diff options
| -rw-r--r-- | docs/contents.html | 1 | ||||
| -rw-r--r-- | docs/dispatch.html | 274 |
2 files changed, 275 insertions, 0 deletions
diff --git a/docs/contents.html b/docs/contents.html index 7a91aa62f0..9f09df9737 100644 --- a/docs/contents.html +++ b/docs/contents.html @@ -73,6 +73,7 @@ a:visited { <li><a href="fbdev-dri.html" target="MainFrame">fbdev/DRI Environment</a> <li><a href="glfbdev-driver.html" target="MainFrame">glFBDev Driver</a> <LI><A HREF="custom.html" target="MainFrame">Custom Development</A> +<LI><A HREF="dispatch.html" target="MainFrame">GL Dispatch</A> </ul> <b>Links</b> diff --git a/docs/dispatch.html b/docs/dispatch.html new file mode 100644 index 0000000000..b9ea8822e6 --- /dev/null +++ b/docs/dispatch.html @@ -0,0 +1,274 @@ +<HTML> +<HEAD> +<TITLE>GL Dispatch in Mesa</TITLE> +<LINK REL="stylesheet" TYPE="text/css" HREF="mesa.css"> +</HEAD> + +<BODY> +<H1>GL Dispatch in Mesa</H1> + +<p>Several factors combine to make efficient dispatch of OpenGL functions +fairly complicated. This document attempts to explain some of the issues +and introduce the reader to Mesa's implementation. Readers already familiar +with the issues around GL dispatch can safely skip ahead to the <A +HREF="#overview">overview of Mesa's implementation</A>.</p> + +<H2>1. Complexity of GL Dispatch</H2> + +<p>Every GL application has at least one object called a GL <em>context</em>. +This object, which is an implicit parameter to ever GL function, stores all +of the GL related state for the application. Every texture, every buffer +object, every enable, and much, much more is stored in the context. Since +an application can have more than one context, the context to be used is +selected by a window-system dependent function such as +<tt>glXMakeContextCurrent</tt>.</p> + +<p>In environments that implement OpenGL with X-Windows using GLX, every GL +function, including the pointers returned by <tt>glXGetProcAddress</tt>, are +<em>context independent</em>. This means that no matter what context is +currently active, the same <tt>glVertex3fv</tt> function is used.</p> + +<p>This creates the first bit of dispatch complexity. An application can +have two GL contexts. One context is a direct rendering context where +function calls are routed directly to a driver loaded within the +application's address space. The other context is an indirect rendering +context where function calls are converted to GLX protocol and sent to a +server. The same <tt>glVertex3fv</tt> has to do the right thing depending +on which context is current.</p> + +<p>Highly optimized drivers or GLX protocol implementations may want to +change the behavior of GL functions depending on current state. For +example, <tt>glFogCoordf</tt> may operate differently depending on whether +or not fog is enabled.</p> + +<p>In multi-threaded environments, it is possible for each thread to have a +differnt GL context current. This means that poor old <tt>glVertex3fv</tt> +has to know which GL context is current in the thread where it is being +called.</p> + +<A NAME="overview"/> +<H2>2. Overview of Mesa's Implementation</H2> + +<p>Mesa uses two per-thread pointers. The first pointer stores the address +of the context current in the thread, and the second pointer stores the +address of the <em>dispatch table</em> associated with that context. The +dispatch table stores pointers to functions that actually implement +specific GL functions. Each time a new context is made current in a thread, +these pointers a updated.</p> + +<p>The implementation of functions such as <tt>glVertex3fv</tt> becomes +conceptually simple:</p> + +<ul> +<li>Fetch the current dispatch table pointer.</li> +<li>Fetch the pointer to the real <tt>glVertex3fv</tt> function from the +table.</li> +<li>Call the real function.</li> +</ul> + +<p>This can be implemented in just a few lines of C code. The file +<tt>src/mesa/glapi/glapitemp.h</tt> contains code very similar to this.</p> + +<blockquote> +<table border="1"> +<tr><td><pre> +void glVertex3f(GLfloat x, GLfloat y, GLfloat z) +{ + const struct _glapi_table * const dispatch = GET_DISPATCH(); + + (*dispatch->Vertex3f)(x, y, z); +}</pre></td></tr> +<tr><td>Sample dispatch function</td></tr></table> +</blockquote> + +<p>The problem with this simple implementation is the large amount of +overhead that it adds to every GL function call.</p> + +<p>In a multithreaded environment, a niave implementation of +<tt>GET_DISPATCH</tt> involves a call to <tt>pthread_getspecific</tt> or a +similar function. Mesa provides a wrapper function called +<tt>_glapi_get_dispatch</tt> that is used by default.</p> + +<H2>3. Optimizations</H2> + +<p>A number of optimizations have been made over the years to diminish the +performance hit imposed by GL dispatch. This section describes these +optimizations. The benefits of each optimization and the situations where +each can or cannot be used are listed.</p> + +<H3>3.1. Dual dispatch table pointers</H3> + +<p>The vast majority of OpenGL applications use the API in a single threaded +manner. That is, the application has only one thread that makes calls into +the GL. In these cases, not only do the calls to +<tt>pthread_getspecific</tt> hurt performance, but they are completely +unnecessary! It is possible to detect this common case and avoid these +calls.</p> + +<p>Each time a new dispatch table is set, Mesa examines and records the ID +of the executing thread. If the same thread ID is always seen, Mesa knows +that the application is, from OpenGL's point of view, single threaded.</p> + +<p>As long as an application is single threaded, Mesa stores a pointer to +the dispatch table in a global variable called <tt>_glapi_Dispatch</tt>. +The pointer is also stored in a per-thread location via +<tt>pthread_setspecific</tt>. When Mesa detects that an application has +become multithreaded, <tt>NULL</tt> is stored in <tt>_glapi_Dispatch</tt>.</p> + +<p>Using this simple mechanism the dispatch functions can detect the +multithreaded case by comparing <tt>_glapi_Dispatch</tt> to <tt>NULL</tt>. +The resulting implementation of <tt>GET_DISPATCH</tt> is slightly more +complex, but it avoids the expensive <tt>pthread_getspecific</tt> call in +the common case.</p> + +<blockquote> +<table border="1"> +<tr><td><pre> +#define GET_DISPATCH() \ + (_glapi_Dispatch != NULL) \ + ? _glapi_Dispatch : pthread_getspecific(&_glapi_Dispatch_key) +</pre></td></tr> +<tr><td>Improved <tt>GET_DISPATCH</tt> Implementation</td></tr></table> +</blockquote> + +<H3>3.2. ELF TLS</H3> + +<p>Starting with the 2.4.20 Linux kernel, each thread is allocated an area +of per-thread, global storage. Variables can be put in this area using some +extensions to GCC. By storing the dispatch table pointer in this area, the +expensive call to <tt>pthread_getspecific</tt> and the test of +<tt>_glapi_Dispatch</tt> can be avoided.</p> + +<p>The dispatch table pointer is stored in a new variable called +<tt>_glapi_tls_Dispatch</tt>. A new variable name is used so that a single +libGL can implement both interfaces. This allows the libGL to operate with +direct rendering drivers that use either interface. Once the pointer is +properly declared, <tt>GET_DISPACH</tt> becomes a simple variable +reference.</p> + +<blockquote> +<table border="1"> +<tr><td><pre> +extern __thread struct _glapi_table *_glapi_tls_Dispatch + __attribute__((tls_model("initial-exec"))); + +#define GET_DISPATCH() _glapi_tls_Dispatch +</pre></td></tr> +<tr><td>TLS <tt>GET_DISPATCH</tt> Implementation</td></tr></table> +</blockquote> + +<p>Use of this path is controlled by the preprocessor define +<tt>GLX_USE_TLS</tt>. Any platform capable of using TLS should use this as +the default dispatch method.</p> + +<H3>3.3. Assembly Language Dispatch Stubs</H3> + +<p>Many platforms has difficulty properly optimizing the tail-call in the +dispatch stubs. Platforms like x86 that pass parameters on the stack seem +to have even more difficulty optimizing these routines. All of the dispatch +routines are very short, and it is trivial to create optimal assembly +language versions. The amount of optimization provided by using assembly +stubs varies from platform to platform and application to application. +However, by using the assembly stubs, many platforms can use an additional +space optimization (see <A HREF="#fixedsize">below</A>).</p> + +<p>The biggest hurdle to creating assembly stubs is handling the various +ways that the dispatch table pointer can be accessed. There are four +different methods that can be used:</p> + +<ol> +<li>Using <tt>_glapi_Dispatch</tt> directly in builds for non-multithreaded +environments.</li> +<li>Using <tt>_glapi_Dispatch</tt> and <tt>_glapi_get_dispatch</tt> in +multithreaded environments.</li> +<li>Using <tt>_glapi_Dispatch</tt> and <tt>pthread_getspecific</tt> in +multithreaded environments.</li> +<li>Using <tt>_glapi_tls_Dispatch</tt> directly in TLS enabled +multithreaded environments.</li> +</ol> + +<p>People wishing to implement assembly stubs for new platforms should focus +on #4 if the new platform supports TLS. Otherwise, implement #2 followed by +#3. Environments that do not support multithreading are uncommon and not +terribly relevant.</p> + +<p>Selection of the dispatch table pointer access method is controlled by a +few preprocessor defines.</p> + +<ul> +<li>If <tt>GLX_USE_TLS</tt> is defined, method #4 is used.</li> +<li>If <tt>PTHREADS</tt> is defined, method #3 is used.</li> +<li>If any of <tt>PTHREADS</tt>, <tt>USE_XTHREADS</tt>, +<tt>SOLARIS_THREADS</tt>, <tt>WIN32_THREADS</tt>, or <tt>BEOS_THREADS</tt> +is defined, method #2 is used.</li> +<li>If none of the preceeding are defined, method #1 is used.</li> +</ul> + +<p>Two different techniques are used to handle the various different cases. +On x86 and SPARC, a macro called <tt>GL_STUB</tt> is used. In the preamble +of the assembly source file different implementations of the macro are +selected based on the defined preprocessor variables. The assmebly code +then consists of a series of invocations of the macros such as: + +<blockquote> +<table border="1"> +<tr><td><pre> +GL_STUB(Color3fv, _gloffset_Color3fv) +</pre></td></tr> +<tr><td>SPARC Assembly Implementation of <tt>glColor3fv</tt></td></tr></table> +</blockquote> + +<p>The benefit of this technique is that changes to the calling pattern +(i.e., addition of a new dispatch table pointer access method) require fewer +changed lines in the assembly code.</p> + +<p>However, this technique can only be used on platforms where the function +implementation does not change based on the parameters passed to the +function. For example, since x86 passes all parameters on the stack, no +additional code is needed to save and restore function parameters around a +call to <tt>pthread_getspecific</tt>. Since x86-64 passes parameters in +registers, varying amounts of code needs to be inserted around the call to +<tt>pthread_getspecific</tt> to save and restore the GL function's +parameters.</p> + +<p>The other technique, used by platforms like x86-64 that cannot use the +first technique, is to insert <tt>#ifdef</tt> within the assembly +implementation of each function. This makes the assembly file considerably +larger (e.g., 29,332 lines for <tt>glapi_x86-64.S</tt> versus 1,155 lines for +<tt>glapi_x86.S</tt>) and causes simple changes to the function +implementation to generate many lines of diffs. Since the assmebly files +are typically generated by scripts (see <A HREF="#autogen">below</A>), this +isn't a significant problem.</p> + +<p>Once a new assembly file is created, it must be inserted in the build +system. There are two steps to this. The file must first be added to +<tt>src/mesa/sources</tt>. That gets the file built and linked. The second +step is to add the correct <tt>#ifdef</tt> magic to +<tt>src/mesa/main/dispatch.c</tt> to prevent the C version of the dispatch +functions from being built.</p> + +<A NAME="fixedsize"/> +<H3>3.4. Fixed-Length Dispatch Stubs</H3> + +<p>To implement <tt>glXGetProcAddress</tt>, Mesa stores a table that +associates function names with pointers to those functions. This table is +stored in <tt>src/mesa/glapi/glprocs.h</tt>. For different reasons on +different platforms, storing all of those pointers is inefficient. On most +platforms, including all known platforms that support TLS, we can avoid this +added overhead.</p> + +<p>If the assembly stubs are all the same size, the pointer need not be +stored for every function. The location of the function can instead be +calculated by multiplying the size of the dispatch stub by the offset of the +function in the table. This value is then added to the address of the first +dispatch stub.</p> + +<p>This path is activated by adding the correct <tt>#ifdef</tt> magic to +<tt>src/mesa/glapi/glapi.c</tt> just before <tt>glprocs.h</tt> is +included.</p> + +<A NAME="autogen"/> +<H2>4. Automatic Generation of Dispatch Stubs</H2> + +</BODY> +</HTML> |