Hardware interrupt service routines and deferred procedure calls can be the silent killers of system performance. ISRs and DPCs are the highest priority code that runs in the system - they cannot be pre-empted by the OS and run to completion. ISRs and DPCs that run too long - or too often - can eat up significant amounts of CPU time and cause overall system performance to suffer. They can cause audio and video glitches, mouse freezes, UI hangs, and any number of other system wide problems.
From a user's standpoint, problems with long or frequent ISRs and DPCs often first show up as audio pops and drops (usually just called 'glitches'). This has long been a problem for PC audio: problematic ISRs or DPCs can cause enough delays in applications or in the audio engine so that audio data simply fails to get to the hardware in time causing audible glitches. Even Vista's new audio sub-system isn't immune to interference from ISR's and DPCs. The human ear is very sensitive to even the slightest problem in the audio stream. Chances are, if you are hearing regular audio glitches, long ISRs or DPCs are the problem.
We've even seen a few extreme cases of really awful DPC behavior; one bug in a new WDDM graphics driver has been particularly persistent. We first noticed it a few months ago when users reported periodic mouse freezes on some laptops. Taking a trace on the problem system quickly helped us find the problem: the graphics driver was periodically waking up to check for an external monitor change. This specific hardware doesn't provide an interrupt when a monitor is attached or removed, so the driver needs to periodically poll the adapter for the monitor state. It did this every few seconds. This would normally be fine, but the driver was spending several hundred milliseconds in a DPC every time it did this. Users saw this as a mouse freeze - in reality, it was a system freeze as the CPU was spending all its time doing nothing but polling for a monitor for a few hundred milliseconds. We worked with the vendor to get this fixed, only to have it show up a couple of more times as the vendor worked on the architecture of their driver.
Network interface cards (NICs) are a frequent culprit of ISR and DPC problems. Their type and age tend to vary widely – even between the same models of computers. Some NICs are well behaved, only interrupting the OS when absolutely necessary, such as when receive buffers are getting full. This is called interrupt aggregation. This lets the OS handle received packets with as few interrupts as possible. Other NICs are very badly behaved, interrupting the CPU for every packet received. Luckily, newer NICs support interrupt aggregation. The 'bad NICs' we see tend to be older, circa 2003, 2004.
I call ISRs and DPCs problems the 'silent killers' for three reasons
The OS has no control over their execution. ISR's are triggered by physical hardware signals from devices to the CPU. When a device signals the CPU that it needs attention, the CPU immediately jumps to the driver's interrupt service routine. DPCs are scheduled by interrupt handlers and run at a priority only exceed by hardware interrupt service routines.
ISR and DPC activity usually increases with system activity. As a system becomes more active, interrupts and DPCs will generally become more frequent, taking up more CPU time. This can get to the point where they visibly (or audibly) affect system performance. In these cases, no single ISR or DPC routine is the problem - it is their cumulative usage of CPU time.
It is rarely obvious that ISRs and DPCs are causing performance problems. There is really only one place in the OS where information about them is visible surfaced, in the performance monitor. This UI is really useful, but a little hard to get to.
ISR's and DPC's can cause other secondary performance problems as well such as dirtying the processor caches and increasing interrupt and DPC latency.
Tracking down Interrupt and DPC problems is further complicated because their behavior is highly dependent on hardware configuration and drivers. A problem seen on one system may only be reproducible on that particular system.
The performance monitor UI is a bit buried, but once found provides a good run-time summary of ISR and DPC execution. My favorite way to get to the performance monitor is by entering 'perfmon' into the system run command dialog. Just press the Win-R key and type 'perfmon'. You can then use the 'performance monitor' tab to monitor ISR and DPC activity using these counters: % Interrupt Time, % DPC Time, Interrupts/Sec, DPC Rate, and DPCs Queued/Sec (see). This is quite handy as it is 'in-box' - available on every Windows XP and Vista system. This UI can help spot patently bad ISR and DPC activity relatively easily.
However, the performance counters only tell part of the story. They don't identify the modules that are generating the ISRs and DPCs, and they can’t be used to look at ISR and DPC execution times. The best way to get a clear picture of the CPU time spent in ISRs and DPCs is to use the kernel's ETW events which can be enabled and analyzed using ETW based performance tools.
Event Tracing for Windows (ETW) is a fundamental operating system feature that provides a very efficient method for logging events, mechanisms for controlling event providers (components that generate events, also often called 'loggers' ) and collecting those events for post processing. ETW is the primary tool we use to measure system performance. Probably the most important logger is the kernel logger itself which can generate events for DPCs, ISRs, context switches, disk I/O, hard and soft faults, process start/stop, thread start/stop, file I/O, disk I/O, TCP/IP and UDP traffic, registry access, and image (executable) loads. All of these can be controlled by the tracelog tool provided in the Windows Vista WDK.
Using the kernel logger to look at ISR and DPC execution statistics is very easy - here's how:
The isrdpc.xml report file will contain a nicely formatted report that lists the % utilization, counts, and histograms of the ISR and DPC execution time by module. This data is really helpful in finding problematic ISR and DPC activity.
Driver developers can take things a step further and add ETW based event logging directly to their driver. This is straight forward to do. Even better, ETW is very light weight and the logging features can be shipped in production (non-debug) drivers. This makes it much easier to debug performance problems as driver tracing can be enabled on any system. Since developers can define their own events, the driver can log additional information such as internal driver or hardware state.
It may even be appropriate for a driver developer to publish the drivers trace GUID, event flags and event structures so others can independently diagnose problems.
I’ve included some useful related links and other information below.
Other Details
Useful Links
Scheduling, Thread Context, and IRQL: This document describes all the gory detail behind how hardware and software interrupts are scheduled and handled by the operating system.
Event Tracing from the WDK docs: Here is a good place to start on understanding the fundamental ETW infrastructure in windows.
Tools for Software Tracing
CPU Performance Counters: This page describes the various CPU performance counters in windows.
TraceLog Documentation
Measuring ISR and DPC Time: A related article.
Device-Driver Performance Considerations for Multimedia Platforms
Event Tracing APIs in the Platform SDK
Windows Vista Display Driver Model: A detailed discussion of the virtues of WDDM.
WDDM In the WDK: The WDDM SDK - how to write a WDDM driver.
The virtues of WDDM Drivers
Windows Driver Kit (WDK) Introduction
WinHEC Presentation on generating your own events
RATTV3:This is a tool that we provided during WinHEC 2005 that allows 24 x 7 logging of ISR and DPC activity. Note, this currently only works on XP, we need to get this updated for Vista.
If your computer is suffering from high CPU usageHow to Fix High CPU Usage in WindowsHow to Fix High CPU Usage in WindowsDoes your PC suffer from high CPU usage up to 100%? Here's how to fix high CPU usage in Windows 10.Read More and the culprit process is called “system interrupts”, then you are dealing with a hardware or driver issue.
In this post, we explain what system interrupts are and how you can find and fix the underlying cause of their high CPU usage.
What Is “System Interrupts”?
System interrupts appears as a Windows process in your Task Manager, but it’s not really a process. Rather, it’s a kind of representative that reports the CPU usage of all interrupts that happen on a lower system level.
Interrupts can originate from software or hardware, including the processor itself. Wikipedia explains:
An interrupt alerts the processor to a high-priority condition requiring the interruption of the current code the processor is executing. The processor responds by suspending its current activities, saving its state, and executing a function called an interrupt handler to deal with the event.
When the interrupt handler task is completed, the processor resumes the state at which it was interrupted.
Interrupts are a form of communication between software and hardware with the CPU. For example, when you type on your keyboard, the respective hardware and software sends interrupts to the CPU to inform it about the task at hand and to trigger the necessary processing.
Try moving your mouse and watch what happens to the CPU usage of system interrupts to understand what that means.
Interrupts can signal to the CPU that an error occurred and this can cause the CPU usage of system interrupts to increase. On a healthy system, system interrupts will hover between 0.1% and 2% of CPU usage, depending on the CPU frequency, running software, and attached hardware.
Even peaks of 3% to 7% can be considered within the normal range, depending on your system setup.
How to Fix the High CPU Usage
If system interrupts constantly hogs more than 5% to 10% of your CPU, something is wrong and you’re most likely dealing with a hardware issue. We’ll help you get to the bottom of this.
The first fix you should always try is to reboot your computerWhy Does Rebooting Your Computer Fix So Many Issues?Why Does Rebooting Your Computer Fix So Many Issues?'Have you tried rebooting?' It's technical advice that gets thrown around a lot, but there's a reason: it works. Not just for PCs, but a wide range of devices. We explain why.Read More.
System Interrupts Deferred Procedure Calls And Interrupt Service Routines Windows 101. Check Hardware Drivers
To quickly check whether you’re dealing with a driver issue, you can run the DPC Latency Checker. Deferred Procedure Call (DPC) is a process related to system interrupts. When the interrupt handler needs to defer a lower priority task until later, it calls on the DPC.
DPC Latency Checker was designed to analyze whether your system can properly handle real-time audio or video streaming by checking the latency of kernel-mode device drivers. It’s is a quick way to reveal issues and the tool requires no installation.
Download the last of us pc highly compressed download. If you see red bars, i.e. drop-outs due to high latency, something is off.
You can either try to find the culprit or — if the problem first occurred recently — roll back recent driver updates (Windows 10)Take Back Control Over Driver Updates in Windows 10Take Back Control Over Driver Updates in Windows 10A bad Windows driver can ruin your day. In Windows 10, Windows Update automatically updates hardware drivers. If you suffer the consequences, let us show you how to roll back your driver and block future..Read More or update your driversHow to Find & Replace Outdated Windows DriversHow to Find & Replace Outdated Windows DriversYour drivers might be outdated and need updating, but how are you to know? Here's what you need to know and how to go about it.Read More with standard versions. Drivers that caused issues in the pastHow to Find & Fix AMD or ATI Display Drivers in WindowsHow to Find & Fix AMD or ATI Display Drivers in WindowsAfter being acquired by AMD, the ATI brand name hasn't been around for years, but old graphics cards are still around. If you're still using one, here are some maintenance tips.Read More were AMD SATA, HD audio device, and missing Bluetooth drivers.
Alternatively, you can install and run LatencyMon, a latency monitor, to find the driver files with the highest DPC count. Press the Start / Play button, then switch to the Drivers tab, and sort the driver files by DPC count.
Drivers with a high DPC count potentially cause a high number of interruptions.
2. Disable Internal Devices
Rather than randomly updating drivers, or if you have found potential offenders, you can disable individual device drivers to identify the culprit.
Go to the Start Menu, search for and open the Device Manager (also found in the Control Panel), expand the peripherals listed below, right-click a device and select Disable.
Do this for one device at a time, check the CPU usage of system interrupts or re-run DPC Latency Checker, then right-click the device and select Enable before moving on to the next device.
These devices are the most likely culprits:
If none of these are to blame, you can proceed with disabling (and re-enabling) other non-essential drivers.
Never disable any drivers necessary to run your system, including anything listed under Computer, Processors, and System device.
Also don’t try to disable the display adapters, the disk drive that runs your system, IDE controllers, your keyboard or mouse (unless you have an alternative input device, such as a touch pad), or your monitor.
3. Unplug or Disable External Devices
DPC Latency Checker didn’t find anything? Maybe the problem is caused by USB hardware. You can either unplug it or — while you’re in the Device Manager (see above) — disable USB Root Hubs, i.e. blocking external hardware from interrupting the CPU.
In the Device Manager, find the entry Universal Serial Bus controllers and disable any USB Root Hub entry you can find.
If you’re using an external keyboard or a USB (Bluetooth) mouse, they might stop functioning. Be sure to have an alternative method of re-enabling the device!
4. Exclude Failing Hardware
If a corrupt driver can cause system interrupts, so can failing hardware. In that case, updating your drivers won’t solve the issue. But if disabling the entire device fixed it, you should follow our guide to test your PC for failing hardwareHow to Test Your PC for Failing Hardware: Tips and Tools to KnowHow to Test Your PC for Failing Hardware: Tips and Tools to KnowWe show you computer hardware and PC diagnostic tests to help you scan your system and spot failing hardware before it's too late.Read More.
Note: System interrupts could also be caused by a faulty power supplyEvery Computer Dies In The End: Learn What Parts Can Fail, & What To Do About ItEvery Computer Dies In The End: Learn What Parts Can Fail, & What To Do About ItMost computers develop problems over time. Learning what they are and how you can deal with them is important if you don't want to be paying through the teeth for professional repairs. Don't worry though..Read More or laptop charger. Try to replace or unplug that, too.
5. Disable Sound Effects
If you’re on Windows 7, this may be the solution you’re looking for.
Right-click the speaker icon in your system tray, select Playback devices, double-click your Default Device (speaker) to open Properties, head to the Enhancements tab, and Disable all sound effects. Confirm with OK and check how system interrupts is doing now.
6. Update Your BIOS
The BIOSDiscover Your BIOS & Learn How to Make the Most of ItDiscover Your BIOS & Learn How to Make the Most of ItWhat the heck is the BIOS, anyway? Is it really that important to know? We think so and fortunately it's pretty easy. Let us introduce you.Read More is the first piece of software that is executed when you turn on your computer. It helps your operating system to boot. First, identify your BIOS version and check the manufacturer’s website for updates and installation instructions.
To find out your BIOS version, press Windows key + R, type cmd, hit Enter, and execute the following two commands, one after the other:
1. systeminfo | findstr /I /c:bios
2. wmic bios get manufacturer, smbiosbiosversion
Note that the I in /I is a capital i, not a lower case L.
Note: Updating the BIOS shouldn’t be taken lightly. Make sure to back up your systemHow to Create an ISO Image of Your Windows SystemHow to Create an ISO Image of Your Windows SystemNeed to backup and restore Windows without backup tools? It's time to learn how to make an ISO image of your Windows PC.Read More first.
System Interrupts Can Be Tricky
System interrupts can have many different causes. Did you reboot your computer as instructed above? We hope you were able to fix the issue.
What brought the relief in your case and how did you track down the issue? Please share your solution with fellow sufferers in the comments.
Explore more about: Computer Processor, CPU, Drivers, Task Management, Troubleshooting.
In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, software interrupt instructions, or software exceptions, and are used for implementing device drivers or transitions between protected modes of operation, such as system calls.
The traditional form of interrupt handler is the hardware interrupt handler. Hardware interrupts arise from electrical conditions or low-level protocols implemented in digital logic, are usually dispatched via a hard-coded table of interrupt vectors, asynchronously to the normal execution stream (as interrupt masking levels permit), often using a separate stack, and automatically entering into a different execution context (privilege level) for the duration of the interrupt handler's execution. In general, hardware interrupts and their handlers are used to handle high-priority conditions that require the interruption of the current code the processor is executing.[1][2]
Later it was found convenient for software to be able to trigger the same mechanism by means of a software interrupt (a form of synchronous interrupt). Rather than using a hard-coded interrupt dispatch table at the hardware level, software interrupts are often implemented at the operating system level as a form of callback function.
Interrupt handlers have a multitude of functions, which vary based on what triggered the interrupt and the speed at which the interrupt handler completes its task. For example, pressing a key on a computer keyboard,[1] or moving the mouse, triggers interrupts that call interrupt handlers which read the key, or the mouse's position, and copy the associated information into the computer's memory.[2]
An interrupt handler is a low-level counterpart of event handlers. However, interrupt handlers have an unusual execution context, many harsh constraints in time and space, and their intrinsically asynchronous nature makes them notoriously difficult to debug by standard practice (reproducible test cases generally don't exist), thus demanding a specialized skillset—an important subset of system programming—of software engineers who engage at the hardware interrupt layer.
Interrupt flags[edit]
Unlike other event handlers, interrupt handlers are expected to set interrupt flags to appropriate values as part of their core functionality.
Even in a CPU which supports nested interrupts, a handler is often reached with all interrupts globally masked by a CPU hardware operation. In this architecture, an interrupt handler would normally save the smallest amount of context necessary, and then reset the global interrupt disable flag at the first opportunity, to permit higher priority interrupts to interrupt the current handler. It is also important for the interrupt handler to quell the current interrupt source by some method (often toggling a flag bit of some kind in a peripheral register) so that the current interrupt isn't immediately repeated on handler exit, resulting in an infinite loop.
Exiting an interrupt handler with the interrupt system in exactly the right state under every eventuality can sometimes be an arduous and exacting task, and its mishandling is the source of many serious bugs, of the kind that halt the system completely. These bugs are sometimes intermittent, with the mishandled edge case not occurring for weeks or months of continuous operation. Formal validation of interrupt handlers is tremendously difficult, while testing typically identifies only the most frequent failure modes, thus subtle, intermittent bugs in interrupt handlers often ship to end customers.
Execution context[edit]
In a modern operating system, upon entry the execution context of a hardware interrupt handler is subtle.
For reasons of performance, the handler will typically be initiated in the memory and execution context of the running process, to which it has no special connection (the interrupt is essentially usurping the running context—process time accounting will often accrue time spent handling interrupts to the interrupted process). However, unlike the interrupted process, the interrupt is usually elevated by a hard-coded CPU mechanism to a privilege level high enough to access hardware resources directly.
Stack space considerations[edit]
In a low-level microcontroller, the chip might lack protection modes and have no memory management unit (MMU). In these chips, the execution context of an interrupt handler will be essentially the same as the interrupted program, which typically runs on a small stack of fixed size (memory resources have traditionally been extremely scant at the low end). Nested interrupts are often provided, which exacerbates stack usage. A primary constraint on the interrupt handler in this programming endeavour is to not exceed the available stack in the worst-case condition, requiring the programmer to reason globally about the stack space requirement of every implemented interrupt handler and application task.
When allocated stack space is exceeded (a condition known as a stack overflow), this is not normally detected in hardware by chips of this class. If the stack is exceeded into another writable memory area, the handler will typically work as expected, but the application will fail later (sometimes much later) due to the handler's side effect of memory corruption. If the stack is exceeded into a non-writable (or protected) memory area, the failure will usually occur inside the handler itself (generally the easier case to later debug).
In the writable case, one can implement a sentinel stack guard—a fixed value right beyond the end of the legal stack whose value can be overwritten, but never will be if the system operates correctly. It is common to regularly observe corruption of the stack guard with some kind of watch dog mechanism. This will catch the majority of stack overflow conditions at a point in time close to the offending operation.
In a multitasking system, each thread of execution will typically have its own stack. If no special system stack is provided for interrupts, interrupts will consume stack space from whatever thread of execution is interrupted. These designs usually contain an MMU, and the user stacks are usually configured such that stack overflow is trapped by the MMU, either as a system error (for debugging) or to remap memory to extend the space available. Memory resources at this level of microcontroller are typically far less constrained, so that stacks can be allocated with a generous safety margin.
In systems supporting high thread counts, it is better if the hardware interrupt mechanism switches the stack to a special system stack, so that none of the thread stacks need account for worst-case nested interrupt usage. Tiny CPUs as far back as the 8-bit Motorola 6809 from 1978 have provided separate system and user stack pointers.
Constraints in time and concurrency[edit]
For many reasons, it is highly desired that the interrupt handler execute as briefly as possible, and it is highly discouraged (or forbidden) for a hardware interrupt to invoke potentially blocking system calls. In a system with multiple execution cores, considerations of reentrancy are also paramount. If the system provides for hardware DMA, concurrency issues can arise even with only a single CPU core. (It is not uncommon for a mid-tier microcontroller to lack protection levels and an MMU, but still provide a DMA engine with many channels; in this scenario, many interrupts are typically triggered by the DMA engine itself, and the associated interrupt handler is expected to tread carefully.)
A modern practice has evolved to divide hardware interrupt handlers into front-half and back-half elements. The front-half (or first level) receives the initial interrupt in the context of the running process, does the minimal work to restore the hardware to a less urgent condition (such as emptying a full receive buffer) and then marks the back-half (or second level) for execution in the near future at the appropriate scheduling priority; once invoked, the back-half operates in its own process context with fewer restrictions and completes the handler's logical operation (such as conveying the newly received data to an operating system data queue).
Divided handlers in modern operating systems[edit]
In several operating systems—Linux, Unix, macOS, Microsoft Windows, z/OS, DESQview and some other operating systems used in the past—interrupt handlers are divided into two parts: the First-Level Interrupt Handler (FLIH) and the Second-Level Interrupt Handlers (SLIH). FLIHs are also known as hard interrupt handlers or fast interrupt handlers, and SLIHs are also known as slow/soft interrupt handlers, or Deferred Procedure Calls in Windows.
A FLIH implements at minimum platform-specific interrupt handling similar to interrupt routines. In response to an interrupt, there is a context switch, and the code for the interrupt is loaded and executed. The job of a FLIH is to quickly service the interrupt, or to record platform-specific critical information which is only available at the time of the interrupt, and schedule the execution of a SLIH for further long-lived interrupt handling.[2]
FLIHs cause jitter in process execution. FLIHs also mask interrupts. Reducing the jitter is most important for real-time operating systems, since they must maintain a guarantee that execution of specific code will complete within an agreed amount of time.To reduce jitter and to reduce the potential for losing data from masked interrupts, programmers attempt to minimize the execution time of a FLIH, moving as much as possible to the SLIH. With the speed of modern computers, FLIHs may implement all device and platform-dependent handling, and use a SLIH for further platform-independent long-lived handling.
FLIHs which service hardware typically mask their associated interrupt (or keep it masked as the case may be) until they complete their execution. An (unusual) FLIH which unmasks its associated interrupt before it completes is called a reentrant interrupt handler. Reentrant interrupt handlers might cause a stack overflow from multiple preemptions by the same interrupt vector, and so they are usually avoided. In a priority interrupt system, the FLIH also (briefly) masks other interrupts of equal or lesser priority.
A SLIH completes long interrupt processing tasks similarly to a process. SLIHs either have a dedicated kernel thread for each handler, or are executed by a pool of kernel worker threads. These threads sit on a run queue in the operating system until processor time is available for them to perform processing for the interrupt. SLIHs may have a long-lived execution time, and thus are typically scheduled similarly to threads and processes.
In Linux, FLIHs are called upper half, and SLIHs are called lower half or bottom half. This is different from naming used in other Unix-like systems, where both are a part of bottom half.[1][2]
See also[edit]
References[edit]
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