1 00:00:00,660 --> 00:00:01,680 In this lesson, 2 00:00:01,680 --> 00:00:03,690 you're going to explore some key concepts 3 00:00:03,690 --> 00:00:06,180 and components that make up the Linux kernel 4 00:00:06,180 --> 00:00:07,470 that you need to be able to troubleshoot 5 00:00:07,470 --> 00:00:10,530 and provide solution to, as you work through the system. 6 00:00:10,530 --> 00:00:13,170 Now there's some aspects of Linux that are consistent 7 00:00:13,170 --> 00:00:15,840 but those are typically handled by the kernel. 8 00:00:15,840 --> 00:00:18,990 Now, the kernel is the core of the Linux operating system 9 00:00:18,990 --> 00:00:21,090 and it handles various crucial functions, 10 00:00:21,090 --> 00:00:24,300 such as, the system initialization, process scheduling 11 00:00:24,300 --> 00:00:26,610 and memory and hardware management. 12 00:00:26,610 --> 00:00:28,860 The core of the operating system is called the kernel, 13 00:00:28,860 --> 00:00:29,940 like I just said, 14 00:00:29,940 --> 00:00:33,090 and all the other components are going to rely upon it. 15 00:00:33,090 --> 00:00:36,510 The kernel manages, file system access, it's memory, 16 00:00:36,510 --> 00:00:38,670 its processes, its devices 17 00:00:38,670 --> 00:00:41,670 and the other resource allocation on the system. 18 00:00:41,670 --> 00:00:44,220 The kernel also controls all the hardware devices 19 00:00:44,220 --> 00:00:46,530 that are plugged into a given system. 20 00:00:46,530 --> 00:00:49,470 It is one of the first elements to be loaded up on startup 21 00:00:49,470 --> 00:00:51,510 and it remains the main thing in memory 22 00:00:51,510 --> 00:00:53,580 during your computer's operation. 23 00:00:53,580 --> 00:00:56,430 The kernel also contains system level commands 24 00:00:56,430 --> 00:00:59,790 and other functions that are normally hidden from users. 25 00:00:59,790 --> 00:01:01,800 Now kernel's tend to divide software, 26 00:01:01,800 --> 00:01:04,170 that's running in memory into two spaces. 27 00:01:04,170 --> 00:01:07,590 Namely, the kernel space and the user space. 28 00:01:07,590 --> 00:01:09,750 The kernel space is simply where the kernel 29 00:01:09,750 --> 00:01:12,390 executes the services that it provides. 30 00:01:12,390 --> 00:01:15,240 The user space, on the other hand, is the area of memory 31 00:01:15,240 --> 00:01:18,390 that includes everything outside of the kernel space. 32 00:01:18,390 --> 00:01:21,090 This can include everything from high level applications 33 00:01:21,090 --> 00:01:23,970 that the users interacting with directly, to processes 34 00:01:23,970 --> 00:01:25,410 that are running in the background, 35 00:01:25,410 --> 00:01:27,960 to various low level system libraries. 36 00:01:27,960 --> 00:01:30,960 The split between these two memory regions is useful 37 00:01:30,960 --> 00:01:32,550 because it provides greater stability 38 00:01:32,550 --> 00:01:34,440 and security for our system. 39 00:01:34,440 --> 00:01:37,560 Software in one space cannot necessarily interfere 40 00:01:37,560 --> 00:01:39,990 with software in the other space. 41 00:01:39,990 --> 00:01:42,630 Now kernels can be classified as monolithic 42 00:01:42,630 --> 00:01:44,160 or microkernels. 43 00:01:44,160 --> 00:01:46,950 In a monolithic kernel all system modules, 44 00:01:46,950 --> 00:01:50,040 such as device drivers or file systems run inside 45 00:01:50,040 --> 00:01:51,450 the kernel space. 46 00:01:51,450 --> 00:01:54,600 As a result, a monolithic kernel can interact quickly 47 00:01:54,600 --> 00:01:56,400 with all the different devices. 48 00:01:56,400 --> 00:01:59,610 However, its main disadvantage is its size, 49 00:01:59,610 --> 00:02:02,340 which tends to lead to higher consumption of Ram. 50 00:02:02,340 --> 00:02:05,490 In addition, a failure and a device driver can actually 51 00:02:05,490 --> 00:02:07,800 lead to system instability if you're using 52 00:02:07,800 --> 00:02:09,509 a monolithic kernel. 53 00:02:09,509 --> 00:02:12,240 So, instead a lot of people like to use 54 00:02:12,240 --> 00:02:14,370 a microkernel architecture. 55 00:02:14,370 --> 00:02:16,830 Now when you're using a microkernel architecture, 56 00:02:16,830 --> 00:02:19,830 the kernel itself runs the minimum amount of resources 57 00:02:19,830 --> 00:02:21,810 that are necessary to actually implement 58 00:02:21,810 --> 00:02:24,150 a fully functional operating system. 59 00:02:24,150 --> 00:02:26,040 Compared to a monolithic kernel, 60 00:02:26,040 --> 00:02:28,560 microkernels have smaller kernel spaces 61 00:02:28,560 --> 00:02:31,290 and instead have much larger user spaces. 62 00:02:31,290 --> 00:02:34,980 This means that microkernels are smaller and overised size 63 00:02:34,980 --> 00:02:37,650 and they actually consume much less memory. 64 00:02:37,650 --> 00:02:40,200 In addition, they're actually considered much more stable 65 00:02:40,200 --> 00:02:41,310 and secure. 66 00:02:41,310 --> 00:02:44,940 However, microkernels also tend to offer worse performance 67 00:02:44,940 --> 00:02:47,130 than a monolithic kernel because there's a lot more 68 00:02:47,130 --> 00:02:50,460 switching of things into or out of memory. 69 00:02:50,460 --> 00:02:53,400 The next thing we need to talk about is device drivers. 70 00:02:53,400 --> 00:02:54,810 Essentially, when we talk about 71 00:02:54,810 --> 00:02:58,110 a Linux Kernel device driver, this is a software program 72 00:02:58,110 --> 00:03:00,840 that enables a computer's operating system to identify 73 00:03:00,840 --> 00:03:04,170 the characteristics and functions of a given hardware device 74 00:03:04,170 --> 00:03:05,610 to be able to communicate with it 75 00:03:05,610 --> 00:03:07,500 and control its operations. 76 00:03:07,500 --> 00:03:09,570 The device driver acts as an interface 77 00:03:09,570 --> 00:03:12,540 between the operating system and the hardware devices. 78 00:03:12,540 --> 00:03:14,760 This can include things like, your storage drives, 79 00:03:14,760 --> 00:03:18,570 your printers, your scanners, your monitors, your keyboards 80 00:03:18,570 --> 00:03:20,220 and your network devices. 81 00:03:20,220 --> 00:03:22,890 Device drivers can be included in the operating system 82 00:03:22,890 --> 00:03:26,130 or you can install them on demand afterwards. 83 00:03:26,130 --> 00:03:28,590 Now, when we talk about the Linux kernel, it's important 84 00:03:28,590 --> 00:03:32,280 to remember it's a free and open source monolithic kernel 85 00:03:32,280 --> 00:03:34,950 in general and it manages all the resources 86 00:03:34,950 --> 00:03:36,780 on the operating system. 87 00:03:36,780 --> 00:03:39,720 As a monolithic kernel, device drivers run within 88 00:03:39,720 --> 00:03:43,380 the kernel space and they have full access to the hardware. 89 00:03:43,380 --> 00:03:45,780 The architecture of the Linux Kernel, does provide 90 00:03:45,780 --> 00:03:47,880 for many useful features, including, 91 00:03:47,880 --> 00:03:49,350 virtual memory management, 92 00:03:49,350 --> 00:03:52,710 support for TCP/IP networking, shared libraries 93 00:03:52,710 --> 00:03:54,210 and much more. 94 00:03:54,210 --> 00:03:56,430 Another important quality of the Linux Kernel 95 00:03:56,430 --> 00:03:58,050 is its modularity. 96 00:03:58,050 --> 00:04:00,540 This enables users to configure and extend kernel 97 00:04:00,540 --> 00:04:03,690 functionality to meet their specific needs. 98 00:04:03,690 --> 00:04:05,880 Now there are a lot of changes and new features 99 00:04:05,880 --> 00:04:08,280 merged into the Linux Kernel. 100 00:04:08,280 --> 00:04:11,370 For versions 2.6.39 and prior, 101 00:04:11,370 --> 00:04:15,450 the kernel number format was w.x.y.z. 102 00:04:15,450 --> 00:04:18,877 Where "W" is the major version, such as, version 2. 103 00:04:18,877 --> 00:04:22,837 "X" was the major revision version, such as, 2.6. 104 00:04:22,837 --> 00:04:27,300 "Y' is the minor revision number, 2.6.39 105 00:04:27,300 --> 00:04:29,550 and "Z" is the patch number. 106 00:04:29,550 --> 00:04:33,420 But after version 2.6.39 they actually decided 107 00:04:33,420 --> 00:04:35,400 to shorten the version number format. 108 00:04:35,400 --> 00:04:40,170 And so the next version was version 3.0 after 3.19 109 00:04:40,170 --> 00:04:44,400 rather than going to 3.20, they jumped up to 4.0 110 00:04:44,400 --> 00:04:46,440 and this was for readability purposes, 111 00:04:46,440 --> 00:04:48,990 not due to any real major technical advances 112 00:04:48,990 --> 00:04:51,390 like we used to have with major versions. 113 00:04:51,390 --> 00:04:53,730 Newer versions of the kernel will continue this trend 114 00:04:53,730 --> 00:04:56,910 of avoiding large minor numbers with multiple decimals 115 00:04:56,910 --> 00:05:01,910 like 2.6.39.2 and instead using something like 3.20 or 4.0. 116 00:05:03,750 --> 00:05:06,540 So, how can you figure out what kernel you're running? 117 00:05:06,540 --> 00:05:09,930 Well to do that you use a command known as, uname. 118 00:05:09,930 --> 00:05:13,470 By default, uname will print the name of the kernel Linux 119 00:05:13,470 --> 00:05:15,840 but you can also view the kernel version of the number 120 00:05:15,840 --> 00:05:20,040 to the system you're using by typing in uname-r. 121 00:05:20,040 --> 00:05:24,150 If you enter uname-i, you can view the hardware platform 122 00:05:24,150 --> 00:05:26,340 and to print all the information on the system. 123 00:05:26,340 --> 00:05:29,463 Type in uname-a at the command prompt. 124 00:05:30,330 --> 00:05:33,120 Now going back to our kernel, remember that our kernel 125 00:05:33,120 --> 00:05:35,850 performs various functions to control and manage 126 00:05:35,850 --> 00:05:37,860 the operations of a system. 127 00:05:37,860 --> 00:05:40,380 It's going to be composed of several layers that operate 128 00:05:40,380 --> 00:05:42,090 in the kernel space. 129 00:05:42,090 --> 00:05:45,180 We call this the SCI kernel layer, and this is, 130 00:05:45,180 --> 00:05:47,760 the System Call Interface kernel layer. 131 00:05:47,760 --> 00:05:50,190 It's going to handle the system calls, sent from user 132 00:05:50,190 --> 00:05:52,980 applications that need to go to the kernel. 133 00:05:52,980 --> 00:05:56,340 This enables the user space applications to request services 134 00:05:56,340 --> 00:05:59,280 from the kernel space, things like processing time 135 00:05:59,280 --> 00:06:02,520 and memory allocation and enables the kernel to schedule 136 00:06:02,520 --> 00:06:03,960 and process system calls 137 00:06:03,960 --> 00:06:07,440 and manage multiple system calls simultaneously. 138 00:06:07,440 --> 00:06:09,210 The process management kernel layer, 139 00:06:09,210 --> 00:06:11,340 handles different processes by allocating 140 00:06:11,340 --> 00:06:14,160 separate execution space on the processor 141 00:06:14,160 --> 00:06:16,560 and ensuring that the running of one process 142 00:06:16,560 --> 00:06:19,440 does not interfere with the other processes. 143 00:06:19,440 --> 00:06:22,020 Our memory management kernel layer is going to manage 144 00:06:22,020 --> 00:06:24,030 the computer's memory, which is one of the most 145 00:06:24,030 --> 00:06:27,150 complex tasks that's performed by the kernel. 146 00:06:27,150 --> 00:06:28,800 Just like processor sharing, 147 00:06:28,800 --> 00:06:30,930 the system's memory also needs to be shared 148 00:06:30,930 --> 00:06:33,630 among different user space resources. 149 00:06:33,630 --> 00:06:35,610 The file system management kernel layer, 150 00:06:35,610 --> 00:06:37,470 is going to manage the file system. 151 00:06:37,470 --> 00:06:40,620 This involves storing, organizing and tracking files 152 00:06:40,620 --> 00:06:42,810 and data on a given computer. 153 00:06:42,810 --> 00:06:45,600 The kernel also supports a virtual file system 154 00:06:45,600 --> 00:06:48,750 or VFS that provides this abstraction layer 155 00:06:48,750 --> 00:06:51,120 of the underlying data that's being organized 156 00:06:51,120 --> 00:06:52,620 under complex structures. 157 00:06:52,620 --> 00:06:55,200 So it appears to be a single structure. 158 00:06:55,200 --> 00:06:58,170 The device management kernel layer is going to manage devices 159 00:06:58,170 --> 00:06:59,790 by controlling device access 160 00:06:59,790 --> 00:07:02,100 and interfacing between user applications 161 00:07:02,100 --> 00:07:04,470 and hardware devices on the computer. 162 00:07:04,470 --> 00:07:07,290 When a user space application sends a system call 163 00:07:07,290 --> 00:07:09,900 the kernel is going to read the request and then pass it on 164 00:07:09,900 --> 00:07:11,790 to the drivers that manage the activities 165 00:07:11,790 --> 00:07:13,770 of that particular device. 166 00:07:13,770 --> 00:07:14,700 As you can see, 167 00:07:14,700 --> 00:07:17,520 the kernel really is at the center of everything 168 00:07:17,520 --> 00:07:18,780 and it's where we're going to do all of 169 00:07:18,780 --> 00:07:22,413 the major device handling and processing for our systems.