The Linux kernel is an open source monolithic Unix-like operating system kernel first released by Linus Torvalds in 1991. It has been under active development ever since by a global community of programmers. The Linux kernel powers a large number of devices today, from smartphones and tablets to supercomputers. Here are some key technical aspects of the Linux kernel in more detail:
- Modular monolithic design:
The Linux kernel has a modular monolithic architecture. It is monolithic because the entire kernel codebase is compiled into a single statically linked binary executable running in kernel space. But it also allows code modules like device drivers and filesystems to be dynamically loaded and unloaded as needed at runtime. This provides the performance benefits of a monolithic kernel while retaining some flexibility. The module mechanism uses object code files known as loadable kernel modules (LKMs). These load dynamically into the running kernel via APIs like init_module() and delete_module().
- Virtual filesystem layer:
The Linux kernel provides the virtual filesystem (VFS) layer, which abstracts away interactions with concrete filesystem implementations like ext3, ext4, XFS, Btrfs, and others. VFS provides a set of common APIs that filesystems can hook into and implement. It handles operations like file creation/deletion, opening files, closing files, writing, reading data, and more. VFS maintains metadata caches, enforces access permissions, handles hard and soft links, manages in-memory and on-disk inodes, and implements advanced features like journaling, logical volume management, encryption etc. This abstracted design allows easy addition of new filesystem types modularly over time.
- Hardware support:
A major strength of Linux is its device driver model which allows it to support a vast range of hardware ranging from embedded systems to supercomputers. The kernel provides interfaces like platform drivers, bus drivers, and device drivers. Vendors can implement drivers for hardware like sound cards, wireless cards, graphics cards etc. and users can dynamically load/unload these drivers as kernel modules. The driver model also has facilities for hot-plugging of USB and PCI devices. Linux has good support for buses like ATA, SATA, SCSI, PCI, USB etc. There are thousands of open source drivers already available for common hardware. The kernel codebase itself is written in portable C and can be compiled for many processor architectures.
- Memory management:
Linux has advanced memory management capabilities like virtual memory using paging and swapping, multiple disk caching strategies, shared memory APIs, several allocation schemes like SLAB/SLOB, NUMA support, and features like control groups to manage resources. The buddy memory allocation scheme is used to manage free memory regions efficiently. The virtual memory system uses paging to map virtual addresses used by processes into physical RAM addresses. It can use swapping to move inactive pages out to disk storage. There are APIs for inter-process communication using shared memory.
- Process management:
The Linux kernel supports spawning and termination of processes, parent-child relationships using process hierarchies, signaling between processes, inter-process communication mechanisms like pipes, sockets, file descriptors etc. Process scheduling is done using Completely Fair Scheduler (CFS) which allocates time slices to processes fairly. Real-time processes get priority. The kernel lets processes wait on events and be woken up later. Features like cgroups allow resource limits to be set per group of processes. The clone() system call is used to create new threads and processes efficiently.
- Networking stack:
Linux has a full-featured networking stack including protocol implementations for TCP, UDP, DNS, DHCP, ICMP, HTTP, FTP, Bluetooth, WiFi and more. Network devices have associated device driver modules in the kernel. There are APIs for socket programming by applications. Routing tables determine where packets get forwarded. Advanced capabilities like firewalling, VPNs, network namespaces for virtualization, traffic shaping, QoS etc. are also supported. Optimizations like Sendfile and Zero-copy help improve throughput. Older IPv4 and newer IPv6 protocols are both fully supported.
- Security features:
Linux provides user-based access control for resources like files/directories at the kernel level. Each user has an owner ID and group ID which determine permissions. There is also process isolation provided by the kernel between user space applications. Capabilities and SECCOMP can restrict what resources a process can access. SELinux and AppArmor allow setting granular access policies. There is also support for encrypting filesystems, block devices, and network communications in the kernel leveraging cryptographic libraries. Firewalling, sandboxing, and seccomp profiles further lock down the kernel.
- Preemption and real-time support:
The Linux kernel supports both preemptive multitasking and real-time workloads. Kernel preemption allows high priority tasks to preempt lower priority ones as soon as they become runnable. This reduces latency and improves interactivity of UI and applications. For real-time workloads, PREEMPT_RT patches exist to provide near millisecond latency. There are real-time schedulers like SCHED_FIFO, SCHED_RR, and deadline scheduling policy also.
- Symmetric multiprocessing:
Linux supports both symmetric and asymmetric multiprocessing. On systems with multiple identical CPUs, Linux can distribute processes equally across CPUs for load balancing. Synchronization primitives like atomic instructions, mutexes, semaphores, spinlocks etc. are used to coordinate safely between CPUs. The Big Kernel Lock (BKL) was finally removed to improve SMP scalability. Affinity can be set to bind processes to CPUs. Features like CFS, NUMA scheduling, and preemption all improve SMP performance.
This combination of features, along with an active open source community of thousands of developers has made Linux the most widely used kernel today powering everything from Android phones to supercomputers. The evolution of the Linux kernel has been one of the greatest success stories of open source software collaboration.