hardware.

For example, if you open Task Manager and review the logical processors on your server, and they seem to be underutilized for receive traffic, you can try increasing the number of RSS queues from the default of two to the maximum that your network adapter supports. Your network adapter might have options to change the number of RSS queues as part of the driver.

Increasing network adapter resources

For network adapters that allow you to manually configure resources such as receive and send buffers, you should increase the allocated resources.

Some network adapters set their receive buffers low to conserve allocated memory from the host. The low value results in dropped packets and decreased performance. Therefore, for receive-intensive scenarios, we recommend that you increase the receive buffer value to the maximum.

Note

If a network adapter does not expose manual resource configuration, either it dynamically configures the resources, or the resources are set to a fixed value that cannot be changed.

Enabling interrupt moderation

To control interrupt moderation, some network adapters expose different interrupt moderation levels, different buffer coalescing parameters (sometimes separately for send and receive buffers), or both.

You should consider interrupt moderation for CPU-bound workloads. When using interrupt moderation, consider the trade-off between the host CPU savings and latency versus the increased host CPU savings because of more interrupts and less latency. If the network adapter does not perform interrupt moderation, but it does expose buffer coalescing, you can improve performance by increasing the number of coalesced buffers to allow more buffers per send or receive.

Performance tuning for low-latency packet processing

Many network adapters provide options to optimize operating system-induced latency. Latency is the elapsed time between the network driver processing an incoming packet and the network driver sending the packet back. This time is usually measured in microseconds. For comparison, the transmission time for packet transmissions over long distances is usually measured in milliseconds (an order of magnitude larger). This tuning will not reduce the time a packet spends in transit.

Following are some performance tuning suggestions for microsecond-sensitive networks.

• Set the computer BIOS to High Performance, with C-states disabled. However, note that this is system and BIOS dependent, and some systems will provide higher performance if the operating system controls power management. You can check and adjust your power management settings from Settings or by using the powercfg command. For more information, see Powercfg Command-Line Options.

• Set the operating system power management profile to High Performance System.

  Note

  This setting does not work properly if the system BIOS has been set to disable operating system control of power management.

• Enable static offloads. For example, enable the UDP Checksums, TCP Checksums, and Send Large Offload (LSO) settings.

• If the traffic is multi-streamed, such as when receiving high-volume multicast traffic, enable RSS.

• Disable the Interrupt Moderation setting for network card drivers that require the lowest possible latency. Remember, this configuration can use more CPU time and it represents a tradeoff.

• Handle network adapter interrupts and DPCs on a core processor that shares CPU cache with the core that is being used by the program (user thread) that is handling the packet. CPU affinity tuning can be used to direct a process to certain logical processors in conjunction with RSS configuration to accomplish this. Using the same core for the interrupt, DPC, and user mode thread exhibits worse performance as load increases because the ISR, DPC, and thread contend for the use of the core.

System management interrupts

Many hardware systems use System Management Interrupts (SMI) for a variety of maintenance functions, such as reporting error correction code (ECC) memory errors, maintaining legacy USB compatibility, controlling the fan, and managing BIOS-controlled power settings.

The SMI is the highest-priority interrupt on the system, and places the CPU in a management mode. This mode preempts all other activity while SMI runs an interrupt service routine, typically contained in BIOS.

Unfortunately, this behavior can result in latency spikes of 100 microseconds or more.

If you need to achieve the lowest latency, you should request a BIOS version from your hardware provider that reduces SMIs to the lowest degree possible. These BIOS versions are frequently referred to as 'low latency BIOS' or 'SMI free BIOS.' In some cases, it is not possible for a hardware platform to eliminate SMI activity altogether because it is used to control essential functions (for example, cooling fans).

Note

The operating system cannot control SMIs because the logical processor is running in a special maintenance mode, which prevents operating system intervention.

Performance tuning TCP

You can use the following items to tune TCP performance.

TCP receive window autotuning

In Windows Vista, Windows Server 2008, and later versions of Windows, the Windows network stack uses a feature that is named TCP receive window autotuning level to negotiate the TCP receive window size. This feature can negotiate a defined receive window size for every TCP communication during the TCP Handshake.

In earlier versions of Windows, the Windows network stack used a fixed-size receive window (65,535 bytes) that limited the overall potential throughput for connections. The total achievable throughput of TCP connections could limit network usage scenarios. TCP receive window autotuning enables these scenarios to fully use the network.

For a TCP receive window that has a particular size, you can use the following equation to calculate the total throughput of a single connection.

Total achievable throughput in bytes = TCP receive window size in bytes * (1 / connection latency in seconds)

For example, for a connection that has a latency of 10 ms, the total achievable throughput is only 51 Mbps. This value is reasonable for a large corporate network infrastructure. However, by using autotuning to adjust the receive window, the connection can achieve the full line rate of a 1-Gbps connection.

Some applications define the size of the TCP receive window. If the application does not define the receive window size, the link speed determines the size as follows:

• Less than 1 megabit per second (Mbps): 8 kilobytes (KB)
• 1 Mbps to 100 Mbps: 17 KB
• 100 Mbps to 10 gigabits per second (Gbps): 64 KB
• 10 Gbps or faster: 128 KB

For example, on a computer that has a 1-Gbps network adapter installed, the window size should be 64 KB.

This feature also makes full use of other features to improve network performance. These features include the rest of the TCP options that are defined in RFC 1323. By using these features, Windows-based computers can negotiate TCP receive window sizes that are smaller but are scaled at a defined value, depending on the configuration. This behavior the sizes easier to handle for networking devices.

Note

You may experience an issue in which the network device is not compliant with the TCP window scale option, as defined in RFC 1323 and, therefore, doesn't support the scale factor. In such cases, refer to this KB 934430, Network connectivity fails when you try to use Windows Vista behind a firewall device or contact the Support team for your network device vendor.

Review and configure TCP receive window autotuning level

You can use either netsh commands or Windows PowerShell cmdlets to review or modify the TCP receive window autotuning level.

Note

Unlike in versions of Windows that pre-date Windows 10 or Windows Server 2019, you can no longer use the registry to configure the TCP receive window size. For more information about the deprecated settings, see Deprecated TCP parameters.

Note

For detailed information about the available autotuning levels, see Autotuning levels.

To use netsh to review or modify the autotuning level

To review the current settings, open a Command Prompt window and run the following command:

The output of this command should resemble the following:

To modify the setting, run the following command at the command prompt:

Note

In the preceding command, <Value> represents the new value for the auto tuning level.

For more information about this command, see Netsh commands for Interface Transmission Control Protocol.

To use Powershell to review or modify the autotuning level

To review the current settings, open a PowerShell window and run the following cmdlet.

The output of this cmdlet should resemble the following.

To modify the setting, run the following cmdlet at the PowerShell command prompt.

Note

In the preceding command, <Value> represents the new value for the auto tuning level.

For more information about these cmdlets, see the following articles:

Autotuning levels

You can set receive window autotuning to any of five levels. The default level is Normal. The following table describes the levels.

If you use an application to capture network packets, the application should report data that resembles the following for different window autotuning level settings.

• Autotuning level: Normal (default state)

• Autotuning level: Disabled

• Autotuning level: Restricted

• Autotuning level: Highly restricted

• Autotuning level: Experimental

Deprecated TCP parameters

The following registry settings from Windows Server 2003 are no longer supported, and are ignored in later versions.

• TcpWindowSize
• NumTcbTablePartitions
• MaxHashTableSize

All of these settings were located in the following registry subkey:

HKEY_LOCAL_MACHINESystemCurrentControlSetServicesTcpipParameters

Windows Filtering Platform

Windows Vista and Windows Server 2008 introduced the Windows Filtering Platform (WFP). WFP provides APIs to non-Microsoft independent software vendors (ISVs) to create packet processing filters. Examples include firewall and antivirus software.

Note

A poorly-written WFP filter can significantly decrease a server's networking performance. For more information, see Porting Packet-Processing Drivers and Apps to WFP in the Windows Dev Center.

For links to all topics in this guide, see Network Subsystem Performance Tuning.

Applies To: Windows Server 2012

This topic contains the following sections.

Determining the correct tuning settings for your network adapter depend on the following variables:

• The network adapter and its feature set

• The type of workload performed by the server

• The server hardware and software resources

• Your performance goals for the server

If your network adapter provides tuning options, you can optimize network throughput and resource usage to achieve optimum throughput based on the parameters described above.

The following sections describe some of your performance tuning options.

Enabling Offload Features

Turning on network adapter offload features is usually beneficial. Sometimes, however, the network adapter is not powerful enough to handle the offload capabilities with high throughput. For example, enabling segmentation offload can reduce the maximum sustainable throughput on some network adapters because of limited hardware resources. However, if the reduced throughput is not expected to be a limitation, you should enable offload capabilities, even for this type of network adapter.

Note

Some network adapters require offload features to be independently enabled for send and receive paths.

Enabling Receive Side Scaling (RSS) for Web Servers

RSS can improve web scalability and performance when there are fewer network adapters than logical processors on the server. When all the web traffic is going through the RSS-capable network adapters, incoming web requests from different connections can be simultaneously processed across different CPUs.

It is important to note that due to the logic in RSS and Hypertext Transfer Protocol (HTTP) for load distribution, performance might be severely degraded if a non-RSS-capable network adapter accepts web traffic on a server that has one or more RSS-capable network adapters. In this circumstance, you should use RSS-capable network adapters or disable RSS on the network adapter properties Advanced Properties tab. To determine whether a network adapter is RSS-capable, you can view the RSS information on the network adapter properties Advanced Properties tab.

RSS Profiles and RSS Queues

RSS predefined profiles are new in Windows Server 2012.

The default profile is NUMA Static, which changes the default behavior from previous versions of the operating system. To get started with RSS Profiles, you can review the available profiles to understand when they are beneficial and how they apply to your network environment and hardware.

For example, if you open Task Manager and review the logical processors on your server, and they seem to be underutilized for receive traffic, 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 might have options to change the number of RSS queues as part of the driver.

Increasing Network Adapter Resources

For network adapters that allow manual configuration of resources, such as receive and send buffers, you should increase the allocated resources. Some network adapters set their receive buffers low to conserve allocated memory from the host. The low value results in dropped packets and decreased performance. Therefore, for receive-intensive scenarios, we recommend that you increase the receive buffer value to the maximum.

Note

If a network adapter does not expose manual resource configuration, it either dynamically configures the resources, or the resources are set to a fixed value that cannot be changed.

Enabling Interrupt Moderation

To control interrupt moderation, some network adapters expose different interrupt moderation levels, buffer coalescing parameters (sometimes separately for send and receive buffers), or both.

You should consider interrupt moderation for CPU-bound workloads, and consider the trade-off between the host CPU savings and latency versus the increased host CPU savings because of more interrupts and less latency. If the network adapter does not perform interrupt moderation, but it does expose buffer coalescing, increasing the number of coalesced buffers allows more buffers per send or receive, which improves performance.

Performance Tuning for Low Latency Packet Processing

Many network adapters provide options to optimize operating system-induced latency. Latency is the elapsed time between the network driver processing an incoming packet and the network driver sending the packet back. This time is usually measured in microseconds. For comparison, the transmission time for packet transmissions over long distances is usually measured in milliseconds (an order of magnitude larger). This tuning will not reduce the time a packet spends in transit.

Following are some performance tuning suggestions for microsecond-sensitive networks.

• Set the computer BIOS to High Performance, with C-states disabled. However, note that this is system and BIOS dependent, and some systems will provide higher performance if the operating system controls power management. You can check and adjust your power management settings from Control Panel or by using the powercfg command. For more information, see Powercfg Command-Line Options

• Set the operating system power management profile to High Performance System. Note that this will not work properly if the system BIOS has been set to disable operating system control of power management.

• Enable Static Offloads, for example, UDP Checksums, TCP Checksums, and Send Large Offload (LSO).

• Enable RSS if the traffic is multi-streamed, such as high-volume multicast receive.

• Disable the Interrupt Moderation setting for network card drivers that require the lowest possible latency. Remember, this can use more CPU time and it represents a tradeoff.

• Handle network adapter interrupts and DPCs on a core processor that shares CPU cache with the core that is being used by the program (user thread) that is handling the packet. CPU affinity tuning can be used to direct a process to certain logical processors in conjunction with RSS configuration to accomplish this. Using the same core for the interrupt, DPC, and user mode thread exhibits worse performance as load increases because the ISR, DPC, and thread contend for the use of the core.

System Management Interrupts

Many hardware systems use System Management Interrupts (SMI) for a variety of maintenance functions, including reporting of error correction code (ECC) memory errors, legacy USB compatibility, fan control, and BIOS controlled power management. The SMI is the highest priority interrupt on the system and places the CPU in a management mode, which preempts all other activity while it runs an interrupt service routine, typically contained in BIOS.

Unfortunately, this can result in latency spikes of 100 microseconds or more. If you need to achieve the lowest latency, you should request a BIOS version from your hardware provider that reduces SMIs to the lowest degree possible. These are frequently referred to as “low latency BIOS” or “SMI free BIOS.” In some cases, it is not possible for a hardware platform to eliminate SMI activity altogether because it is used to control essential functions (for example, cooling fans).

Note

The operating system can exert no control over SMIs because the logical processor is running in a special maintenance mode, which prevents operating system intervention.

Performance Tuning TCP

You can performance tune TCP using the following items.

Tcp Receive Window Auto-tuning Level 5

Details are provided in the following sections.

TCP Receive Window Auto-Tuning

Prior to Windows Server 2008, the network stack used a fixed-size receive-side window that limited the overall potential throughput for connections. One of the most significant changes to the TCP stack is TCP receive window auto-tuning. You can calculate the total throughput of a single connection when you use this fixed size default as:

Total achievable throughput in bytes = TCP window * (1 / connection latency)

For example, the total achievable throughput is only 51 Mbps on a 1 GB connection with 10 ms latency – which is a reasonable value for a large corporate network infrastructure.

With auto-tuning, however, the receive-side window is adjustable, and it can grow to meet the demands of the sender. It is entirely possible for a connection to achieve the full line rate of a 1 GB connection. Network usage scenarios that might have been limited in the past by the total achievable throughput of TCP connections can now fully use the network.

Windows Filtering Platform

The Windows Filtering Platform (WFP) that was introduced in Windows Vista and Windows Server 2008 provides APIs to non-Microsoft independent software vendors (ISVs) to create packet processing filters. Examples include firewall and antivirus software.

Note

A poorly written WFP filter can significantly decrease a server’s networking performance.

For more information, see Windows Filtering Platform in the Windows Dev Center.

TCP Parameters

Tcp Receive Window Auto-tuning Level 4

The following registry keywords from Windows Server 2003 are no longer supported, and they are ignored in Windows Server 2012, Windows Server 2008 R2, and Windows Server 2008.

Tcp Receive Window Auto-tuning Level Guide

• TcpWindowSize

• NumTcbTablePartitions

• MaxHashTableSize