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OS02: Interrupts and I/O

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Computer Structures and Operating Systems 2021
Dr. Jens Lechtenbörger (License Information)

1 Introduction

1.1 OS Plan

OS course plan, summer 2021

1.2 Today’s Core Questions

  • Recall keyboard handling in Hack.
  • How can we improve I/O processing over polling?
    • Why keep CPU busy in loop when nothing happens?
    • With multitasking such time is wasted as other tasks could make better use of the CPU.
  • Add interrupts as I/O notification mechanism.
    • How to organize I/O then?
    • How much overhead arises?
      • (Overhead: Additional indirect computation time; makes system less efficient)
      • How to deal with “lots” of I/O events?

1.3 Learning Objectives

  • Explain techniques for I/O communication
    • Including sequencing of events under synchronous and asynchronous techniques (polling vs interrupts)
  • Discuss dis/advantages of I/O communication techniques
  • Explain (interrupt) livelock and mitigation via hybrid technique

1.4 A Note on Literature

  • As an exception, this presentation is not based on [Hai19].
    • In [Hai19], Section 2.5 contains some introductory paragraphs on interrupts, while Section 7.3.1 starts with explanations on privilege levels and system calls.
  • Chapter 1 of [Sta14] as well of [TB15] contain introductions on interrupts and I/O, while Section 5.1 of [TB15] has additional explanations.

1.5 Retrieval Practice

  • Before you continue, answer the following; ideally, without outside help.
    • How does the von Neumann architecture look like?
    • How does I/O processing work with Hack?
    • How to talk to an OS kernel?
    • How are programs, processes, and threads related?
    • What is multitasking? What are its advantages?

1.5.1 Von Neumann and Hack Architecture

1.5.2 Computations

Table of Contents

2 Hack vs Modern Computers

2.1 Recall: Von Neumann Architecture

von Neumann Architecture

von Neumann Architecture

Figure by Kapooht under CC BY-SA 3.0; converted from Wikimedia Commons

2.2 Hack vs Modern Computer

         Modern Computer          
      (e.g., PC, smartphone)      
 • RAM for data, ROM for instructions 
 • Physical RAM addresses             
 • No secondary memory                
 • RAM for data and instructions  
 • Physical and virtual addresses 
 • Memory hierarchy (disks)       
 • Single core                        
 • Single mode of execution           
 • Neither cache nor MMU              
 • Multi-core                     
 • Multiple protection domains    
 • Caches and MMU                 
 • No interrupts                      
 • Polling for I/O (recall keyboard)  
 • Interrupts, DMA                
 • Different options for I/O      
 • Language library                   
 • Single thread, no multitasking     
 • No virtual memory                  
 • Real OSs with system calls     
 • Multitasking, scheduling       
 • Virtual memory                 

2.3 CPUs in the Real World

  • Known from Hack
    • Registers (addresses, data, control information)
    • Instruction execution cycle (e.g., fetch, decode, execute)
  • Additionally

    • Caching
      • Cache = Small, fast memory between CPU and RAM
        • (Usually, multiple levels of caches; L1, L2, …)
      • Instructions and data must be in cache before CPU can access them
    • Special instructions and modes
      • Access to memory and devices in kernel mode: Subsequent slide
      • Enter idle state when nothing to do (save power)
    • Interrupts: Later slides

2.4 Privilege Levels/Rings/Modes

  • Hierarchical protection domains of CPUs
    • At least kernel mode vs. user mode (see Sec. 7.3.1 of [Hai19])
      • E.g., 4 rings since Intel 80286
        • Typically, ring 0 is kernel mode (most privileged), ring 3 is user mode (least privileged)
      • Governed by bit pattern in special register
  • Instruction set restricted depending on mode/privilege level
    • Special registers protected
    • I/O, memory management protected
  • OS starts in kernel mode
    • Applications run in user mode
    • Interrupts (system calls, traps, …) lead into kernel mode

2.5 Big Picture

  • I/O devices are components that interact with the OS
    • Some receive requests and deliver results
      • E.g., disk, printer, network card
    • Some generate events on their own
      • E.g., timer/clock, keyboard, network card
  • Alternative types of I/O
    1. OS polls (continuously asks) for events/results
    2. Device triggers interrupt when event occurs or result is ready
      • (Historically, special CPU input pins/bits were used to signal interrupts; now, also existing busses are used, e.g., MSI)
      • CPU interrupts current computation and jumps into OS, which handles interrupt

2.5.1 Types of I/O

  • With polling, I/O is called synchronous
    • OS monitors I/O operation for completion
  • With interrupts, I/O is called asynchronous
    • OS initiates I/O
    • I/O proceeds asynchronously, CPU free to perform other tasks
    • Device triggers interrupt when I/O operation completed
      • CPU interrupted, I/O result handled by OS

3 Polling

3.1 I/O with Polling

I/O happens synchronously while OS polls for result

I/O with Polling image/svg+xml I/O with Polling Jens Lechtenbörger Summer 2017, 2018 Application OS I/O Device time Application invokes system call for I/O OS starts I/O operation on behalf of application I/O operation proceeds, monitored by OS I/O operation finishes OS transfers result to application

3.2 Polling Observations

  • Advantages
    • Simple
    • Fast
      • Result processed as soon as it is available
        • No overhead (compared to interrupts as presented subsequently)
  • Disadvantage
    • Busy waiting = waiting on CPU for event to occur
      • CPU time wasted if I/O is slow or infrequent
      • Bad idea if wait period is “long”

3.2.1 Polling Example: Hack Keyboard

  • Program waits for user to press key
    • Loop, repeatedly reading keyboard memory location until key pressed
    • Recall Fill.asm
  • CPU executes instructions all the time
  • Even if Hack had OS with multitasking, nothing else could be executed
    • CPU time in loop is wasted

4 Interrupts

4.1 Fundamental Idea

  • Interrupt: Signal to CPU
    • Generated externally to CPU (signal on bus or separate pin)
      • CPU stops doing whatever it did
      • CPU jumps (resets program counter) to interrupt handler instead (details on following slides)
  • If I/O devices generate interrupts, CPU does not need to wait for I/O completion
    • OS initiates I/O operation at device
      • CPU is free to do something else asynchronously during I/O execution
    • At later point, I/O operation completes and device triggers an interrupt
      • OS interrupt handler acts accordingly

4.2 I/O with Interrupts – Overview

  • Asynchronous processing of I/O
    • External notifications via interrupts

I/O with Interrupts image/svg+xml I/O with Interrupts Jens Lechtenbörger Summer 2017, 2018 time Application OS I/O Device Interrupt Handler Application invokes system call for I/OOS starts I/O operation on behalf of application I/O operation proceeds, asynchronously to CPU Whether same or differentapplication continues, depends on type of I/O I/O operation finishedDevice triggers interrupt,application interruptedInterrupt handled by OSsubsequently

4.2.1 Hypothetical Interrupt Example: Hack Keyboard (1/3)

  • Suppose Hack had multitasking OS with threads and interrupts (which it does not)
    • Multiple programs could run concurrently in separate threads
  • Again, wait for user to press key
    • This time, thread invokes (blocking) system call, asking OS for next pressed key
    • OS remembers to inform that thread about keyboard input, blocks it
      • OS takes that thread aside
      • OS dispatches different thread to execute on CPU

4.2.2 Hypothetical Interrupt Example: Hack Keyboard (2/3)

  • Eventually, key gets pressed
    • Keyboard triggers interrupt
      • CPU interrupts whatever it does, jumps to interrupt handler, which interacts with hardware to obtain value representing key
      • OS records that value as return value of system call
        • OS unblocks blocked thread
    • Scheduling decision by OS determines what (unblocked) thread to continue next
      • At some point in time, maybe right now, OS chooses thread that invoked the keyboard system call
        • Thread continues with return value from system call

4.2.3 Hypothetical Interrupt Example: Hack Keyboard (3/3)

  • Notice: Latency (delay) between system call and processing of its return value
    • Latency between system call and key press
      • Cannot be avoided
      • In contrast to polling, with interrupts something useful can happen on the CPU during this period
    • Between key press (interrupt) and return of key’s value into thread
      • Processing of interrupt with overhead
        • Discussed subsequently
      • Resulting delay does not exist for polling

4.3 Dijkstra on Interrupts

4.4 Interrupts, Traps, Faults, Exceptions

  • Interruption of ordinary CPU execution
  • Hardware-specific, terminology not unified
  • Classification
    • Asynchronous, triggered externally
    • Synchronous, triggered internally
      • Software interrupt

        • After execution of instruction (e.g., overflow)

4.5 Interrupts and CPUs

  • OS specifies a handler for each type of interrupt and exception
    • Handler = function
    • Type of interrupt determined by number
  • E.g., x86 processors
    • Addresses of handlers stored by OS in in-memory table, the Interrupt Descriptor Table (IDT) (synonym: Interrupt Vector (Table))
      • Each table entry points to one handler/function
    • CPU (each core) contains an Interrupt Descriptor Table Register (IDTR)
      • OS initializes IDTR with start address of IDT

4.6 Interrupt Handling

  • Upon interrupt of type n:
    • A context switch takes place, and (in kernel mode) the CPU
      • saves state of current execution,
      • uses IDTR to access IDT,
      • looks up entry n in IDT, and invokes corresponding handler/function.
      • Afterwards, state is restored, previous execution continued.
  • Context switch comes with overhead

4.7 Aside: Interrupts for System Calls

  • Recall: System calls = API provided by OS kernel
    • Implementation is OS and hardware specific
  • Hardware specific methods to enter kernel mode
    • int 0x80 (generic), SYSCALL (AMD), SYSENTER (Intel)
  • E.g., Linux, Intel IA-32, int 0x80

4.8 JiTT Quiz

This task is available for self-study in Learnweb.

  • Consider a networked machine that receives incoming messages. Each of those messages requires about 4 µs CPU time for processing. If interrupts are used, each interrupt introduces a delay of about 6 µs (caused by different types of overhead).
    • How many messages can be processed per second with polling, how many with interrupts?
    • How much time is wasted (waiting for messages to arrive) with polling in the worst case? How much spent on overhead processing with interrupts?

5 Interrupts and I/O Communication

5.1 Recall: I/O with Interrupts

  • Asynchronous processing of I/O
    • External notifications via interrupts

I/O with Interrupts

5.2 Blocking vs Non-Blocking I/O

  • Previous slide left open which application continues after I/O system call
    • OS provides blocking and non-blocking system calls
  • Blocking system call
    • Application has to wait (is blocked) until I/O completed
    • However, a different application may continue
      • Scheduling, context switch, overhead
  • Non-blocking system call
    • OS initiates I/O and returns incomplete result to application
    • Application continues (and is informed of or needs to check for I/O completion at later point in time)
    • (Notice: This is impossible with polling)

6 I/O Processing

6.1 Latency Example (1/2)

  • Goal: Explain interrupt overhead as serious challenge if interrupts are frequent
  • See [LSHK09]
    • Two PCs with Intel Xeon processors (2.13 GHz)
    • 1 Gbps Ethernet networking cards connected via PCIe
    • 1 – 2 frames may arrive per 1 µs (1 µs = one millionth second)
      • For the curious
        • Ethernet’s unit of transfer: frame with minimum size of 512 b
        • At 1 Gbps, 1000 b need 1 µs for transfer, plus propagation and queueing delays
        • Thus, 1 – 2 frames may arrive per 1 µs
    • Interrupt per frame arrival!?
      • What about 10 Gbps networking?

6.2 Latency Example (2/2)

  • Numbers from [LSHK09]
    • Processing of single frame takes total of 7.7 µs
    • Latency breakdown according to different sources
      • Hardware: ≈ 0.6 µs
      • Interrupt processing: > 3 µs
      • Processing of data: > 3 µs
  • If one or two frames arrive per 1 µs and each frame needs 7.7 µs processing time, something is seriously wrong
    • Network data will be dropped because it arrives too fast
      • The system could even crash
    • Interrupt per arrival does not work

6.3 Interrupt Livelocks

  • Livelock: Situation in which computations take place but (almost) no progress is made
    • Computation time is mostly wasted on overhead
  • Interrupt livelock
    • Interrupts arrive so fast that they cannot be processed any longer
      • Also, not enough CPU time left for other tasks
        • Interrupts served with high priority
      • Context switching, cache pollution
      • Nothing useful happens any more
    • Prevent by hybrid of polling and interrupts
      • E.g., NAPI

6.3.1 Starvation

  • Interrupt livelock is special case of starvation
  • Starvation = continued denial/lack of resource
    • Under interrupt livelock, threads do not receive resource CPU (in sufficient quantities for progress) as long as “too many” interrupts are triggered
  • Starvation revisiting in later presentations on scheduling and challenges for mutual exclusion

6.4 NAPI

  • Linux “New API” for networking (2001), see [SOK01]
  • Hybrid scheme
    • Use interrupts under low load
      • Utilize CPUs better
        • Avoid polling for devices without data
    • Switch to polling under high load
      • Avoid I/O overhead
        • Data will be available anyways

6.5 JiTT: Questions and Feedback

  • What did you find difficult or confusing about the contents of the presentation? Please be as specific as possible. For example, you could describe your current understanding (which might allow us to identify misunderstandings), ask questions that allow us to help you, or suggest improvements (maybe on GitLab). Please use the session’s shared document or MoodleOverflow. Most questions turn out to be of general interest; please do not hesitate to ask and answer where others can benefit. If you created additional original content that might help others (e.g., a new exercise, an experiment, explanations concerning relationships with different courses, …), please share.

7 Outlook

7.1 When to Poll?

Measurements for DRAM-based storage prototype (data from cite:YMH12)

Measurements for DRAM-based storage prototype (data from [YMH12])

7.2 I/O Processing – Then and Now

  • Then: Disks are slow
    • Mechanical devices
    • Delivered data is processed immediately by CPU
    • Latency before data arrives
      → Interrupts beneficial
  • Now: Nonvolatile memory/SCMs are fast, see [NSWW16]
    • Mechanics eliminated
    • Operation at network/bus speed (PCIe)
    • Data can be delivered faster than processed
      → Polling beneficial
    • Need to rethink previous techniques
      • Balancing, scheduling, scaling, tiering

7.3 Call for Research

  • [B17]: Attack of the Killer Microseconds
    • Nanosecond latency (DRAM access when data not in CPU cache) is hidden by CPU hardware
      • Out-of-order execution, branch prediction, multithreading (two threads per core)
      • (However, also ongoing research to address Killer Nanoseconds [J18])
    • Millisecond latency (disk I/O) is hidden by OS
      • Multitasking
    • What about microseconds of new generation of fast I/O devices?
      • E.g., Gbps networking, flash memory
      • Paper describes datacenter challenges experienced at Google

8 Conclusions

8.1 Summary

  • Interrupt handling is major OS task
    • System call implementation
    • I/O processing
    • Timers, to be revisited for scheduling
  • Polling vs interrupt-driven I/O
    • Efficiency trade-off
    • Interrupt livelocks


License Information

This document is part of an Open Educational Resource (OER) course on Operating Systems. Source code and source files are available on GitLab under free licenses.

Except where otherwise noted, the work “OS02: Interrupts and I/O”, © 2017-2021 Jens Lechtenbörger, is published under the Creative Commons license CC BY-SA 4.0.

No warranties are given. The license may not give you all of the permissions necessary for your intended use.

In particular, trademark rights are not licensed under this license. Thus, rights concerning third party logos (e.g., on the title slide) and other (trade-) marks (e.g., “Creative Commons” itself) remain with their respective holders.