├── Networking.assets ├── CN.png ├── e-slow.gif ├── Figure_13.gif ├── CSF-Images.4.13.png ├── Microsoft-Office-365.png ├── image-20200623215735002.png ├── image-20200623215753306.png ├── image-20200623220050056.png ├── image-20200623220114430.png ├── image-20200623220136817.png ├── image-20200623220713752.png ├── image-20200623220804780.png ├── image-20200623221254485.png ├── image-20200623221341633.png ├── image-20200623221818361.png ├── image-20200623225505230.png ├── image-20200623225921742.png ├── image-20200623230329887.png ├── image-20200623230359117.png ├── image-20200623230401603.png ├── image-20200623231046428.png ├── image-20200623232824349.png ├── image-20200623233004254.png ├── image-20200623233007383.png ├── image-20200623233011163.png ├── image-20200623233037433.png ├── image-20200623234129193.png ├── image-20200623234145520.png ├── image-20200623234208358.png ├── image-20200623234336910.png ├── image-20200623235249999.png ├── image-20200624000208734.png ├── image-20200624000224471.png ├── image-20200624000409444.png ├── image-20200624000505461.png ├── image-20200624000844032.png ├── image-20200624013054670.png ├── image-20200624013057568.png ├── image-20200624013215039.png ├── image-20200624014246763.png ├── image-20200624014313909.png ├── image-20200624014720330.png ├── image-20200624014723711.png ├── image-20200624014730469.png ├── image-20200624020007685.png ├── image-20200624020818665.png ├── image-20200624020820491.png ├── image-20200624020902473.png ├── image-20200624021749594.png ├── image-20200624022117233.png ├── image-20200624092825490.png ├── image-20200624095142329.png ├── image-20200624103101848.png ├── image-20200624103110424.png ├── image-20200624105028886.png ├── image-20200624110227422.png ├── image-20200624110229861.png ├── image-20200624110324352.png ├── image-20200624110326739.png ├── image-20200624110332545.png ├── image-20200624230205165.png ├── image-20200624230216414.png ├── image-20200624230221945.png ├── image-20200624230232871.png ├── image-20200624230252693.png ├── image-20200624234647683.png ├── image-20200625010213278.png ├── image-20200625012528149.png ├── image-20200625013331756.png ├── image-20200625013731030.png ├── image-20200625013824168.png ├── image-20200625013827980.png ├── image-20200625014342187.png ├── image-20200625020409241.png ├── image-20200625020930095.png ├── image-20200625022044519.png ├── image-20200625022234903.png ├── image-20200625023952760.png ├── image-20200625030243438.png ├── image-20200625031330489.png ├── image-20200625031457858.png ├── image-20200625031535963.png ├── image-20200625031619492.png ├── image-20200625031828464.png ├── image-20200625031830643.png ├── image-20200625032052124.png ├── image-20200625032105171.png ├── image-20200625032744198.png ├── image-20200625101247108.png ├── image-20200625101306541.png ├── image-20200625101542460.png ├── image-20200625111613411.png ├── image-20200625111622412.png ├── image-20200625112423391.png ├── image-20200625113042048.png ├── image-20200625113510142.png ├── image-20200625113538154.png ├── image-20200625113610404.png ├── image-20200625114141046.png ├── image-20200625114326058.png ├── image-20200625120140330.png ├── image-20200625121157561.png ├── image-20200625125306632.png ├── image-20200626092159605.png ├── image-20200626092221770.png ├── image-20200626093138846.png ├── image-20200626093228471.png ├── image-20200626094059288.png ├── image-20200626095513464.png ├── image-20200626100604463.png ├── image-20200626105312271.png ├── image-20200626105514915.png ├── image-20200626110423209.png ├── image-20200626110932352.png ├── image-20200626113259904.png ├── image-20200626145459240.png ├── image-20200625111613411-1593233219026.png └── yn9gnMaPU-8h3ZeV4wX7ZnLCksH1kZXaaEo_QtGWlxIQ-1DKpTM1_z9hbyudKjsBHOmzhzvMc_v8uxbuxSPsJ69FWpDJRSxJj3m0weqIQUtKTqhhmA ├── Thread.assets ├── image-20200628125209890.png ├── image-20200628130034111.png ├── image-20200628130255794.png ├── image-20200628132052016.png ├── image-20200628132500027.png └── image-20200628133924576.png ├── CloudComputing.assets ├── image-20200627143513778.png └── image-20200627144308729.png ├── RaceCondition.assets ├── image-20200629155744723.png ├── image-20200629160702944.png ├── image-20200629162016751.png ├── image-20200629153013564-1593412086483.png └── image-20200629154527507-1593412088182.png ├── CloudComputing.md ├── RaceCondition.md ├── Thread.md └── LICENSE /Networking.assets/CN.png: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/Haswf/ComputerSystemNote/HEAD/Networking.assets/CN.png -------------------------------------------------------------------------------- /Networking.assets/e-slow.gif: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/Haswf/ComputerSystemNote/HEAD/Networking.assets/e-slow.gif 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https://raw.githubusercontent.com/Haswf/ComputerSystemNote/HEAD/Networking.assets/yn9gnMaPU-8h3ZeV4wX7ZnLCksH1kZXaaEo_QtGWlxIQ-1DKpTM1_z9hbyudKjsBHOmzhzvMc_v8uxbuxSPsJ69FWpDJRSxJj3m0weqIQUtKTqhhmA -------------------------------------------------------------------------------- /CloudComputing.md: -------------------------------------------------------------------------------- 1 | # The Future of Computer Systems 2 | 3 | ## Trends 4 | 5 | ![image-20200627144308729](CloudComputing.assets/image-20200627144308729.png) 6 | 7 | ### End-user 8 | 9 | - Desktop -> Laptop -> Tablet 10 | - Commoditised hardware 11 | - Online services 12 | - Desktop as a Service (DaaS) 13 | 14 | ### Impact on Development 15 | 16 | - Web first approach 17 | - Collaborative tools 18 | - Multiple platforms 19 | 20 | ## Service Model 21 | 22 | ![image-20200627143513778](CloudComputing.assets/image-20200627143513778.png) 23 | 24 | ## Software as a Service (SaaS) 25 | 26 | - **Service**: Enterprise applications delivered through the browser 27 | - Everything from Email, Customer Relationship Management, Collaboration 28 | - Streamlines enterprise IT management 29 | - Vendor is responsible for maintaining the OS, runtimes, patches, backup, hardware 30 | 31 | - **Examples** include: Google Apps, Salesforce, Citrix GoToMeeting, Office 365, GitHub, GitLab 32 | 33 | ![Microsoft Office 365 | Melbourne Geelong Werribee](Note.assets/Microsoft-Office-365.png) 34 | 35 | ## Infrastructure as a Service (IaaS) 36 | 37 | - **Service**: virtualized access to hardware instances 38 | - **Example**: Microsoft Azure/AWS EC2/Google cloud is one of the biggest examples 39 | - Various hardware specifications available 40 | - Both shared and dedicated instances 41 | - Dedicated instances provide a greater degree of security and predictability of performance 42 | 43 | - **Advantage** 44 | - Reduce hardware purchase and support costs 45 | - Allows rapid rescaling of resources 46 | - scale up efficiently on peak demand at a low cost 47 | 48 | - **Responsibility** 49 | - Have to install patches 50 | - keep OS to date 51 | - manage software and applications 52 | 53 | - **Security Issues** 54 | - Replication makes scaling easy but also can lead to replication of vulnerabilities 55 | - Old images not being updated 56 | - Poorly configured server instance replicated 100’s of times 57 | - Incorrect security settings leading to massive data loss 58 | - **Misconfiguration** 59 | - Amazon Simple Cloud Storage Service (S3) 60 | - Nothing inherently insecure about it, just far too easy to setup incorrectly 61 | - In 2013 Rapid7 survey 12,000 S3 buckets and found that 1 in 6 where left open to the public 62 | - AWS EC2 63 | - By default AWS instances will have some degree of insecurity 64 | - Globally accessible 65 | - Password based access 66 | - Publicly accessible metadata available at /latest/meta-data/ 67 | - **Compromised Credential** 68 | - AWS Credential compromise is a growing problem 69 | - Never hard code your AWS credentials or store them in a git repository 70 | - Incurs financial cost 71 | 72 | ## Platform as a Service (PaaS) 73 | 74 | - Provides a framework for rapid development and deployment of applications 75 | - Potentially across multiple platforms 76 | - Select frameworks to add to application, then develop the application 77 | - No managing the installation or updating of the underlying frameworks 78 | - Generally provides **auto-scaling** 79 | - Often uses Git or Kubernetes to deploy applications 80 | - **Example** 81 | - Salesforce Heroku 82 | - AWS Elastic Beanstalk 83 | - Microsoft Azure, 84 | - RedHat OpenShift 85 | - Google App Engine 86 | 87 | ## Serverless - Function as a Service 88 | 89 | - Developer takes no interest in infrastructure or software stack, just the **functionality of the application** 90 | - Development is simplified, no low level handling of requests, functions are small pieces of code expected to run in milliseconds 91 | - **Content delivery**: Static content served via Content Delivery Network 92 | - **Example**: Alexa 93 | - Allows easy access to advanced APIs and functionality 94 | - Including Amazon Alexa 95 | - Using some configuration and single JavaScript function it is possible to implement an Alexa Skill which users can then interact with 96 | - All delivered without configuring a server or installing a software library 97 | 98 | - **Advantage** 99 | - Allows companies to focus on what differentiates them, which is their product/application, and not on infrastructure, which is largely commoditised anyway 100 | 101 | ## What does this mean for a System Administrator? 102 | 103 | - The role isn’t going to disappear anytime soon 104 | - But the skillset is changing 105 | - Configuration vs. Installation 106 | - Serverless may well dominate in the application development space, but it is not suited to long running or high performance tasks 107 | - Could see a shift away from PaaS to Serverless, but IaaS will always be around 108 | - Awareness of best practice, network configuration, security, and privacy will be paramount 109 | 110 | -------------------------------------------------------------------------------- /RaceCondition.md: -------------------------------------------------------------------------------- 1 | [TOC] 2 | 3 | ## Race condition 4 | 5 | - Situations like this, where two or more processes are reading or writing some **shared data** and **the final result depends** on who runs precisely when, are called **race conditions**. 6 | - Debugging programs containing race conditions is no fun at all. 7 | 8 | - Example 9 | - ![image-20200629153013564](RaceCondition.assets/image-20200629153013564-1593412086483.png) 10 | - **Setting**: 11 | - When a process wants to print a file, it enters the file name in a special spooler directory. Another process 12 | - the printer daemon, periodically checks to see if there are any files to be printed, and if there are, it prints them and then removes their names from the directory. 13 | - Process A reads in and stores the value, 7, in a local variable called next free slot. 14 | - Just then a clock interrupt occurs and the CPU decides that process A has run long enough, so it switches to process B. 15 | - Process B also reads in and also gets a 7. 16 | - It, too, stores it in its local variable next free slot. At this instant both processes think that the next available slot is 7. 17 | - Process B now continues to run. It stores the name of its file in slot 7 and updates in to be an 8. 18 | - Eventually, process A runs again, starting from the place it left off. It looks at next free slot, finds a 7 there, and writes its file name in slot 7, erasing the name that process B just put there. 19 | - Process B will never receive any output. 20 | 21 | ### Critical Regions 22 | 23 | - **Mutual exclusion**: if one process is using a shared variable or file, the other processes will be excluded from doing the same thing. 24 | - **Critical region**: 25 | - The part of the program where the shared memory is accessed is called the **critical region** or **critical section**. 26 | - If we could arrange matters such that no two processes were ever in their critical regions at the same time, we could avoid races. 27 | - **Assumptions** 28 | - No two processes may be simultaneously inside their critical regions. 29 | - No assumptions may be made about speeds or the number of CPUs. 30 | - No process running outside its critical region may block any process. 31 | - No process should have to wait forever to enter its critical region. 32 | 33 | ![image-20200629154527507](RaceCondition.assets/image-20200629154527507-1593412088182.png) 34 | 35 | ### Disabling Interrupts 36 | 37 | - On a single-processor system, the simplest solution is to have each process **disable** 38 | **all interrupts** just after entering its critical region and re-enable them just before 39 | leaving it. 40 | - Once a process has disabled interrupts, it can examine and update the shared 41 | memory without fear that any other process will intervene. 42 | 43 | - **Problem** 44 | - Unwise to give user processes the power to turn off interrupts. 45 | - What if one of processes turn off interrupts, and never turned them on again? 46 | - If the system is a multiprocessor (with two or more CPUs) disabling interrupts affects only the CPU that executed the disable instruction. 47 | - The other ones will continue running and can access the shared memory. 48 | - **Conclusion** 49 | - Disabling interrupts often a useful technique within the operating system itself but is not appropriate as a general mutual exclusion mechanism for user processes. 50 | - In a multicore system disabling the interrupts of one CPU does not prevent other CPUs from interfering with operations the first CPU is performing. 51 | 52 | ### Lock Variables 53 | 54 | - When a process wants to enter its critical region, it first tests the lock. 55 | 56 | - If the lock is 0, the process sets it to 1 and enters the critical region. 57 | - If the lock is already 1, the process just waits until it becomes 0. 58 | - Thus, a 0 means that no process is in its critical region, and a 1 means that some 59 | process is in its critical region. 60 | - When a process wants to enter its critical region, 61 | it first tests the lock. If the lock is 0, the process sets it to 1 and enters the 62 | critical region. If the lock is already 1, the process just waits until it becomes 0. 63 | Thus, a 0 means that no process is in its critical region, and a 1 means that some 64 | process is in its critical region. 65 | - **Problem** 66 | - Suppose that one process reads the lock and sees that it is 0. 67 | - Before it can set the lock to 1, another process is scheduled, runs, and sets the lock to 1 68 | - When the first process runs again, it will also set the lock to 1, and two processes will be in their critical regions at the same time. 69 | - The race now occurs **if the second process modifies the lock just after the first process has finished its second check.** 70 | 71 | ### Strict Alternation 72 | 73 | ![image-20200629155744723](RaceCondition.assets/image-20200629155744723.png) 74 | 75 | - In Fig. 2-23, the integer variable `turn`, initially 0, keeps track of whose turn it is to enter the critical region and examine or update the shared memory. 76 | 77 | - **Problem** 78 | 79 | - The faster process will loop until the slower process sets turn to 0. 80 | - taking turns is not a good idea when one of the processes is much slower than the other 81 | - This situation violates condition 3 set out above: 82 | - process 0 is being blocked by a process not in its critical region. 83 | - requires that the two processes strictly alternate in entering their critical regions 84 | 85 | 86 | 87 | ### Test and Set Lock (TSL) 88 | 89 | - It reads the contents of the memory word lock into register RX and then stores a nonzero value at the memory address lock. 90 | - The operations of reading the word and storing into it are guaranteed to be indivisible 91 | - The CPU executing the TSL instruction locks the memory bus to prohibit other CPUs from accessing memory until it is done. 92 | - We use a shared variable, `lock`, to coordinate access to shared memory. 93 | - When lock is 0, any process may set it to 1 using the TSL instruction and then read or write the shared memory. 94 | - When it is done, the process sets lock back to 0 using an ordinary move instruction. 95 | 96 | - Before entering its critical region, a process calls enter region, which does busy waiting until the lock is free; 97 | - then it acquires the lock and returns. 98 | - After leaving the critical region the process calls leave region, which stores a 0 in lock. 99 | 100 | ![image-20200629160702944](RaceCondition.assets/image-20200629160702944.png) 101 | 102 | ### Busy Waiting 103 | 104 | - When a process wants to enter a critical section 105 | - it checks if the entry is allowed 106 | - If not, the process executes a loop, checking if it is allowed to enter a critical section 107 | 108 | - Continuously testing a variable until some value appears is called **busy waiting**. 109 | 110 | - It should usually be avoided, since it wastes CPU time. 111 | - A lock that uses busy waiting is called a **spin lock**. 112 | 113 | #### Priority Inversion Problem 114 | 115 | - Consider a computer with two processes, 116 | - H, with high priority, and 117 | - L, with low priority. 118 | - The scheduling rules are such that H is run whenever it is in ready state. 119 | - At a certain moment, 120 | - with L in its critical region, H becomes ready to run, H now begins busy waiting, 121 | - but since L is never scheduled while H is running, L never gets the chance to leave its critical region, so H loops forever. 122 | - This situation is sometimes referred to as the **priority inversion problem**. 123 | 124 | ### Blocking 125 | 126 | - `Sleep` is a system call that causes the caller to block 127 | - that is, be suspended until another process wakes it up. 128 | - The `wakeu`p call has one parameter, the process to be awakened. 129 | 130 | - Approach 131 | - Attempt to enter a critical section 132 | - If critical section available, enter it 133 | - If not, register interest in the critical section and block 134 | - When the critical section becomes available, the OS will unblock a process waiting for the critical section, if one exists 135 | 136 | - Using blocking constructs improves the CPU utilization 137 | 138 | ## Deadlock 139 | 140 | **Resources** 141 | 142 | - A major class of deadlocks involves resources to which some process has been granted exclusive access. 143 | - These resources include devices, data records, files, and so forth. 144 | - To make the discussion of deadlocks as general as possible, we will refer 145 | to the objects granted as **resources.** 146 | 147 | - **Deadlock** 148 | - A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause 149 | - In other words, each member of the set of deadlocked processes is waiting for a resource that is owned by a deadlocked process. 150 | - This kind of deadlock is called a resource deadlock. 151 | - ![image-20200629162016751](RaceCondition.assets/image-20200629162016751.png) 152 | - **Deadlock Modelling** 153 | - A directed arc from a process to a resource means that the process is currently 154 | blocked waiting for that resource. 155 | - A directed arc from a resource node (square) to a process node (circle) means that the resource has previously been requested by, granted to, and is currently held by that process. 156 | - A cycle in the graph means that there is a deadlock involving the processes and resources in 157 | the cycle (assuming that there is one resource of each kind). 158 | - **resource graphs** are a tool that lets us see if a given request/release sequence leads to deadlock. 159 | 160 | - **Strategies** 161 | - Just ignore the problem. Maybe if you ignore it, it will ignore you. 162 | 2. Detection and recovery. Let them occur, detect them, and take action. 163 | 3. Dynamic avoidance by careful resource allocation. 164 | 4. Prevention, by structurally negating one of the four conditions. -------------------------------------------------------------------------------- /Thread.md: -------------------------------------------------------------------------------- 1 | [TOC] 2 | 3 | # Thread 4 | 5 | ## What is a thread? 6 | 7 | - **Process** 8 | - it is a way to group related resource together 9 | - address space containing program text and data 10 | - other resources(open files, child processes, pending alarms, signal handlers) 11 | - **Thread** 12 | 13 | - **Definition**: a sequential execution stream within the process. 14 | 15 | - **Threads** are the entities scheduled for execution on the CPU. 16 | - The CPU switches rapidly back and forth among the threads, providing the 17 | illusion that the threads are running in parallel, albeit on a slower CPU than the 18 | real one. 19 | - **Threads** are sometimes called lightweight processes. 20 | - Because threads have some of the properties of processes, 21 | - **Multithreading** 22 | - describe the situation of allowing multiple threads in the same process. 23 | 24 | ## What's shared between threads? 25 | 26 | image-20200628132500027 27 | 28 | - All threads have exactly the same **address space**, which means that they also share the same **global** **variables**. 29 | - In addition to sharing an address space, all the threads can share the same **set of open files**, **child processes**, **alarms**, and **signals**, an so on, as shown in Fig. 2-12 30 | - The ability for multiple threads of execution to share a set of resources so that they can work **together** closely to perform some task. 31 | 32 | - Each thread has its own **state** 33 | - `running`: A running thread currently has the CPU and is active. 34 | - `blocked`: a blocked thread is waiting for some event to unblock it. 35 | - `ready`: a ready thread is scheduled to run and will as soon as its turn comes up. 36 | - `terminated` 37 | - Each thread has its own **stack** 38 | - Each thread will generally call different procedures and thus have a different execution history. This is why each thread needs its own stack. 39 | - image-20200628132052016 40 | 41 | | Per-process Items | Per-thread items | 42 | | -------------------------- | ------------------------------------------- | 43 | | Address space | Program counter | 44 | | Global variables | Registers | 45 | | Open files | Stack(local variables, function call stack) | 46 | | Child processes | State | 47 | | Pending alarms | | 48 | | Signal and signal handlers | | 49 | | Accounting information | | 50 | 51 | - **Problem** 52 | - What happens if one thread closes a file while another one is still reading from it? 53 | 54 | ## Why do we need thread? 55 | 56 | 57 | - **Address Space sharing** 58 | - the ability for the parallel entities to share an address space and all of its data among themselves. 59 | - **Easy to create and destroy** 60 | - Threads are easier (i.e., faster) to create and destroy than processes. 61 | - because they are lighter weight than processes, 62 | - **Performance gain on I/O bound application** 63 | - Threads yield no performance gain when all of them are CPU bound, 64 | - but when there is substantial computing and also substantial I/O, having threads allows these activities to overlap, thus speeding up the application. 65 | - **Real parallelism** 66 | - Useful on systems with multiple CPUs, where real parallelism is possible. 67 | 68 | image-20200628125209890 69 | 70 | ## Word Processor Example 71 | 72 | - **Scenario**: word processor automatically saving the entire file to disk** 73 | - **Problem** 74 | - if the program were single-threaded, then whenever a disk backup started, 75 | commands from the keyboard and mouse would be ignored until the backup was 76 | finished. 77 | - **Solution** 78 | - The first thread just interacts with the user. 79 | - The second thread writes the contents of RAM to disk periodically. 80 | - **Having multiple process would not work** 81 | - because all three threads need to operate on the document. 82 | 83 | ## Web Server Example 84 | 85 | ### Multi-threaded 86 | 87 | image-20200628130034111 88 | 89 | - **Assumption**: a system call blocks only the calling thread, not the entire process. 90 | - Web servers use `cache` to improve performance by maintaining a collection of heavily used pages 91 | in main memory. 92 | - the **dispatcher** 93 | - reads incoming requests for work from the network 94 | - After examining the request, it chooses an idle (i.e., blocked) worker thread and hands it the request 95 | - the **worker** 96 | - wakes up 97 | - checks to see if the request can be satisfied from the Web page cache 98 | - If not, it starts a read operation to get the page from the disk and blocks until the disk operation completes. 99 | - When the thread blocks on the disk operation, another thread is chosen to run, possibly 100 | the dispatcher 101 | - This model allows the server to be written as a collection of sequential threads. 102 | - ![image-20200628130255794](Thread.assets/image-20200628130255794.png) 103 | - The dispatcher’s program consists of an infinite loop for getting a work request and 104 | handing it off to a worker. 105 | - Each worker’s code consists of an infinite loop consisting of accepting a request from the dispatcher and checking the Web cache to see if the page is present. 106 | 107 | ### **Single threaded web server** 108 | 109 | - The main loop of the Web server gets a request, examines it, and carries it out to completion before getting the next one. 110 | 111 | - the CPU is simply idle while the Web server is waiting for the disk. 112 | - result is many fewer requests/sec can be processed. 113 | 114 | ## POSIX Threads 115 | 116 | - To make it possible to write portable threaded programs, IEEE has defined a standard for threads in IEEE standard 1003.1c. The threads package it defines is called `Pthreads`. 117 | 118 | - **Attribute Structure**: All pthreads threads have certain properties. Each one has an identifier, a set of 119 | registers (including the program counter), and a set of attributes, which are stored 120 | in a structure. 121 | 122 | ### TL;DR 123 | 124 | | Thread call | Description | 125 | | ---------------------- | ---------------------------------------------------- | 126 | | `pthread create` | Create a new thread | 127 | | `pthread_exit` | Terminate the calling thread | 128 | | `pthread_join` | Wait for a specific thread to exit | 129 | | `pthread_yield` | Release the CPU to let another thread run | 130 | | `pthread_attr_init` | Create and initialize a thread’s attribute structure | 131 | | `pthread_attr_destroy` | Remove a thread’s attribute structure | 132 | 133 | ### `pthread create` 134 | 135 | - A new thread is created using the` pthread_create` call. 136 | - The thread identifier of the newly created thread is returned as the function value. 137 | 138 | ### `pthread_exit` 139 | 140 | - When a thread has finished the work it has been assigned, it can terminate by 141 | calling `pthread_exit`. 142 | - This call stops the thread and releases its stack. 143 | 144 | ### `pthread_join` 145 | 146 | - The thread that is waiting calls `pthread_join` to wait for a specific other thread to terminate. 147 | - The thread identifier of the thread to wait for is given as a parameter. 148 | 149 | ## `pthread_yield` 150 | 151 | - Sometimes it happens that a thread is not logically blocked, but feels that it has 152 | run long enough and wants to give another thread a chance to run. 153 | - It can accomplish this goal by calling `pthread_yield`. 154 | 155 | ### `pthread_attr_init` 156 | 157 | - `pthread_attr_init` creates the attribute structure associated with a thread and initializes it to the default values. 158 | - These values (such as the priority) can be changed by manipulating fields in the attribute structure. 159 | 160 | ### `pthread_attr_destroy` 161 | 162 | - `pthread_attr_destroy` removes a thread’s attribute structure, freeing up its memory. 163 | - It does not affect threads using it; they continue to exist. 164 | 165 | ### Example 166 | 167 | - When a thread is created, it prints a one-line message announcing itself, then it 168 | exits. 169 | - The order in which the various messages are interleaved is nondeterminate and may vary on consecutive runs of the program. 170 | 171 | ```c 172 | #include 173 | #include 174 | #include 175 | 176 | void* print_hello(void* _) { 177 | printf("Hello\n"); 178 | } 179 | 180 | void* spawn(void* _) { 181 | pthread_t tid1, tid2, tid3; 182 | pthread_create(&tid1, NULL, print_hello, NULL); 183 | pthread_create(&tid2, NULL, print_hello, NULL); 184 | pthread_create(&tid3, NULL, print_hello, NULL); 185 | 186 | // comment/uncomment 187 | pthread_join(tid1, NULL); 188 | pthread_join(tid2, NULL); 189 | pthread_join(tid3, NULL); 190 | } 191 | 192 | int main() { 193 | pthread_t tid1, tid2; 194 | pthread_create(&tid1, NULL, spawn, NULL); 195 | pthread_create(&tid2, NULL, spawn, NULL); 196 | 197 | // comment/uncomment 198 | pthread_join(tid1, NULL); 199 | pthread_join(tid2, NULL); 200 | // usleep(700); 201 | return 0; 202 | } 203 | ``` 204 | 205 | ## Implementing Threads in User Space 206 | 207 | ![image-20200628133924576](Thread.assets/image-20200628133924576.png) 208 | 209 | - The first method is to put the threads package entirely in user space. 210 | - The kernel knows nothing about them. 211 | - As far as the kernel is concerned, it is managing ordinary, single-threaded processes. 212 | 213 | - **OS-independent** 214 | - A user-level threads package can be implemented on an operating system that does not support threads. 215 | - With this approach, threads are implemented by a library 216 | - **Thread table**: 217 | - When threads are managed in user space, each process needs its **own** private thread table to keep track of the threads in that process. 218 | - keeps track only of the per-thread properties, such as each thread’s program counter, stack pointer, registers, state, and so forth. 219 | - managed by the runtime system 220 | - When a thread is moved to ready state or blocked state, the information needed to restart it is stored in the thread table 221 | 222 | - **Advantage** 223 | 224 | - **Thread switching** : Doing thread switching is at least an order of magnitude faster than trapping to the kernel 225 | - If the machine happens to have an instruction to store all the registers and another 226 | one to load them all, the entire thread switch can be done in just a handful of instructions. 227 | 228 | - **Thread scheduling** 229 | - The procedure that saves the thread’s state and the scheduler are just local procedures, 230 | so invoking them is much more efficient than making a kernel call. 231 | - Among other issues, no trap is needed, no context switch is needed, the memory cache need not be flushed, and so on. 232 | 233 | - **Customized scheduling Algorithm** 234 | - User space threads allow each process to have its own customized scheduling algorithm. 235 | - **Scalability**: User space threads also scale better 236 | - since kernel threads invariably require some table space and stack space in the kernel, which 237 | can be a problem if there are a very large number of threads. 238 | 239 | - **Disadvantage** 240 | - **Blocking system call stop all threads** 241 | - If one thread makes a system call, all the threads/the entire process will be stopped. 242 | - Changing to non-blocking system call? 243 | - Requiring changes to the operating system 244 | - require changes to many user proglem 245 | - User-level threads could run with existing operating systems. 246 | - When this call is present, the library procedure read can be replaced with a new one that first does a select call and then does the read call only if it is safe (i.e., will not block). 247 | - If the read call will block, the call is not made **Instead, another thread is run** 248 | - This approach requires rewriting parts of the system call library, and is inefficient 249 | and inelegant, but there is little choice. 250 | - **Page fault stop all threads** 251 | - If a thread causes a page fault, the kernel blocks the entire process until the disk I/O is complete 252 | - even though other threads might be runnable 253 | - **If a thread starts running, no other thread in that process will ever run unless the first thread voluntarily gives up the CPU.** 254 | - Unless a thread enters the run-time system of its own free will, the scheduler will never get a 255 | chance. 256 | - **Possible solution** 257 | - have the run-time system request a clock signal (interrupt) once a second to give it control, 258 | but this, too, is crude and messy to program. 259 | 260 | ## Implementing Threads in the Kernel 261 | 262 | - No run-time system is needed in each process 263 | 264 | - No thread table in each process 265 | - the kernel has a **thread table** that keeps track of all the threads in the system. 266 | - The kernel’s thread table holds each thread’s registers, state, and other information. 267 | - All calls that might block a thread are implemented as system calls 268 | - at considerably greater cost than a call to a run-time system procedure. 269 | - **Thread recycling** 270 | - When a thread is destroyed, it is marked as not runnable, but its kernel data structures are not otherwise affected. 271 | - When a new thread must be created, an old thread is reactivated, saving some overhead. 272 | - Also possible for user-level threads, but since the thread-management overhead is much 273 | smaller, there is less incentive to do this. 274 | - **Advantage** 275 | - Kernel threads do not require any new, nonblocking system calls. 276 | - If one thread in a process causes a page fault, the kernel can easily check to see if 277 | the process has any other runnable threads 278 | - if so, run one of them while waiting for the required page to be brought in from the disk 279 | - **Disadvantage** 280 | - The cost of a system call is substantial 281 | - if thread operations (creation, termination, etc.) a common, much more overhead will be incurred. 282 | 283 | - **Problem** 284 | - What happens when a multithreaded process forks? 285 | - Does the new process have as many threads as the old one did, or does it have just one? 286 | - When a signal comes in, which thread should handle it? 287 | - What happens if two or more threads register for the same signal? 288 | 289 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | GNU GENERAL PUBLIC LICENSE 2 | Version 3, 29 June 2007 3 | 4 | Copyright (C) 2007 Free Software Foundation, Inc. 5 | Everyone is permitted to copy and distribute verbatim copies 6 | of this license document, but changing it is not allowed. 7 | 8 | Preamble 9 | 10 | The GNU General Public License is a free, copyleft license for 11 | software and other kinds of works. 12 | 13 | The licenses for most software and other practical works are designed 14 | to take away your freedom to share and change the works. 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If the Program does not specify a version number of the 576 | GNU General Public License, you may choose any version ever published 577 | by the Free Software Foundation. 578 | 579 | If the Program specifies that a proxy can decide which future 580 | versions of the GNU General Public License can be used, that proxy's 581 | public statement of acceptance of a version permanently authorizes you 582 | to choose that version for the Program. 583 | 584 | Later license versions may give you additional or different 585 | permissions. However, no additional obligations are imposed on any 586 | author or copyright holder as a result of your choosing to follow a 587 | later version. 588 | 589 | 15. Disclaimer of Warranty. 590 | 591 | THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY 592 | APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT 593 | HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY 594 | OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, 595 | THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 596 | PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM 597 | IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF 598 | ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 599 | 600 | 16. Limitation of Liability. 601 | 602 | IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING 603 | WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS 604 | THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY 605 | GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE 606 | USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF 607 | DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD 608 | PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), 609 | EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF 610 | SUCH DAMAGES. 611 | 612 | 17. Interpretation of Sections 15 and 16. 613 | 614 | If the disclaimer of warranty and limitation of liability provided 615 | above cannot be given local legal effect according to their terms, 616 | reviewing courts shall apply local law that most closely approximates 617 | an absolute waiver of all civil liability in connection with the 618 | Program, unless a warranty or assumption of liability accompanies a 619 | copy of the Program in return for a fee. 620 | 621 | END OF TERMS AND CONDITIONS 622 | 623 | How to Apply These Terms to Your New Programs 624 | 625 | If you develop a new program, and you want it to be of the greatest 626 | possible use to the public, the best way to achieve this is to make it 627 | free software which everyone can redistribute and change under these terms. 628 | 629 | To do so, attach the following notices to the program. It is safest 630 | to attach them to the start of each source file to most effectively 631 | state the exclusion of warranty; and each file should have at least 632 | the "copyright" line and a pointer to where the full notice is found. 633 | 634 | 635 | Copyright (C) 636 | 637 | This program is free software: you can redistribute it and/or modify 638 | it under the terms of the GNU General Public License as published by 639 | the Free Software Foundation, either version 3 of the License, or 640 | (at your option) any later version. 641 | 642 | This program is distributed in the hope that it will be useful, 643 | but WITHOUT ANY WARRANTY; without even the implied warranty of 644 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 645 | GNU General Public License for more details. 646 | 647 | You should have received a copy of the GNU General Public License 648 | along with this program. If not, see . 649 | 650 | Also add information on how to contact you by electronic and paper mail. 651 | 652 | If the program does terminal interaction, make it output a short 653 | notice like this when it starts in an interactive mode: 654 | 655 | Copyright (C) 656 | This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 657 | This is free software, and you are welcome to redistribute it 658 | under certain conditions; type `show c' for details. 659 | 660 | The hypothetical commands `show w' and `show c' should show the appropriate 661 | parts of the General Public License. Of course, your program's commands 662 | might be different; for a GUI interface, you would use an "about box". 663 | 664 | You should also get your employer (if you work as a programmer) or school, 665 | if any, to sign a "copyright disclaimer" for the program, if necessary. 666 | For more information on this, and how to apply and follow the GNU GPL, see 667 | . 668 | 669 | The GNU General Public License does not permit incorporating your program 670 | into proprietary programs. If your program is a subroutine library, you 671 | may consider it more useful to permit linking proprietary applications with 672 | the library. If this is what you want to do, use the GNU Lesser General 673 | Public License instead of this License. But first, please read 674 | . 675 | --------------------------------------------------------------------------------