[examples] comments in multi-threaded parse example

examples/integration/multi_threaded.c

@@ -1,7 +1,17 @@
 /* 
 ** Example: multi threaded parse
 **
-** TODO
+** This example shows how to arrange a multi-threaded Metadesk parser. The
+** strategy used is to make each *.mdesk file into an independent task.
+**
+** The goal in this example is to have all of the files parsed and visible at
+** the same time by the end. Another conceivable way to make a multi-threaded
+** Metadesk parser would be to use each parse as a temporary and throw them
+** away after extracting out the important parts.
+**
+** This example depends directly on OS headers, intrinsics, and functions,
+** so you'll have to re-interpret those parts to whatever OS you are targeting.
+**
 */
 
 //~ includes and globals //////////////////////////////////////////////////////
@@ -9,7 +19,11 @@
 #include "md.h"
 #include "md.c"
 
-#include <Windows.h>
+#if MD_OS_WINDOWS
+# include <Windows.h>
+#else
+# error Not implemented for this OS
+#endif
 
 //~ multi-threaded parse setup ////////////////////////////////////////////////
 
@@ -18,21 +32,35 @@
 #if MD_OS_WINDOWS
 # define atomic_inc_then_eval_u64(p) InterlockedIncrement64((LONG64*)p)
 #else
-# error Not implemented for this compiler
+# error Not implemented for this OS
 #endif
 
+// @notes We use one structure that describes the whole multi-threaded parse
+//  work load, and carries the shared state values for synchronizing the
+//  worker threads.
 typedef struct TaskData
 {
+    // the set of tasks
     MD_u64 task_max;
     char **tasks;
     
+    // synchronization
     volatile MD_u64 task_counter;
     volatile MD_u64 thread_counter;
 } TaskData;
 
+// @notes Each thread gets it's own thread data.
+//
+// The library doesn't make any of it's data structures thread safe but all of
+// the calls are thread safe so long as different threads are operating on
+// different data structures. So we arrange for each worker thread to get it's
+// own arena, and set of lists for collecting parse results.
 typedef struct ThreadData
 {
+    // shared
     TaskData *task;
+    
+    // unique-to-thread
     MD_Arena *arena;
     MD_Node *list;
     MD_MessageList errors;
@@ -44,17 +72,21 @@ parse_worker_loop(ThreadData *thread_data)
     TaskData *task = thread_data->task;
     for (;;)
     {
+        // atomically get the next unhandled task index
         MD_u64 task_index = atomic_inc_then_eval_u64(&task->task_counter) - 1;
         if (task_index >= task->task_max)
         {
             break;
         }
+        
+        // load and parse the file specified by this task.
         MD_String8 file_name = MD_S8CString(task->tasks[task_index]);
         MD_ParseResult parse = MD_ParseWholeFile(thread_data->arena, file_name);
         MD_MessageListConcat(&thread_data->errors, &parse.errors);
         MD_PushNewReference(thread_data->arena, thread_data->list, parse.node);
     }
     
+    // atomically count the threads as they finish
     atomic_inc_then_eval_u64(&task->thread_counter);
 }
 
@@ -98,6 +130,7 @@ main(int argc, char **argv)
     }
     
     // launch the worker threads
+    //  (no worker thread 0)
     for (int i = 1; i < THREAD_COUNT; i += 1)
     {
 #if MD_OS_WINDOWS
@@ -108,6 +141,7 @@ main(int argc, char **argv)
 #endif
     }
     
+    // let the main thread act as thread 0
     parse_worker_loop(&threads[0]);
     
     // wait for all threads to be finished
@@ -148,6 +182,24 @@ main(int argc, char **argv)
         }
     }
     
+    // @notes In this example we are done, but in some cases it might be useful
+    //  to merge the results of a multi-threaded parse to make it as if it was
+    //  a single threaded parse, and it turns out this is quite easy to do from
+    //  here.
+    //
+    //  We merge all of the arenas together so that we can handle all of the
+    //  memory with a single arena moving forward. This relies on the default
+    //  arena implementation which has an 'absorb' operation. If you plug in 
+    //  your own arena via oerrides it's up to that implementation what your
+    //  options are for this part.
+    //
+    //  The list of roots and the list of messages can be concatenated.
+    //
+    //  All of these operations (absorb and the concat operations) move
+    //  information out of the right hand operand and into the left hand
+    //  operand, leaving the individual pieces from the worker threads invalid
+    //  after the merge.
+    
     // combine results
     MD_Arena *arena = threads[0].arena;
     MD_Node *list = threads[0].list;