... or another attempt to improve latencies Little bit of theory: From "Windows Internals" by Mark Russinovich, David A. Solomon, Alex Ionescu Line-Based vs. Message Signaled-Based Interrupts Shared interrupts are often the cause of high interrupt latency and can also cause stability issues. They are typically undesirable and a side effect of the limited number of physical interrupt lines on a computer. For example, in the previous example of the 7-in-1 media card reader, a much better solution is for each device to have its own interrupt and for one driver to manage the different interrupts knowing which device they came from. However, consuming four IRQ lines for a single device quickly leads to IRQ line exhaustion. Additionally, PCI devices are each connected to only one IRQ line anyway, so the media card reader cannot use more than one IRQ in the first place. Other problems with generating interrupts through an IRQ line is that incorrect management of the IRQ signal can lead to interrupt storms or other kinds of deadlocks on the machine, because the signal is driven “high” or “low” until the ISR acknowledges it. (Furthermore, the interrupt controller must typically receive an EOI signal as well.) If either of these does not happen due to a bug, the system can end up in an interrupt state forever, further interrupts could be masked away, or both. Finally, line-based interrupts provide poor scalability in multiprocessor environments. In many cases, the hardware has the final decision as to which processor will be interrupted out of the possible set that the Plug and Play manager selected for this interrupt, and there is little device drivers can do. A solution to all these problems is a new interrupt mechanism first introduced in the PCI 2.2 standard called message-signaled interrupts (MSI). Although it remains an optional component of the standard that is seldom found in client machines, an increasing number of servers and workstations implement MSI support, which is fully supported by the all recent versions of Windows. In the MSI model, a device delivers a message to its driver by writing to a specific memory address. This action causes an interrupt, and Windows then calls the ISR with the message content (value) and the address where the message was delivered. A device can also deliver multiple messages (up to 32) to the memory address, delivering different payloads based on the event. Because communication is based across a memory value, and because the content is delivered with the interrupt, the need for IRQ lines is removed (making the total system limit of MSIs equal to the number of interrupt vectors, not IRQ lines), as is the need for a driver ISR to query the device for data related to the interrupt, decreasing latency. Due to the large number of device interrupts available through this model, this effectively nullifies any benefit of sharing interrupts, decreasing latency further by directly delivering the interrupt data to the concerned ISR. Finally, MSI-X, an extension to the MSI model, which is introduced in PCI 3.0, adds support for 32-bit messages (instead of 16-bit), a maximum of 2048 different messages (instead of just 32), and more importantly, the ability to use a different address (which can be dynamically determined) for each of the MSI payloads. Using a different address allows the MSI payload to be written to a different physical address range that belongs to a different processor, or a different set of target processors, effectively enabling nonuniform memory access (NUMA)-aware interrupt delivery by sending the interrupt to the processor that initiated the related device request. This improves latency and scalability by monitoring both load and closest NUMA node during interrupt completion. Now practice. Checking for PCI devices working in MSI-mode. Go to Device Manager. Click in menu "View -> Resources by type". Expand "Interrupt request (IRQ)" node of the tree. Scroll down to "(PCI) 0x... (...) device name" device nodes. Devices with positive number for IRQ (like "(PCI) 0x00000011 (17) ...") are in Line-based interrupts-mode. Devices with negative number for IRQ (like "(PCI) 0xFFFFFFFA (-6) ...") are in Message Signaled-based Interrupts-mode. Trying to switch device to MSI-mode. You must locate device`s registry key. Invoke device properties dialog. Switch to "Details" tab. Select "Device Instance Path" in "Property" combo-box. Write down "Value" (for example "PCI\VEN_1002&DEV_4397&SUBSYS_1609103C&REV_00\3&11583659&0&B0"). This is relative registry path under the key "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\". Go to that device`s registry key ("HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\PCI\VEN_1002&DEV_4397&SUBSYS_1609103C&REV_00\3&11583659&0&B0") and locate down the subkey "Device Parameters\Interrupt Management". For devices working in MSI-mode there will be subkey "Device Parameters\Interrupt Management\MessageSignaledInterruptProperties" and in that subkey there will be DWORD value "MSISupported" equals to "0x00000001". To switch device from legacy- to MSI-mode just add these subkey and value. Before adding these key and value (or changing "MSISupported" to "0x00000001" in case subkey and value already exist) you have to perform safety steps like doing backup (creating system restore point at least). Do tweak one device -> reboot and check (1) if it is displayed in Device Manager as correctly working device; (2) if its IRQ became negative -> if no (1) and no (2) then either remove subkey "MessageSignaledInterruptProperties" (if you added it) or change "MSISupported" to "0x00000000" and reboot. Theoretically if device driver (and platform = chipset) unable to perform in MSI-mode it should ignore mentioned subkey and value. But who knows... Afaik most intensive/active IRQ devices are network controller, SATA controller and USB controller. I have found that network controller is already working in MSI-mode (both at work and at home). I have successfully switched SATA controllers, video- and sound-cards into MSI-mode (both at work and at home). USB 2 controllers ignored "MSISupported" (both at ...), but USB 3 controllers at parents` PC switched to MSI-mode. And Intel MEI device at home rig became incorrect after adding "MSISupported", so I reverted it. And most important is that at work rig (which is old and weak) LatencyMon showed stable level of latencies (at heavy load) which is almost twice lower then before these tweaks, and usual stutters at heavy loads (VisualStudio + VMWare + SmartSVN + Skype + mp3-player) are gone. Edit: If you have old rig and there are no devices working in MSI-mode, then may be you should not experiment with this tweak. As this tweak is risky then you probably should not tweak devices which can prevent Windows to boot (by their failure) - SATA controllers, videocards. Especially if they do not share their IRQ with another devices. Edit: Troubleshooting topic with info from "Windows Internals": Spoiler Accepting the Boot and Last Known Good Besides starting services, the system charges the Sevice Control Manager (SCM) with determining when the system’s registry configuration, HKLM\SYSTEM\CurrentControlSet, should be saved as the last known good control set. The CurrentControlSet key contains the Services key as a subkey, so CurrentControlSet includes the registry representation of the SCM database. It also contains the Control key, which stores many kernel-mode and user-mode subsystem configuration settings. By default, a successful boot consists of a successful startup of auto-start services and a successful user logon. A boot fails if the system halts because a device driver crashes the system during the boot or if an auto-start service with an ErrorControl value of SERVICE_ERROR_SEVERE or SERVICE_ERROR_CRITICAL reports a startup error. The SCM obviously knows when it has completed a successful startup of the auto-start services, but Winlogon (%SystemRoot%\System32\Winlogon.exe) must notify it when there is a successful logon. Winlogon invokes the NotifyBootConfigStatus function when a user logs on, and NotifyBootConfigStatus sends a message to the SCM. Following the successful start of the auto-start services or the receipt of the message from NotifyBootConfigStatus (whichever comes last), the SCM calls the system function NtInitializeRegistry to save the current registry startup configuration. Windows maintains several copies of CurrentControlSet, and CurrentControlSet is really a symbolic registry link that points to one of the copies. The control sets have names in the form HKLM\SYSTEM\ControlSetnnn, where nnn is a number such as 001 or 002. The HKLM\SYSTEM\Select key contains values that identify the role of each control set. For example, if CurrentControlSet points to ControlSet001, the Current value under Select has a value of 1. The LastKnownGood value under Select contains the number of the last known good control set, which is the control set last used to boot successfully. Another value that might be on your system under the Select key is Failed, which points to the last control set for which the boot was deemed unsuccessful and aborted in favor of an attempt at booting with the last known good control set. Figure 4-15 displays a system’s control sets and Select values. NtInitializeRegistry takes the contents of the last known good control set and synchronizes it with that of the CurrentControlSet key’s tree. If this was the system’s first successful boot, the last known good won’t exist and the system will create a new control set for it. If the last known good tree exists, the system simply updates it with differences between it and CurrentControlSet. Last known good is helpful in situations in which a change to CurrentControlSet, such as the modification of a system performance-tuning value under HKLM\SYSTEM\Control or the addition of a service or device driver, causes the subsequent boot to fail. Users can press F8 early in the boot process to bring up a menu that lets them direct the boot to use the last known good control set, rolling the system’s registry configuration back to the way it was the last time the system booted successfully. So if you choose to try and switch PCI device into the message-mode, you better do add "MSISupproted" registry value to one device and reboot. If Windows failed to boot you restart and press F8 and choose boot option "last known good configuration" (which must be the configuration without "MSISupported"). Edit: Improper Propagation of SATA Message Signaled Interrupts on AMD SB950 Spoiler Description The SATA Message Signaled Interrupt (MSI) from higher port numbers may not properly propagate to port 0 before being sent to the driver. Potential Effect on System On a platform with SATA MSI enabled and activity occurring on SATA devices connected to three or more ports, if the failing condition occurs, the driver will not receive the interrupt and the system will hang. Suggested Workaround Do not expose the MSI capability in the SATA controller. This change is implemented in CIMx version 220.127.116.11. Fix Planned No Edit: Here is Intel`s paper (thanks to n1hilist): http://www.intel.co.za/content/dam/...hite-papers/msg-signaled-interrupts-paper.pdf Edit: PCI Express overview Edit: Here is utility for switching MSI-mode http://www.mediafire.com/download/hrxw4rsb9xnw1ei/MSI_util.zip Edit: MSI utility v2. http://www.mediafire.com/file/2kkkvko7e75opce/MSI_util_v2.zip Change log: - New devices grid layout with 4 columns - name, irq, msi, limit, where msi is checkbox column (for MsiSupported value), limit is textbox column (for MessageNumberLimit value) which accepts either empty string or integer value in range of 0..2048 (where 0 has the same meaning as empty string - no limit). Edit: Here is the script for switching MSI-mode http://www.mediafire.com/download/rcgtxc73fp00x26/MSI_mode_utils.zip Open it in text editor to read the help or execute in PowerShell: get-help .\MSI_mode_utils.ps1 -full Remind you that to be able to execute PowerShell scripts you should specify execution policy by: Set-ExecutionPolicy -ExecutionPolicy Unrestricted or Set-ExecutionPolicy -ExecutionPolicy RemoteSigned And I strongly recommend to use script with argument 'reg' first to make the backup reg-file before turning MSI-mode on.