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This chapter is from the book

This chapter is from the book

3.3 Kernels

The following sections discuss Unix-like kernel implementation details with a focus on performance. As background, the performance features of earlier kernels are discussed: Unix, BSD, and Solaris. The Linux kernel is discussed in more detail in Section 3.4, Linux.

Kernel differences can include the file systems they support (see Chapter 8, File Systems), the system call (syscall) interfaces, network stack architecture, real-time support, and scheduling algorithms for CPUs, disk I/O, and networking.

Table 3.3 shows Linux and other kernel versions for comparison, with syscall counts based on the number of entries in section 2 of the OS man pages. This is a crude comparison, but enough to see some differences.

Table 3.3 Kernel versions with documented syscall counts

Kernel Version


UNIX Version 7


SunOS (Solaris) 5.11


FreeBSD 12.0


Linux 2.6.32-21-server


Linux 2.6.32-220.el6.x86_64


Linux 3.2.6-3.fc16.x86_64


Linux 4.15.0-66-generic


Linux 5.3.0-1010-aws


These are just the syscalls with documentation; more are usually provided by the kernel for private use by operating system software.

UNIX had twenty system calls at the very first, and today Linux—which is a direct descendant—has over a thousand . . . I just worry about the complexity and the size of things that grow.

Ken Thompson, ACM Turing Centenary Celebration, 2012

Linux is growing in complexity and exposing this complexity to user-land by adding new system calls or through other kernel interfaces. Extra complexity makes learning, programming, and debugging more time-consuming.

3.3.1 Unix

Unix was developed by Ken Thompson, Dennis Ritchie, and others at AT&T Bell Labs during 1969 and the years that followed. Its exact origin was described in The UNIX Time-Sharing System [Ritchie 74]:

The first version was written when one of us (Thompson), dissatisfied with the available computer facilities, discovered a little-used PDP-7 and set out to create a more hospitable environment.

The developers of UNIX had previously worked on the Multiplexed Information and Computer Services (Multics) operating system. UNIX was developed as a lightweight multitasked operating system and kernel, originally named UNiplexed Information and Computing Service (UNICS), as a pun on Multics. From UNIX Implementation [Thompson 78]:

The kernel is the only UNIX code that cannot be substituted by a user to his own liking. For this reason, the kernel should make as few real decisions as possible. This does not mean to allow the user a million options to do the same thing. Rather, it means to allow only one way to do one thing, but have that way be the least-common divisor of all the options that might have been provided.

While the kernel was small, it did provide some features for high performance. Processes had scheduler priorities, reducing run-queue latency for higher-priority work. Disk I/O was performed in large (512-byte) blocks for efficiency and cached in an in-memory per-device buffer cache. Idle processes could be swapped out to storage, allowing busier processes to run in main memory. And the system was, of course, multitasking—allowing multiple processes to run concurrently, improving job throughput.

To support networking, multiple file systems, paging, and other features we now consider standard, the kernel had to grow. And with multiple derivatives, including BSD, SunOS (Solaris), and later Linux, kernel performance became competitive, which drove the addition of more features and code.

3.3.2 BSD

The Berkeley Software Distribution (BSD) OS began as enhancements to Unix 6th Edition at the University of California, Berkeley, and was first released in 1978. As the original Unix code required an AT&T software license, by the early 1990s this Unix code had been rewritten in BSD under a new BSD license, allowing free distributions including FreeBSD.

Major BSD kernel developments, especially performance-related, include:

  • Paged virtual memory: BSD brought paged virtual memory to Unix: instead of swapping out entire processes to free main memory, smaller least-recently-used chunks of memory could be moved (paged). See Chapter 7, Memory, Section 7.2.2, Paging.

  • Demand paging: This defers the mapping of physical memory to virtual memory to when it is first written, avoiding an early and sometimes unnecessary performance and memory cost for pages that may never be used. Demand paging was brought to Unix by BSD. See Chapter 7, Memory, Section 7.2.2, Paging.

  • FFS: The Berkeley Fast File System (FFS) grouped disk allocation into cylinder groups, greatly reducing fragmentation and improving performance on rotational disks, as well as supporting larger disks and other enhancements. FFS became the basis for many other file systems, including UFS. See Chapter 8, File Systems, Section 8.4.5, File System Types.

  • TCP/IP network stack: BSD developed the first high-performance TCP/IP network stack for Unix, included in 4.2BSD (1983). BSD is still known for its performant network stack.

  • Sockets: Berkeley sockets are an API for connection endpoints. Included in 4.2BSD, they have become a standard for networking. See Chapter 10, Network.

  • Jails: Lightweight OS-level virtualization, allowing multiple guests to share one kernel. Jails were first released in FreeBSD 4.0.

  • Kernel TLS: As transport layer security (TLS) is now commonly used on the Internet, kernel TLS moves much of TLS processing to the kernel, improving performance14 [Stewart 15].

While not as popular as Linux, BSD is used for some performance-critical environments, including for the Netflix content delivery network (CDN), as well as file servers from NetApp, Isilon, and others. Netflix summarized FreeBSD performance on its CDN in 2019 as [Looney 19]:

“Using FreeBSD and commodity parts, we achieve 90 Gb/s serving TLS-encrypted connections with ~55% CPU on a 16-core 2.6-GHz CPU.”

There is an excellent reference on the internals of FreeBSD, from the same publisher that brings you this book: The Design and Implementation of the FreeBSD Operating System, 2nd Edition [McKusick 15].

3.3.3 Solaris

Solaris is a Unix and BSD-derived kernel and OS created by Sun Microsystems in 1982. It was originally named SunOS and optimized for Sun workstations. By the late 1980s, AT&T developed a new Unix standard, Unix System V Release 4 (SVR4) based on technologies from SVR3, SunOS, BSD, and Xenix. Sun created a new kernel based on SVR4, and rebranded the OS under the name Solaris.

Major Solaris kernel developments, especially performance-related, include:

  • VFS: The virtual file system (VFS) is an abstraction and interface that allows multiple file systems to easily coexist. Sun initially created it so that NFS and UFS could coexist. VFS is covered in Chapter 8, File Systems.

  • Fully preemptible kernel: This provided low latency for high-priority work, including real-time work.

  • Multiprocessor support: In the early 1990s, Sun invested heavily in multiprocessor operating system support, developing kernel support for both asymmetric and symmetric multiprocessing (ASMP and SMP) [Mauro 01].

  • Slab allocator: Replacing the SVR4 buddy allocator, the kernel slab memory allocator provided better performance via per-CPU caches of preallocated buffers that could be quickly reused. This allocator type, and its derivatives, has become the standard for kernels including Linux.

  • DTrace: A static and dynamic tracing framework and tool providing virtually unlimited observability of the entire software stack, in real time and in production. Linux has BPF and bpftrace for this type of observability.

  • Zones: An OS-based virtualization technology for creating OS instances that share one kernel, similar to the earlier FreeBSD jails technology. OS virtualization is now in widespread use as Linux containers. See Chapter 11, Cloud Computing.

  • ZFS: A file system with enterprise-level features and performance. It is now available for other OSes, including Linux. See Chapter 8, File Systems.

Oracle purchased Sun Microsystems in 2010, and Solaris is now called Oracle Solaris. Solaris is covered in more detail in the first edition of this book.

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