394 lines
17 KiB
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394 lines
17 KiB
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<TITLE>Galapagos - Implementation</TITLE>
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<P>
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<H1 ALIGN=CENTER><A NAME="top"></A>IMPLEMENTATION</H1>
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<UL>
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<LI><A HREF="implementation.html#GARBAGE">Garbage Collection</A></LI>
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<LI><A HREF="implementation.html#BOARDS">Boards</A></LI>
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<LI><A HREF="implementation.html#TURTLES">Turtles</A></LI>
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<LI><A HREF="implementation.html#VISION">Vision</A></LI>
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<LI><A HREF="implementation.html#INTERRUPTS">Interrupts</A></LI>
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<LI><A HREF="implementation.html#CLASS">Class Organization</A></LI>
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</UL>
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<HR>
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<P>Galapagos was written using Microsoft Visual C++, and is designed to
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run under Windows 95. We chose Windows 95 because it seems to have the
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largest potential Galapagos users base. The following sections describe
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some implementation details of Galapagos.<BR>
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</P>
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<P>The SCM interpreter we used as a base is written in C. Most of the original
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code we added was written in C++. The parts we used from SCM are almost
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identical to the original. In fact, by changing the scmconfig.h file (which
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contain machine-specific configuration) and #defining THREAD to be null,
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the C files should become equivalent to the sources we used.<BR>
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</P>
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<P>We have implemented two MT-safe FIFO message queue classes. Both will
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block when trying to read from an empty queue. <B>CMsgQx</B>, the extended
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message queue, supports the same interface as the one provided in Scheme,
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plus an additional support for "Urgent Messages". These take
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precedence over all other messages. <B>CMessageQueue</B> is message queue
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with exactly the same interface as the Scheme level message queues, but
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which contain internal logic to handle "Urgent" messages used
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to deal with cases where synchronous respond is needed, such as I/O, Garbage
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collection, and Scheme-level inter-thread communications. <BR>
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</P>
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<H2><A NAME="FITTING SCM INTO"></A>FITTING SCM INTO WINDOWS<BR>
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</H2>
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<P>Galapagos is based on SCM, which is a single-thread, read-evaluate-print-loop
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(repl) based Scheme interpreter. The most important issue in migrating
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SCM was how to maintain the interpreter's natural repl-based approach,
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yet allow for multiple threads to interact, and for Windows messages to
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be processed quickly.<BR>
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</P>
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<P>We used WIN32's multithreading capabilities to solve these problems.
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A single thread handles all aspects of the GUI - in a sense, "all
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that is Windows": graphic boards, turtles, consoles, menus and so
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on. Each interpreter runs in a thread of its own, interacting with the
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GUI using a message queue similar to the one provided at the Scheme level.<BR>
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</P>
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<CENTER><P><IMG border=0 SRC="img00011.gif" HEIGHT=182 WIDTH=530><BR>
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</P></CENTER>
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<P>The GUI thread manages both commands received from the OS and from the
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different interpreters running on their own threads. To ensure as fast
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responses as possible, priorities are used: OS messages (such as windows
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updates and input devices) gets highest priority; Console (text messages)
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come second, and graphics messages are last. This allows the interpreters
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to run interactively in a satisfactory manner. <BR>
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</P>
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<P>SCM interpreter threads each run in the old-fashion repl mode. When
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a computation is over, the interpreter blocks until new input comes from
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the GUI thread. All blocking functions were modified to allow synchronous
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messages (such as the one generated by <B>tell-thread</B>) to work. In
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addition, SCM's "poll routine" is used to force checking for
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such messages even during computations.<BR>
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</P>
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<P>An additional thread is used for Garbage Collection. It is described
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in detail in the section dealing with garbage collection.<BR>
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</P>
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<H2><A NAME="GARBAGE"></A>GARBAGE COLLECTION</H2>
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<P>In this section we will briefly describe SCM's garbage collector, and
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then discuss the modifications done to adapt it to Galapagos's multithreading
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computations. It should be noted that the garbage collector used is a portable
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garbage collector taken from "SCHEME IN ONE DEFUN, BUT IN C THIS TIME",
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by George J Carrette.<BR>
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</P>
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<P>SCM uses a conservative Mark & Sweep garbage collector (GC). All
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Scheme data objects share some common configuration (called "cells"):
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They are 8-byte aligned; they reside is specially-allocated memory segments
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(called hplims); they are the size of two pointers (so a scheme cons is
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exactly a cell); and they contain a special GC bit used by the garbage
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collector. This bit is 0 during actual computations. When a new cell is
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needed and all the hplims are used, garbage collection is initiated. If
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it does not free enough space to pass a certain threshold, a new hplim
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is allocated.<BR>
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</P>
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<P>The first stage in garbage collection is marking all cells which are
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not to be deleted. Some terminology might be helpful here: <BR>
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</P>
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<UL>
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<LI>A cell (or any data object) is called <B><I>alive</I></B> if it may
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in some way influence the future of the computation. Needless to say, discovering
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which cells are alive and which are not is impossible, because of the very
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nature of the future. </LI>
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</UL>
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<UL>
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<LI>A cell is called <B><I>reachable</I></B> if the computation can read
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its value. Some data is <I>immediately reachable</I>: The data on the machine's
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stack or in the CPU registers, for example; some interpreters store some
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information in a fixed location so it's permanently reachable. In SCM the
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array <TT><FONT FACE="Courier New">sys_protects[]</FONT></TT> is used for
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this propose. The set of reachable cells is the union of all immediately
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reachable cells, and all those cells pointed by reachable cells, recursively.
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</LI>
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</UL>
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<P>Obviously, all unreachable data is dead. Conservative garbage collectors
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treat all reachable data as alive. <BR>
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</P>
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<P>The Mark stage of the garbage collector scans the <TT><FONT FACE="Courier New">sys_protects[]</FONT></TT>
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array and the machine's stack and registers for anything that looks like
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a valid cell. All cell pointer have their 3 least significant bits zero,
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and are in one of few known ranges (the hplims). The garbage collector
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searches for anything matching a cell's bit pattern, and treats it as an
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immediately reachable cell pointer. In some cases, this may mean an integer,
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for example, happens to match the binary pattern and thus be interpreted
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as a cell pointer. However, this will only mean some cell or cells are
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marked as reachable though they are not such. Because of the uniform structure
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of the cell and its limited range of possible locations, such an accident
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is guarantied not to corrupt memory. Furthermore, if we accept the assumption
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that integers are usually relatively small, and memory addresses are relatively
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big, we conclude that such accidents are not very likely to happen often
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anyway.<BR>
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</P>
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<P>During mark stage, the garbage collector recursively finds (a superset
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of) all live cells, and marks them by setting their special GC bit to 1.
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The second stage is the Sweep stage, in which all the hplims are scanned
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linearly, and every cell which is not marked is recognized as dead, and
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as such is reclaimed as free. Marked cells get unmarked so they are ready
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for the next garbage collection. <BR>
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</P>
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<P>Mark & Sweep garbage collection has two main disadvantages: One,
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that it requires all computation to stop while garbage collection is in
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progress. In Galapagos, since all threads use a shared memory heap, it
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means all threads must synchronize and halt while garbage is collected.
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Second, Mark & Sweep is very likely to cause memory fragmentation.
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However, since cells are equally sized, fragmentation is only rarely a
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problem.<BR>
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</P>
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<P>We chose to stick with Mark & Sweep in Galapagos because of its
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two major advantages: Simplicity and efficiency. Mark & Sweep GC does
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not affect computation speed, because direct pointers are used. Most concurrent
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garbage collectors work by making all pointers indirect, which may slow
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computations down considerably. The need to halt all threads for GC is
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accepted. Since memory is shared, it would only be fair to stop all threads
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when GC is needed: Threads will probably halt anyway since cells are allocated
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continuously during computations.<BR>
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</P>
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<P>Two major issues are introduced when trying to multithread the garbage
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collector. One is the synchronization of the different threads, which run
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almost completely unaware one of the other; the second is the need to mark
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data from every thread's specific stack, registers, and <TT><FONT FACE="Courier New">sys_protects[]</FONT></TT>.
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We solved these two issues by combining them to one.<BR>
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</P>
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<P>The intuitive approach might be to let each thread mark its own information,
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and then sweep centrally. However, since synchronization of threads is
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mandatory, letting every thread mark its own data will lead only to redundant
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marking and to excessive context switches, since each threads has to become
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active. Therefore we created the "<B>Garbage Collection daemon</B>"
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(GCd), which runs in a distinct thread and lasts for the whole Galapagos
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session. The GCd is not an interpreter, but a mechanism which keeps track
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of live threads and their need of GC. The GCd thread is always blocked,
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except when a thread notifies it on its birth or death, or when a thread
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indicates the need for garbage collection. Since the GC daemon is blocked
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whenever it is not needed, and then becomes the exclusive running thread
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during actual GC (with the exception of the GUI thread), its existence
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does not hurt performance. <BR>
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</P>
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<P>To explain how the GCd synchronizes all threads, let us examine the
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three-way protocol involved. <TT><FONT FACE="Courier New">freelist</FONT></TT>
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is a global pointer which holds a linked list of free cells - it can be
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either a cell pointer, a value indicating "busy" (thus implementing
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busy/wait protection over it) or "end of memory" which is found
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at the end of the linked list. MIB stands for <B><I>Memory Information
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Block</I></B>, which is a block of memory containing all of a thread's
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immediately reachable data.<BR>
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</P>
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<P><B><U>GCd scenario:</U></B> GCd is blocked until a threads sends a GC
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request. </P>
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<OL>
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<LI>GCd scans through its list of active threads, and sends each a <I>MIB
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request</I>. </LI>
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<LI>It then blocks until all MIB blocks are received. GCd ignores further
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GC request messages it get. </LI>
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<LI>At this point all threads are blocked. The GCd has gained, therefore,
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exclusive access to the hplims. The GCd now marks all reachable cells,
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inspecting each MIB block for immediately reachable cells and proceeding
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recursively. Then, it sweeps. </LI>
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<LI>If needed, the GCd allocated a fresh hplim. </LI>
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<LI>GCd sends every thread a message allowing it to resume. Then it blocks
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waiting for the next time. </LI>
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</OL>
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<P><B><U>Scenario 1:</U></B> A thread needs to allocate a cell but can't.
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</P>
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<OL>
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<LI>The thread sends GCd a<I> GC request message.</I> </LI>
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<LI>Then it suspends until GCd sends it an <I>MIB request</I>. </LI>
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<LI>When one arrives, the thread generates and sends a MIB block to the
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GCd. </LI>
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<LI>And blocks again until GCd notifies it that GC is done. </LI>
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<LI>At this point free cells are available and the computation can resume.
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</LI>
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</OL>
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<P><B><U>Scenario 2:</U></B> A thread receives a<I> MIB request</I>. This
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may happen within a computation or when considered otherwise blocked -
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waiting for input, for example. </P>
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<OL>
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<LI>The thread generates and sends a MIB block to the GCd. </LI>
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<LI>And blocks until GCd notifies it that GC is done. </LI>
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</OL>
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<P>The important thing to note about this protocol is its indifference
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to the GC "initiator". Several threads can "initiate"
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GC, and each request is "satisfied", although of course only
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one GC takes place. The GCd itself is unaware of the initiating thread
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identity, and completely ignores any further GC requests. It treats all
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threads identically. This is important because it allows each thread meeting
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a low memory condition to initiate GC immediately. This is in fact the
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mechanism which saves us from explicitly checking for a third-party GC
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request during computation: If a thread runs out of memory, the <TT><FONT FACE="Courier New">freelist</FONT></TT>
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variable is kept at "out of memory" state, causing any other
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thread trying to allocate memory to initiate GC as well. This simplifies
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the GC protocol (technically, if not conceptually), and does it with almost
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no affect on computation speed.<BR>
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</P>
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<H2><A NAME="BOARDS"></A>BOARDS<BR>
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</H2>
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<P>A board or a view as it called in MFC is the environment where a turtle
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moves and interact with. It hold two main data structures. The first is
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the bitmap of the drawing. It is a 800X600 bitmap. Every time a turtle
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draws on the board it makes its pen the active pen on the board and draws
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on it. Every time the picture needs refreshing (as signaled by the operating
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system) it is the board's duty to copy the relevant section from the bitmap
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to the screen. The second data object is the turtles list, an expandable
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array of turtles which holds pointers to all turtles on the specific board.<BR>
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</P>
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<P>The most important part in the board's work is to notify the turtles
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on any event that happened on the board such as drawing, changing background
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or moving of a turtle. If for example a user draws a line on the board,
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the board (after drawing the line) goes through the turtles list and tell
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each one that some event happened at a rectangle that contains the line.
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Each turtle will decide if this has any importance to it or not.<BR>
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</P>
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<P>Apart from that, the board handles all the user interface from the menus
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and the toolbar. The most obvious example is the move turtle button, which,
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when pressed, causes the board to find a turtle close enough to the click's
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location. Then, on every movement of the mouse it gives the turtle a command
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to move to this point. <BR>
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</P>
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<P>In order to support scrolling of the picture, we derived the <B>CBoardView</B>
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class from <B>CScrollView</B>. The interface with the interpreter threads
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is done via a message queue. The main function is <TT><FONT FACE="Courier New">ReadAndEval,</FONT></TT>
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which gets a message and then interprets its and act upon the result.<BR>
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</P>
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<H2><A NAME="TURTLES"></A>TURTLES<BR>
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</H2>
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<P>In addition to a pointer to its current board, and to inner-state variable
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which affects its graphical aspects, every turtle holds an expandable array
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of interrupts. When a turtle gets from the window that a message signifying
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that some change has happened, it sends this change to each of its interrupts
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(only if the interrupt flag is on) and the interrupt is responsible to
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send an appropriate message if necessary. The turtle's location are stored
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as floating points on x,y axes, to allow for accuracy on the turtle's location
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and heading.<BR>
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</P>
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<P>The turtles interacts with the interpreter thread using a message queue.
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As with the board, the main function here is the <TT><FONT FACE="Courier New">ReadAndEval,</FONT></TT>
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which translates these messages to valid function calls.<BR>
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</P>
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<H2><A NAME="VISION"></A>VISION<BR>
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</H2>
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<P>Every turtle holds a pointer to the bitmap it is drawing on. When it
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is "looking" for a color it calculates the minimal rectangle
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that holds the desired area. Then it iterates on all the pixels in this
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rectangle. First it checks if the pixel is in the vision area using the
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sign rule to determine if a point is clockwise or anti clockwise from a
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line, and then check for distance. If the point is in the relevant area
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the turtle gets its color from the bitmap and compares it with the sought
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color. <BR>
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</P>
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<P>When looking for a specific turtle, the turtle gets this turtle's position
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and calculates if this location is in the relevant area using the same
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algorithm. When looking for any turtle, the turtle passes the relevant
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arguments to the view, which then uses the same algorithm for each turtle
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on its turtles array.<BR>
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</P>
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<H2><A NAME="INTERRUPTS"></A>INTERRUPTS<BR>
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</H2>
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<P>Each turtle holds an expandable array of interrupts. Each interrupts
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is an object that is much like a turtle vision. The interrupts has the
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view area argument and what it is looking for. It also has the message
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it needs to send and a pointer to the given queue. When the turtle notifies
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the interrupt that a change has happened, the interrupt first checks if
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the change is an area which is of interest to it. If so it calls the turtle
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look function with its location and the sought object. According to the
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turtle's answer and to the data stored inside the interrupt, the interrupt
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sends the needed message, if any.<BR>
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<BR>
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</P>
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<H2><A NAME="CLASS"></A>CLASS ORGANIZATION<BR>
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</H2>
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<P><CENTER><IMG border=0 SRC="img00012.gif" HEIGHT=626 WIDTH=693><BR>
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</CENTER>
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</P>
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<P>Message-passing mechanisms:<BR>
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</P>
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<P>
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