Mesa (computer graphics)

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Mesa
Original author(s) Brian Paul
Developer(s) Currently: Intel, AMD, VMware
Formerly: Tungsten Graphics[1]
Initial release August 1993; 22 years ago (1993-08)[2]
Stable release 12.0.1 / July 8, 2016; 16 days ago (2016-07-08)[3]
Preview release 12.0.0 RC4 / June 21, 2016; 33 days ago (2016-06-21),[4][5]
Development status Active
Written in C, C++, Assembly[6]
Operating system Cross-platform (BSDs, Haiku, Linux, et al.)
Type Graphics library
License MIT License[7]
Website mesa3d.org

Mesa is an open-source implementation of the OpenGL specification— a cross-language, cross-platform, vendor-neutral standard API for interfacing with vendor-specific graphics hardware primitives. Mesa is hosted by freedesktop.org, which is also the home of the X.org and Wayland display servers, and of several open-source display drivers.

On Unix-like systems such as the BSD derivatives, or the Linux distributions, Mesa implements a vendor-independent translation layer between a graphics API such as OpenGL and the graphics hardware drivers in the operating system kernel. Besides 3D applications such as games, modern display servers (X.org's Glamor or Wayland's Weston) use OpenGL/EGL calls to produce the screen image, therefore all graphics typically go through Mesa. Some proprietary graphics drivers replace all of Mesa, providing their own implementation of a graphics API, rather than providing a driver that Mesa talks to.

Mesa is also not specific to Unix-like operating systems: on Windows for example, Mesa provides an OpenGL API over DirectX.

Mesa was initiated in August 1993 by Brian Paul, who is still active in the project. Mesa was subsequently widely adopted, and now contains numerous contributions from various individuals and corporations worldwide, including from the graphics hardware manufacturers of the Khronos Group that administer the OpenGL specification. For Linux, development has also been partially driven by crowdfunding.[8]

Overview[edit]

Video games outsource rendering calculations to the GPU over OpenGL in real-time. Shaders are written in OpenGL Shading Language or SPIR-V and compiled on the CPU. The compiled programs are executed on the GPU.
Illustration of the Linux graphics stack: DRM & libDRM, Mesa 3D. Display server belongs to the windowing system and is not necessary e.g. for gaming.

Implementations of rendering APIs[edit]

The free implementations of Wayland rely upon the Mesa implementation of EGL. The special library called libwayland-EGL, written to accommodate access to the framebuffer, should have been made obsolete by the EGL 1.5 release. On the GDC 2014, AMD was exploring a strategy change towards using DRM instead of their in-kernel blob.[9]

Mesa is known as housing implementation of graphic APIs. Historically the main API that Mesa has implemented is OpenGL, along with other Khronos Group related specifications (like OpenVG, OpenGL ES or recently EGL). But Mesa can implement other APIs and indeed it did with Glide (deprecated) and Direct3D.

The supported version of the different graphic APIs depends on the driver, because each hardware driver has its own implementation (and therefore status). This is specially true for the "classic" drivers, while the Gallium3D drivers share common code that tend to homogenize the supported extensions and versions.

Mesa 10.x complies with OpenGL 3.3, for Intel, AMD/ATI and Nvidia GPU hardware. Mesa 11.0 was announced with some drivers being OpenGL 4.1 compliant.[10] Mesa maintains a support matrix with the status of the current OpenGL 3 and 4 conformance.[11][12]

API Vulkan OpenCL OpenGL OpenGL ES OpenVG EGL GLX Direct3D
Version Date last Update 1.0
2016-02-16
2.1
2015-08-09
4.5
2014-08-11
3.2
2015-08-10
1.1
2008-12-03
1.5
2014-03-19
1.4
2005-12-16
12.0
2015-07-29
Future release: 13.0 N/A N/A 1.0 N/A 4.5 3.1 N/A[13] 1.5 1.4 9.0c[14][15]
Current stable version: 12.0 2016-07-08[16] 12.0.1 4.3[16]
Older version, yet still supported: 11.2 2016-04-04[17] 11.2.2 N/A 4.1[18]
Old version, no longer supported: 11.1 2015-12-15[19] 11.1.4 3.0
Old version, no longer supported: 11.0 2015-09-12[20] 11.0.9
Old version, no longer supported: 10.6 2015-06-15[21] 10.6.9 3.3[22] 1.4
Old version, no longer supported: 10.5 2015-03-06[23] 10.5.9 1.1
Old version, no longer supported: 10.4 2014-12-14[24] 10.4.7
Old version, no longer supported: 10.3 2014-09-19[25] 10.3.7 N/A
Old version, no longer supported: 10.2 2014-06-06[26] 10.2.9
Old version, no longer supported: 10.1 2014-03-04[27] 10.1.6
Old version, no longer supported: 10.0 2013-11-30[28] 10.0.5
Old version, no longer supported: 9.0 2012-10-08 9.0.3, 9.1.7, 9.2.5 3.1[29] 2.0
Old version, no longer supported: 8.0 2012-02-08 8.0.5 3.0
Old version, no longer supported: 7.0 2007-06-22 7.0.4, ..., 7.11.2 2.1 N/A N/A N/A
Old version, no longer supported: 6.0 2004-01-06 6.0.1 1.5 1.3
Old version, no longer supported: 5.0 2002-11-13 5.0.2 1.4
Old version, no longer supported: 4.0 2001-10-22 4.0.4 1.3
Old version, no longer supported: 3.0 1998-09-01 3.1, 3.2.1, 3.4.2.1 1.2
Old version, no longer supported: 2.0 1996-10-01 2.6 1.1
Old version, no longer supported: 1.0 1995-02-01 1.2.8 1.0
Legend:
Old version
Older version, still supported
Latest version
Latest preview version
Future release

There has also been a Direct3D 9 state tracker since July 2013.[30] Note that due to the modularized nature of OpenGL, Mesa actually supports extensions from newer versions of OpenGL without claiming full support for them. For example, as of June 2016, Mesa supports OpenGL 4.3 but also all OpenGL 4.4 and 4.5 extensions except for one.

Vulkan[edit]

The Khronos Group officially announced Vulkan API in March 2015, and officially released Vulkan 1.0 on February 16, 2016. Vulkan breaks compatibility with OpenGL and completely abandons its one giant monolithic state machine concept. The developers of Gallium3D called Vulkan to be something along the lines of Gallium3D 2.0 – Gallium3D separates the code that implements the OpenGL state machine from the code that is specific to the hardware.

As Gallium3D ingests TGSI, Vulkan ingests SPIR-V (Standard Portable Intermediate Representation version "V" as in "Vulkan").

Intel released their implementation of a Vulkan driver for their hardware the day the specification was officially released, but it was only mainlined in April and so it will be part of Mesa 12.0 to be released in June. While already the i965 driver wasn't written according to the Gallium3D specifications, for the Vulkan driver it makes even less sense to flange it on top of Gallium3D. Similarly there is no technical reason to flange it with NIR, but yet Intel's employees implemented their Vulkan driver that way.

It is to be expected that AMD's own proprietary Vulkan driver, which was released in March, and was announced to be released as free and open-source software in the future and be mainlined into Mesa,[citation needed] also abandons Gallium3D.

Nvidia released their proprietary GeForce driver with Vulkan support at launch day and Imagination Technologies (PowerVR), Qualcomm (Adreno) and ARM (Mali) have done the same or at least announced proprietary Vulkan drivers for Android and other operating systems. But when and whether additional free and open-source Vulkan implementations for these GPUs will show up, remains to be seen.

Generic Buffer Management[edit]

Generic Buffer Management (GBM) is an API which provides a mechanism for allocating buffers for graphics rendering tied to Mesa. GBM is intended to be used as a native platform for EGL on drm or openwfd. The handle it creates can be used to initialize EGL and to create render target buffers.[31]

Mesa GBM is an abstraction of the graphics driver specific buffer management APIs (for instance the various libdrm_* libraries), implemented internally by calling into the Mesa GPU drivers.

For example, the Wayland compositor Weston does its rendering using OpenGL ES 2, which it initializes by calling EGL. Since the server runs on the "bare KMS driver", it uses the EGL DRM platform, which could really be called as the GBM platform, since it relies on the Mesa GBM interface.

At XDC2014, Nvidia employee Andy Ritger proposed to enhance EGL in order to replace GBM.[32]

Implementations of video acceleration APIs[edit]

There are three possible ways to do the calculations necessary for the encoding and decoding of video streams:

  1. use a software implementation of a video compression or decompression algorithm (commonly called a CODEC) and execute this software on the CPU
  2. use a software implementation of a video compression or decompression algorithm (commonly called a CODEC) and execute this software on the GPU (the 3D rendering engine)
  3. use a complete (or partial) hardware implementation of a video compression or decompression algorithm; it has become very common to integrate such ASICs into the chip of the GPU/CPU/APU/SoC and therefore abundantly available; for marketing reasons companies have established brands for their ASICs, such as PureVideo (Nvidia), Unified Video Decoder (AMD), Video Coding Engine (AMD), Quick Sync Video (Intel), DaVinci (Texas Instruments), CedarX (Allwinner), Crystal HD (Broadcom); some ASICs are available for licensing as semiconductor intellectual property core; usually different versions implement different video compression and/or video decompression algorithms; support for such ASICs usually belong into the kernel driver, to initialize the hardware and do low-level stuff. Mesa, which runs in user-space, houses the implementations of several APIs for software, e.g. VLC media player, GStreamer, HandBrake, etc., to conveniently access such ASICs:

For example, Nouveau, which has been developed as part of Mesa, but also includes a Linux kernel component, which is being developed as part of the Linux kernel, supports the PureVideo-branded ASICs and provides access to them through VDPAU and partly through XvMC.[33]

The free radeon driver supports Unified Video Decoder and Video Coding Engine through VDPAU and OpenMAX.[34]

Please note, that V4L2 is a kernel-to-user-space interface for video bit streams delivered by webcams or TV tuners.

Device drivers[edit]

Graphics device drivers are implemented using two components: a UMD (user-mode driver) and a KMD (kernel-mode driver). Starting with Linux kernel 3.x AMD Catalyst and Mesa will share the same Linux kernel driver: amdgpu. Amdgpu provides interfaces defined by DRM and KMS.

The available free and open-source device drivers for graphic chipsets are "stewarded" by Mesa (because the existing free and open-source implementation of APIs are developed inside of Mesa). Currently there are two frameworks to write graphics drivers: "classic" and Gallium3D.[35] An overview over some of the drivers available in Mesa is given at mesamatrix.net. The driver vc4 is missing from this site, while the driver freedreno is not part of Mesa.[citation needed]

There are device drivers for AMD/ATI R100 to R800, Intel, and Nvidia cards with 3D acceleration. Previously drivers existed for the IBM/Toshiba/Sony Cell APU of the PlayStation 3, S3 Virge & Savage chipsets, VIA chipsets, Matrox G200 & G400, and more.[36]

The free and open-source drivers compete with proprietary closed-source drivers. Depending on the availability of hardware documentation and man-power, the free and open-source driver lag behind more or less in supporting 3D acceleration of new hardware. Also, 3D rendering performance is usually significantly slower with some notable exceptions,[37][38][39][40] where the free and open-source driver performs better than proprietary drivers.

Direct Rendering Infrastructure (DRI)[edit]

At the time 3D graphics cards became more mainstream for PCs, individuals partly supported by some companies began working on adding more support for hardware-accelerated 3D rendering to Mesa.[when?] The Direct Rendering Infrastructure (DRI) was one of these approaches to interface Mesa, OpenGL and other 3D rendering API libraries with the device drivers and hardware. After reaching a basic level of usability, DRI support was officially added to Mesa. This significantly broadened the available range of hardware support achievable when using the Mesa library.[41]

With adapting to DRI, the Mesa library finally took over the role of the front end component of a full scale OpenGL framework with varying backend components that could offer different degrees of 3D hardware support while not dropping the full software rendering capability. The total system used many different software components.[41]

While the design requires all these components to interact carefully, the interfaces between them are relatively fixed. Nonetheless, as most components interacting with the Mesa stack are open source, experimental work is often done through altering several components at once as well as the interfaces between them. If such experiments prove successful, they can be incorporated into the next major or minor release. That applies e.g. to the update of the DRI specification developed in the 2007-2008 timeframe. The result of this experimentation, DRI2, operates without locks and with improved back buffer support. For this, a special git branch of Mesa was created.[42]

DRI3 should be supported by at least the intel driver.[43][44]

Software renderer[edit]

Software rendering is the misleading term, when shaders are executed on the CPU instead of the GPU. Mesa also contains an implementation of software rendering that allows shaders to run on the CPU as a fallback when no graphics hardware accelerators are present called swrast. The Gallium software rasterizer is known as softpipe or when built with support for LLVM llvmpipe which generates CPU code at runtime.[45][46]

In Mesa 12.0 a new Intel Rasterizer OpenSWR is available with high advantages in clusters for large data sets.[47] OpenGL 3.1+ is now supported.[48] Accelaration values from 29 to 51 related to LLVMPIPE were measured in some examples.[49] Only change to option "GALLIUM_DRIVER="swr" is necessary.

Mega drivers[edit]

The idea of bundling multiple drivers into a single "mega" driver was proposed by Eric Anholt. It allows for a single copy of the shared Mesa code to be used among multiple drivers (instead of it existing in each driver separately) and offering better performance than a separate shared library due to the removal of the internal library interface.[50] The state trackers for VDPAU and XvMC have become separate libraries.[51]

shader-db[edit]

shader-db is a collection of about 20,000 shaders gathered from various computer games and benchmarks as well as some scripts to compile these and collect some statistics. Shader-db shall help validate an optimization.

It was noticed that an unexpected number of shaders are not hand-written but generated. This means these shaders were originally written in HLSL and then translated into GLSL by some translator program, such as e.g. HLSL2GLSL. The problem is, that the generated code is often far from being optimal. Matt Turner said it was much easier to fix this in the translator program, then having to make Mesa's compiler carry the burden of dealing with such bloated shaders.

shader-db cannot be considered free and open-source software. To use it legally, one must have a license for all the computer games, that the shaders are part of. At least the Valve Corporation has granted all Mesa and all Debian developers free personal licenses for all the games distributed on Steam.

Software architecture[edit]

A graphics drivers consists of an implementation of the OpenGL state machine and a compilation stack to compile the shaders into the GPU's machine language. This compilation as well as pretty much anything else is executed on the CPU, then the compiled shaders are sent to the GPU and are executed by it. (SDL = Simple DirectMedia Layer).
The Intermediate Representations (IRs) in Mesa: GLSL IR, Mesa IR, TGSI, and LLVM IR. Missing are HIR, LIR and NIR.
Mesa IR is due to be removed completely.

The so-called "user-mode graphics device drivers" (UMD) in Mesa have very few commonalities with what is generally called a device driver. There are a couple of differences:

  • they are meant to work on top of additionally existent kernel mode graphics device drivers, that are e.g. available as part of the Linux kernel found in the source code under /drivers/gpu/drm/ Each UMD communicates with its kernel mode counterpart with the help of a specific library, name libdrm_specific and a generic one, named libdrm. This section shall look solely on the user-mode part above libdrm
  • there is some implementation of the finite-state machine as specified by e.g. OpenGL; this implementation of the OpenGL state machine may be shared among multiple UMDs or not
  • they consist to a great part of some sort of compiler, that ingests e.g. GLSL and eventually outputs machine code. Parsers and may be shared among multiple UMD or be specific

Mesa's Intermediate Representations[edit]

One goal of Mesa is the optimization of code that is to be executed by the respective GPU. Another is the sharing of code. Instead of documenting the pieces of software, that do this or that, this Wikipedia article shall instead look at the Intermediate Representations used in the process of compiling and optimizing. See Abstract syntax tree (AST) and Static single assignment form (SSA form).

SPIR-V[edit]

SPIR-V is a certain version of the Standard Portable Intermediate Representation. The idea is, that game engines output SPIR-V instead of GLSL. In contrast to the latter, SPIR-V is binary to protect intellectual property and is already the result of general optimizations. A UMD needs only apply optimizations, that are specific to the supported hardware.

GLSL IR[edit]

Mesa IR[edit]

NIR[edit]

TGSI[edit]

The Tungsten Graphics Shader Infrastructure (TGSI) was introduced in 2008 by Tungsten Graphics. All Gallium3D-style UMDs ingest TGSI.

LLVM IR[edit]

The UMDs radeonsi and llvmpipe do not output machine code, but instead LLVM IR. From here on, LLVM does optimizations and the compilation to machine code. This does mean, that a certain minimum version of LLVM has to be installed as well.

Mesa's GLSL compiler[edit]

Mesa's GLSL compiler generates its own IR. Because each driver has very different requirements from a LIR, it differentiates between HIR (high-level IR) and LIR (low-level IR).

Gallium3D[edit]

Main article: Gallium3D
Gallium3D introduced the intermediate language TGSI (Tungsten Graphics Shader Infrastructure). Any shaders written in GLSL (or other shading languages as well) are translated into TGSI, this translation is supported by the hardware driver. The driver translates these TGSI shaders into the instruction set of the GPU it is written for.

Gallium3D was developed by Tungsten Graphics as a means to simplify the writing of device drivers and also to achieve maximum portability of them, without having to rewrite the source code. Gallium3D specification were released in 2008 and as of 24 July 2016, the current version is still 0.4.

Most device drivers inside of Mesa have been rewritten conforming to the new internal APIs defined by Gallium3D, and new drivers written since the introduction of Gallium3D have been written as Gallium3D drivers from the beginning on. A notable exception is the Mesa driver for the Intel 965-family of GPUs.

The main disadvantage is, that by introducing additional interfaces, namely the Gallium3D WinSys Interface, the full capabilities of the underlying hardware can not be accessed by the device drivers.[citation needed]

Performance[edit]

History[edit]

Project initiator Brian Paul was a graphics hobbyist. He thought it would be fun to implement a simple 3D graphics library using the OpenGL API, which he might then use instead of VOGL (very ordinary GL Like Library).[2] Beginning in 1993, he spent eighteen months of part-time development before he released the software on the Internet. The software was well received, and people began contributing to its development. Mesa started off by rendering all 3D computer graphics on the CPU. Despite this, the internal architecture of Mesa was designed to be open for attaching to graphics processor-accelerated 3D rendering. In this first phase, rendering was done indirectly in the display server, leaving some overhead and noticeable speed lagging behind the theoretical maximum. The Diamond Monster 3D, using the Voodoo Graphics chipset, was one of the first 3D hardware devices supported by Mesa.

The first true graphics hardware support was added to Mesa in 1997, based upon the Glide API for the then new 3dfx Voodoo I/II graphics cards and their successors.[41] A major problem of using Glide as the acceleration layer was the habit of Glide to run full screen, which was only suitable for computer games. Further, Glide took the lock of the screen memory, and thus the display server was blocked from doing any other GUI tasks.[52]

See also[edit]

References[edit]

  1. ^ Marshall, David (2008-12-16). "VMware's year end acquisition of Tungsten Graphics". InfoWorld. Retrieved 2011-08-06. 
  2. ^ a b "Mesa Introduction". Mesa Team. Retrieved 2015-06-08. 
  3. ^ "Mesa 12.0.1 release announcement". Retrieved 2016-07-10. 
  4. ^ "Mesa 12.0.0 release candidate 4". Retrieved 2016-06-26. 
  5. ^ https://mesa.freedesktop.org/archive/12.0.0/
  6. ^ "Mesa Languages Page". Open Hub. Retrieved 2015-03-02. 
  7. ^ "Mesa 3D license". Retrieved 2015-06-03. 
  8. ^ "Improve OpenGL support for the Linux Graphics Drivers - Mesa". Indiegogo. 2013-12-11. Retrieved 2015-01-21. 
  9. ^ "AMD exploring new Linux driver Strategy". 2014-03-22. Retrieved 2014-03-23. 
  10. ^ "Mesa 11.0 Has Been Branched, The Release March Begins". 2015-08-22. Retrieved 2015-08-22. 
  11. ^ "mesa/mesa - The Mesa 3D Graphics Library". Retrieved 2015-01-22. 
  12. ^ "The OpenGL vs Mesa matrix". 2015-03-25. Retrieved 2015-03-29. 
  13. ^ Larabel, Michael (4 March 2015). "OpenVG Support Stripped From Gallium3D". Phoronix. Retrieved 11 July 2015. 
  14. ^ "latest patches to "nine" state tracker". 2016-02-04. 
  15. ^ Larabel, Michael (14 December 2014). "Mesa 10.4 Officially Released With Direct3D 9 State Tracker". Phoronix. Retrieved 11 July 2015. 
  16. ^ a b "Mesa 12.0 Released With OpenGL 4.3 Support, Intel Vulkan & Many Other Features". 2016-07-08. Retrieved 2016-07-08. 
  17. ^ "[Mesa-announce] Mesa 11.2.0". Retrieved 2016-04-04. 
  18. ^ http://mesa3d.org/relnotes/11.0.0.html
  19. ^ "[Mesa-announce] Mesa 11.1.0". Retrieved 2015-12-15. 
  20. ^ "[Mesa-announce] Mesa 11.0.0". Retrieved 2015-09-26. 
  21. ^ "[Mesa-announce] Mesa 10.6.0". Retrieved 2015-06-15. 
  22. ^ Larabel, Michael (2013-10-26). "Features To Be Found In Mesa 10.0". Phoronix. 
  23. ^ "[Mesa-announce] Mesa 10.5.0". Retrieved 2015-03-07. 
  24. ^ "[Mesa-announce] Mesa 10.4.0 released". Retrieved 2015-03-07. 
  25. ^ "[Mesa-announce] Mesa 10.3 released". Retrieved 2015-03-07. 
  26. ^ "[Mesa-announce] Mesa 10.2 released". Retrieved 2015-03-07. 
  27. ^ "[Mesa-announce] Mesa 10.1 released". Retrieved 2015-03-07. 
  28. ^ "[Mesa-announce] Mesa 10.0 released". Retrieved 2015-03-07. 
  29. ^ "Mesa 9.0 Release Notes". Mesa. 8 October 2012. Retrieved 11 July 2015. 
  30. ^ http://www.phoronix.com/scan.php?page=news_item&px=MTQxMjk
  31. ^ "libgbm in the Debian repositories". 
  32. ^ "Enabling Alternative Window Systems with a non-Mesa Graphics Driver Implementation". 
  33. ^ "Nouveau Video Acceleration". freedesktop.org. 
  34. ^ "Radeon Feature Matrix". freedesktop.org. 
  35. ^ Toral, Iago (8 August 2014). "Diving into Mesa". Retrieved 19 May 2016. 
  36. ^ "Direct Rendering Infrastructure Status Page". freedesktop.org. 
  37. ^ http://apcmag.com/how-to-improve-gaming-performance-on-your-linux-machine.htm
  38. ^ http://www.geeks3d.com/20120110/linux-mesa-gallium3d-nouveau-and-nvidia-drivers-opengl-test-gtx-280-gtx-480-gtx-580/
  39. ^ http://www.phoronix.com/vr.php?view=18344
  40. ^ http://www.phoronix.com/scan.php?page=article&item=july_2013_gpus&num=8
  41. ^ a b c Paul, Brian (2000-08-10). "Introduction to the Direct Rendering Infrastructure". dri.sourceforge.net. Retrieved 2012-01-25. 
  42. ^ "DRI2". X.org. Retrieved 2012-01-25. 
  43. ^ https://lwn.net/Articles/570082/
  44. ^ https://lists.freedesktop.org/archives/dri-devel/2013-November/048258.html
  45. ^ "LLVMpipe: OpenGL With Gallium3D on Your CPU". Phoronix.com. 2010-04-30. Retrieved 2014-11-04. 
  46. ^ "llvmpipe". mesa3d.org. Retrieved 2015-06-08. 
  47. ^ http://openswr.org
  48. ^ https://mesamatrix.net
  49. ^ http://openswr.org/perf.html
  50. ^ "DRI megadrivers". 2013-09-25. 
  51. ^ "VDPAU & XvMC state trackers are now separate libraries". 2014-06-23. 
  52. ^ "What's the relationship between Glide and DRI?". dri.freedesktop.org. Retrieved 2012-01-25. 

External links[edit]

Various layers within Linux, also showing separation between the userland and kernel space
User mode User applications For example, bash, LibreOffice, Apache OpenOffice, Blender, 0 A.D., Mozilla Firefox, etc.
Low-level system components: System daemons:
systemd, runit, logind, networkd, soundd, ...
Windowing system:
X11, Wayland, Mir, SurfaceFlinger (Android)
Other libraries:
GTK+, Qt, EFL, SDL, SFML, FLTK, GNUstep, etc.
Graphics:
Mesa, AMD Catalyst, ...
C standard library open(), exec(), sbrk(), socket(), fopen(), calloc(), ... (up to 2000 subroutines)
glibc aims to be POSIX/SUS-compatible, uClibc targets embedded systems, bionic written for Android, etc.
Kernel mode Linux kernel stat, splice, dup, read, open, ioctl, write, mmap, close, exit, etc. (about 380 system calls)
The Linux kernel System Call Interface (SCI, aims to be POSIX/SUS-compatible)
Process scheduling
subsystem
IPC
subsystem
Memory management
subsystem
Virtual files
subsystem
Network
subsystem
Other components: ALSA, DRI, evdev, LVM, device mapper, Linux Network Scheduler, Netfilter
Linux Security Modules: SELinux, TOMOYO, AppArmor, Smack
Hardware (CPU, main memory, data storage devices, etc.)