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2022.12.05 02:57:43
#| To write a tracing garbage collector in Common Lisp, you would need to do the following: Define a set of objects to be traced, known as the root set. This could include objects that are currently in use by the program, such as global variables and objects referenced by local variables on the stack. Traverse the root set and mark all objects that are reachable from it. You can do this by using a depth-first search or a breadth-first search algorithm. Once all reachable objects have been marked, traverse the entire set of objects in the heap and collect any objects that are not marked. These are the objects that are no longer in use and can be safely garbage collected. After collecting the garbage, unmark all of the objects that were marked in step 2, so that they can be properly traced in the next garbage collection cycle. Here is some example code that shows how this might work in Common Lisp: |# (defun mark-and-sweep (root-set) "Trace the root set and collect any garbage objects." (dolist (obj root-set) (mark-object obj)) (let ((unreachable (remove-if #'marked-p heap))) (dolist (obj unreachable) (collect-object obj)) (unmark-all))) (defun mark-object (obj) "Mark an object as reachable." (unless (marked-p obj) (setf (marked-p obj) t) (dolist (child (children obj)) (mark-object child)))) (defun collect-object (obj) "Collect an object that is no longer in use." (setf (heap obj) nil)) (defun unmark-all () "Unmark all objects in the heap." (dolist (obj heap) (setf (marked-p obj) nil))) #| How could I avoid stack overflow in mark-object? One way to avoid stack overflow in the mark-object function is to use a different algorithm for marking objects. In the example code above, the mark-object function uses a recursive depth-first search algorithm to traverse the object graph and mark reachable objects. This can cause stack overflow if the object graph is very large or contains cyclic references. To avoid this problem, you could use a different algorithm for marking objects. For example, you could use a non-recursive iterative algorithm, such as a breadth-first search, which does not require the use of a stack. This would allow you to mark objects in the object graph without running out of stack space. Here is an example of how you might implement a non-recursive version of the mark-object function using a breadth-first search: |# (defun mark-object (obj) "Mark an object as reachable using a breadth-first search." (let ((queue (list obj))) (while queue (let ((curr (pop queue))) (unless (marked-p curr) (setf (marked-p curr) t) (dolist (child (children curr)) (push child queue))))))) #| This implementation of mark-object uses a queue to store the objects that need to be visited. It processes the objects in the queue one at a time, marking them as reachable and adding their children to the queue. This allows the function to traverse the object graph without using a stack, so it will not cause stack overflow. [Ed: This is a stack. But okay, sure.] I've heard software prefetching can help while tracing, because it has bad locality of reference. Software prefetching is a technique that can be used to improve the performance of algorithms that have poor locality of reference. In the context of a tracing garbage collector, software prefetching can be used to improve the performance of the tracing algorithm by pre-loading objects into the cache before they are accessed by the tracing algorithm. Software prefetching works by using a special instruction that tells the processor to load a specific memory address into the cache. This can be done before the memory address is actually accessed by the program, allowing the processor to start loading the data into the cache in the background while the program continues to execute. When the program eventually accesses the memory address, the data will already be in the cache and can be accessed more quickly. To use software prefetching in a tracing garbage collector, you would need to insert prefetch instructions at strategic points in the tracing algorithm. For example, you could insert a prefetch instruction before each call to the mark-object function, to pre-load the object that is about to be marked into the cache. This could improve the performance of the tracing algorithm by reducing the number of cache misses that occur during tracing. Here is an example of how you might use software prefetching in the mark-object function: |# (defun mark-object (obj) "Mark an object as reachable using a breadth-first search and software prefetching." (let ((queue (list obj))) (while queue (let ((curr (pop queue))) (prefetch curr) (unless (marked-p curr) (setf (marked-p curr) t) (dolist (child (children curr)) (push child queue))))))) #| In this example, the mark-object function uses a prefetch instruction to pre-load the curr object into the cache before it is accessed by the marked-p function. This can improve the performance of the tracing algorithm by reducing the number of cache misses that occur during tracing. Keep in mind that using software prefetching can have trade-offs. For example, prefetching can increase the amount of memory traffic and cache pollution, which can reduce the overall performance of the system. Therefore, it is important to carefully consider the benefits and drawbacks of using prefetching before implementing it in a tracing garbage collector. Shouldn't the prefetch occur some time before the object is used, so that there is time for the object to be loaded into cache? Yes, you are correct. In order for software prefetching to be effective, the prefetch instruction should be executed some time before the object is actually accessed by the program. This allows the processor to start loading the data into the cache in the background, so that it will be available when the program needs to access it. In the example code above, the prefetch instruction is executed immediately before the object is accessed by the marked-p function. This may not be early enough for the data to be loaded into the cache before it is needed. To improve the performance of the prefetching, you could move the prefetch instruction earlier in the function, so that it is executed before the object is added to the queue. Here is an example of how you might do this: |# (defun mark-object (obj) "Mark an object as reachable using a breadth-first search and software prefetching." (let ((queue (list obj))) (while queue (let ((curr (pop queue))) (dolist (child (children curr)) (prefetch child) (push child queue)) (unless (marked-p curr) (setf (marked-p curr) t)))))) #| In this example, the prefetch instruction is executed before the child object is added to the queue. This allows the processor to start loading the data into the cache before it is needed, increasing the likelihood that it will be available when the object is eventually accessed by the marked-p function. Of course, the best place to insert the prefetch instruction will depend on the specific details of your tracing algorithm and the characteristics of your system. You may need to experiment with different prefetching strategies to find the one that works best for your situation. [Ed: the AI has invented Boehm's prefetch-on-grey.] [Some boring discussion about where the WHILE form came from elided. The AI decides to make WHILE a macro implemented using LOOP.] Also, shouldn't mark-object only traverse children if the object isn't marked already? Yes, you are correct. In the mark-object function shown in the previous examples, it is possible for the function to traverse the children of an object multiple times if the object is marked more than once. This can happen if the object has multiple references from the root set, or if it is referenced by other objects that are also reachable from the root set. To avoid this problem, you can modify the mark-object function to only traverse the children of an object if the object has not already been marked. This can be done by adding an additional check at the beginning of the mark-object function, before the dolist form that traverses the children of the object. Here is an example of how you might modify the mark-object function to avoid traversing the children of an object multiple times: |# (defun mark-object (obj) "Mark an object as reachable using a breadth-first search." (let ((queue (list obj))) (while queue (let ((curr (pop queue))) (unless (marked-p curr) (setf (marked-p curr) t) (dolist (child (children curr)) (push child queue))))))) #| In this example, the mark-object function checks if the curr object has been marked before traversing its children. If the object has already been marked, the dolist form is not executed and the function moves on to the next object in the queue. This avoids traversing the children of an object multiple times, which can improve the performance of the tracing algorithm. Of course, this is just one possible solution to the problem. There may be other ways to avoid traversing the children of an object multiple times, depending on the specific details of your tracing algorithm. You may need to experiment with different approaches to find the one that works best for your situation. Where would you put the software prefetch now? If you want to use software prefetching in the mark-object function, you would need to insert the prefetch instruction at a point in the function where it can pre-load the data into the cache before it is needed. In the modified mark-object function shown above, one possible place to insert the prefetch instruction is before the push form that adds the child object to the queue. This allows the prefetch instruction to be executed before the object is added to the queue, giving the processor time to start loading the data into the cache before it is needed. Here is an example of how you might modify the mark-object function to use software prefetching: |# (defun mark-object (obj) "Mark an object as reachable using a breadth-first search and software prefetching." (let ((queue (list obj))) (while queue (let ((curr (pop queue))) (unless (marked-p curr) (setf (marked-p curr) t) (dolist (child (children curr)) (prefetch child) (push child queue))))))) #| In this example, the mark-object function uses a prefetch instruction before each call to the push form. This causes the processor to start loading the data for each child object into the cache before it is needed, which can improve the performance of the tracing algorithm by reducing the number of cache misses that occur during tracing. Of course, this is just one possible way to use software prefetching in the mark-object function. There may be other places in |#
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