Merge branch 'task1/merged/priority-scheduling' into 'task1/priority-scheduling'

# Conflicts:
#   .gitignore
#   src/threads/synch.c
#   src/threads/thread.c

.gitignore

@@ -4,6 +4,13 @@
 #ignore pdf files (just keep source files)
 *.pdf
 
+#ignore Mac OS generated files
+.DS_Store
+
+#ignore code editor generated directories
+.idea
+.vscode
+
 #ignore junk files from latex output
 *.out
 *.log
@@ -24,6 +31,3 @@
 *.nav
 *.toc
 
-#ignore files from CLion/VSCode IDEs
-.idea
-.vscode

src/threads/fixed-point.h

@@ -1,95 +0,0 @@
-#include <stdint.h>
-#ifndef FIXED_POINT_H
-#define FIXED_POINT_H
-
-typedef struct
-{
-  int32_t raw;
-} fp32_t;
-
-/* Fixed Point Arithmetic bit count constants */
-#define NUM_FRAC_BITS 14
-#define NUM_INT_BITS (31 - NUM_FRAC_BITS)
-#define CONVERSION_CONST (1 << NUM_FRAC_BITS) /* f = 2^q, (2^20) */
-
-/* Fixed Point Arithmetic conversion operations */
-/* Converts an integer n to a fixed point number */
-inline fp32_t
-int_to_fp (int32_t n)
-{
-  return (fp32_t){ n * CONVERSION_CONST };
-}
-
-/* Handles conversion of fixed point to integer. First version truncates, second one rounds */
-inline int32_t
-fp_floor (fp32_t x)
-{
-  return x.raw / CONVERSION_CONST;
-}
-
-inline int32_t
-fp_round (fp32_t x)
-{
-  if (x.raw >= 0)
-    return (x.raw + CONVERSION_CONST / 2) / CONVERSION_CONST;
-  else
-    return (x.raw - CONVERSION_CONST / 2) / CONVERSION_CONST;
-}
-
-/* Add two fixed points */
-inline fp32_t
-fp_add (fp32_t x, fp32_t y)
-{
-  return (fp32_t){ x.raw + y.raw };
-}
-
-/* Subtract two fixed points */
-inline fp32_t
-fp_sub (fp32_t x, fp32_t y)
-{
-  return (fp32_t){ x.raw - y.raw };
-}
-
-/* Add fixed point to integer */
-inline fp32_t
-fp_add_int (fp32_t x, int32_t n)
-{
-  return (fp32_t){ x.raw + n * CONVERSION_CONST };
-}
-
-/* Subtract integer from fixed point */
-inline fp32_t
-fp_sub_int (fp32_t x, int32_t n)
-{
-  return (fp32_t){ x.raw - n * CONVERSION_CONST };
-}
-
-/* Multiple two fixed points */
-inline fp32_t
-fp_mul (fp32_t x, fp32_t y)
-{
-  return (fp32_t){ ((int64_t)x.raw) * y.raw / CONVERSION_CONST };
-}
-
-/* Divide two fixed points */
-inline fp32_t
-fp_div (fp32_t x, fp32_t y)
-{
-  return (fp32_t){ ((int64_t)x.raw) * CONVERSION_CONST / y.raw };
-}
-
-/* Multiply fixed point and integer */
-inline fp32_t
-fp_mul_int (fp32_t x, int32_t n)
-{
-  return (fp32_t){ x.raw * n };
-}
-
-/* Divide fixed point by integer */
-inline fp32_t
-fp_div_int (fp32_t x, int32_t n)
-{
-  return (fp32_t){ x.raw / n };
-}
-
-#endif //FIXED_POINT_H

src/threads/synch.c

@@ -68,7 +68,8 @@ sema_down (struct semaphore *sema)
   old_level = intr_disable ();
   while (sema->value == 0) 
     {
-      list_push_back (&sema->waiters, &thread_current ()->elem);
+      list_insert_ordered(&sema->waiters, &thread_current ()->elem,
+                          priority_more, NULL);
       thread_block ();
     }
   sema->value--;
@@ -113,11 +114,18 @@ sema_up (struct semaphore *sema)
   ASSERT (sema != NULL);
 
   old_level = intr_disable ();
-  if (!list_empty (&sema->waiters)) 
-    thread_unblock (list_entry (list_pop_front (&sema->waiters),
-                                struct thread, elem));
+
+  /* Wake up (unblock) the highest priority thread from the waiters list */
+  struct thread *t = NULL;
+  if (!list_empty (&sema->waiters))
+    {
+      t = list_entry (list_pop_front (&sema->waiters), struct thread, elem);
+      thread_unblock (t);
+    }
+
   sema->value++;
   intr_set_level (old_level);
+  thread_yield ();
 }
 
 static void sema_test_helper (void *sema_);
@@ -233,7 +241,6 @@ lock_release (struct lock *lock)
 
   lock->holder = NULL;
   sema_up (&lock->semaphore);
-  thread_yield ();
 }
 
 /* Returns true if the current thread holds LOCK, false
@@ -254,6 +261,54 @@ struct semaphore_elem
     struct semaphore semaphore;         /* This semaphore. */
   };
 
+/* Function that compares the two *semaphores* associated with the provided
+   list_elem structures. [i.e., takes list_elem of semaphore_elem, and]
+   Returns true if the thread associated with the semaphore associated with a_
+   has a higher priority than that of b_.
+
+    If aux is provided, then it is a pointer to an integer representing the
+    priority of the first semaphore. This is useful when the thread has not been
+    sema'd down yet. */
+static bool
+sema_priority_more(const struct list_elem *a_, const struct list_elem *b_,
+                   void *aux)
+{
+  int a_priority, b_priority;
+
+  /* If an aux is provided, then use it as the priority of the first semaphore.
+     Otherwise, get the priority of the first semaphore. */
+  if (aux != NULL)
+    a_priority = *(int *) aux;
+  else
+    {
+      struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
+
+      /* If waiters list is empty, return false (i.e., a has lower priority) */
+      if (list_empty(&a->semaphore.waiters))
+        return false;
+
+      /* Otherwise, get the thread with the highest priority from the waiters
+         list. By design, this is the first one in the list (See sema_down). */
+      struct thread *a_thread =
+        list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
+
+      a_priority = a_thread->priority;
+    }
+
+  struct semaphore_elem *b = list_entry(b_, struct semaphore_elem, elem);
+
+  /* If waiters list is empty, return true (i.e., a has higher priority) */
+  if (list_empty(&b->semaphore.waiters))
+    return true;
+
+  struct thread *b_thread =
+    list_entry(list_front(&b->semaphore.waiters), struct thread, elem);
+
+  b_priority = b_thread->priority;
+
+  return a_priority > b_priority;
+}
+
 /* Initializes condition variable COND.  A condition variable
    allows one piece of code to signal a condition and cooperating
    code to receive the signal and act upon it. */
@@ -265,6 +320,38 @@ cond_init (struct condition *cond)
   list_init (&cond->waiters);
 }
 
+/* Returns true iff the priority of the only thread in the first singleton
+   semaphore is greater than the priority of the only thread in the second
+   singleton semaphore.
+   
+   Where this function is used for insertion in a singleton semaphore list, the
+   third argument may specify a list_elem * to assume corresponds to the thread
+   waiting for the inserting semaphore. For correctness, ensure this thread
+   calls sema_down () for this semaphore before future list accesses. */
+
+static bool
+singleton_sema_priority_greater (const struct list_elem *a, 
+                                 const struct list_elem *b,
+                                 void *insertingThread)
+{
+  struct list_elem *te_a, *te_b;
+  
+  te_b = list_front (
+    &list_entry (b, struct semaphore_elem, elem)->semaphore.waiters);
+
+  if (insertingThread == NULL)
+    {
+      te_a = list_front (
+        &list_entry (a, struct semaphore_elem, elem)->semaphore.waiters);
+    }
+  else
+    {
+      te_a = insertingThread;
+    }
+
+  return thread_priority_greater (te_a, te_b, NULL);
+}
+
 /* Atomically releases LOCK and waits for COND to be signaled by
    some other piece of code.  After COND is signaled, LOCK is
    reacquired before returning.  LOCK must be held before calling
@@ -296,7 +383,14 @@ cond_wait (struct condition *cond, struct lock *lock)
   ASSERT (lock_held_by_current_thread (lock));
   
   sema_init (&waiter.semaphore, 0);
-  list_push_back (&cond->waiters, &waiter.elem);
+
+  /* Insert the semaphore_elem into the waiters list in order of priority.
+     We pass the priority of the current thread as aux to sema_priority_more
+     because the thread has not been sema'd down yet (See sema_priority_more) */
+  int priority = thread_current ()->priority;
+  list_insert_ordered (&cond->waiters, &waiter.elem, sema_priority_more,
+                       &priority);
+
   lock_release (lock);
   sema_down (&waiter.semaphore);
   lock_acquire (lock);

src/threads/thread.c

@@ -4,8 +4,6 @@
 #include <random.h>
 #include <stdio.h>
 #include <string.h>
-#include "devices/timer.h"
-#include "threads/fixed-point.h"
 #include "threads/flags.h"
 #include "threads/interrupt.h"
 #include "threads/intr-stubs.h"
@@ -51,7 +49,6 @@ struct kernel_thread_frame
 static long long idle_ticks;    /* # of timer ticks spent idle. */
 static long long kernel_ticks;  /* # of timer ticks in kernel threads. */
 static long long user_ticks;    /* # of timer ticks in user programs. */
-static fp32_t load_avg = { 0 }; /* System load average. */
 
 /* Scheduling. */
 #define TIME_SLICE 4            /* # of timer ticks to give each thread. */
@@ -67,14 +64,9 @@ static void kernel_thread (thread_func *, void *aux);
 static void idle (void *aux UNUSED);
 static struct thread *running_thread (void);
 static struct thread *next_thread_to_run (void);
-static void init_thread (struct thread *, const char *name, int nice,
-                         int priority, fp32_t recent_cpu);
+static void init_thread (struct thread *, const char *name, int priority);
 static bool is_thread (struct thread *) UNUSED;
 static void *alloc_frame (struct thread *, size_t size);
-static int calculate_bsd_priority (fp32_t recent_cpu, int nice);
-static void thread_update_recent_cpu (struct thread *t, void *aux UNUSED);
-static bool thread_priority_less (const struct list_elem *a,
-                                  const struct list_elem *b, void *aux UNUSED);
 static void schedule (void);
 void thread_schedule_tail (struct thread *prev);
 static tid_t allocate_tid (void);
@@ -103,9 +95,7 @@ thread_init (void)
 
   /* Set up a thread structure for the running thread. */
   initial_thread = running_thread ();
-  fp32_t initial_thread_recent_cpu = { 0 };
-  init_thread (initial_thread, "main", NICE_DEFAULT, PRI_DEFAULT,
-               initial_thread_recent_cpu);
+  init_thread (initial_thread, "main", PRI_DEFAULT);
   initial_thread->status = THREAD_RUNNING;
   initial_thread->tid = allocate_tid ();
 }
@@ -155,28 +145,6 @@ thread_tick (void)
   else
     kernel_ticks++;
 
-  /* Update system load_avg and all threads recent_cpu every second. */
-  int64_t ticks = timer_ticks ();
-  if (thread_mlfqs && (ticks % TIMER_FREQ == 0))
-    {
-      size_t ready = threads_ready ();
-      if (t != idle_thread)
-        ready++;
-      fp32_t old_coeff = fp_mul (fp_div_int (int_to_fp (59), 60), load_avg);
-      fp32_t new_coeff = fp_div_int (int_to_fp (ready), 60);
-      load_avg = fp_add (old_coeff, new_coeff);
-
-      thread_foreach (thread_update_recent_cpu, NULL);
-    }
-
-  /* Update current thread's recent_cpu. */
-  if (thread_mlfqs && (t != idle_thread))
-    {
-      t->recent_cpu = fp_add_int (t->recent_cpu, 1);
-      if (ticks % 4 == 0) // recent_cpu was updated, update priority.
-        t->priority = calculate_bsd_priority (t->recent_cpu, t->nice);
-    }
-
   /* Enforce preemption. */
   if (++thread_ticks >= TIME_SLICE)
     intr_yield_on_return ();
@@ -224,8 +192,7 @@ thread_create (const char *name, int priority,
     return TID_ERROR;
 
   /* Initialize thread. */
-  struct thread *pt = thread_current ();
-  init_thread (t, name, pt->nice, priority, pt->recent_cpu);
+  init_thread (t, name, priority);
   tid = t->tid = allocate_tid ();
 
   /* Prepare thread for first run by initializing its stack.
@@ -252,6 +219,11 @@ thread_create (const char *name, int priority,
 
   /* Add to run queue. */
   thread_unblock (t);
+  thread_yield ();
+
+  /* Yield if the new thread has a higher priority than the current thread. */
+  if (priority > thread_get_priority ())
+    thread_yield ();
 
   return tid;
 }
@@ -289,10 +261,10 @@ thread_unblock (struct thread *t)
 
   old_level = intr_disable ();
   ASSERT (t->status == THREAD_BLOCKED);
-  if (thread_mlfqs)
-    list_insert_ordered (&ready_list, &t->elem, thread_priority_less, NULL);
-  else
-    list_push_back (&ready_list, &t->elem);
+
+  /* Insert the thread back into the ready list in priority order. */
+  list_insert_ordered(&ready_list, &t->elem, priority_more, NULL);
+
   t->status = THREAD_READY;
   intr_set_level (old_level);
 }
@@ -362,14 +334,11 @@ thread_yield (void)
   ASSERT (!intr_context ());
 
   old_level = intr_disable ();
-  if (cur != idle_thread)
-    {
-      if (thread_mlfqs)
-        list_insert_ordered (&ready_list, &cur->elem, thread_priority_less,
-                             NULL);
-      else
-        list_push_back (&ready_list, &cur->elem);
-    }
+
+  /* Insert the thread back into the ready list in priority order. */
+  if (cur != idle_thread) 
+    list_insert_ordered(&ready_list, &cur->elem, priority_more, NULL);
+
   cur->status = THREAD_READY;
   schedule ();
   intr_set_level (old_level);
@@ -392,13 +361,29 @@ thread_foreach (thread_action_func *func, void *aux)
     }
 }
 
+/* Function that compares the two threads associated with the provided
+   list_elem structures. Returns true if the thread associated with a_ has
+   a higher priority than that of b_. */
+bool
+priority_more (const struct list_elem *a_, const struct list_elem *b_,
+                   void *aux UNUSED)
+{
+  struct thread *a = list_entry (a_, struct thread, elem);
+  struct thread *b = list_entry (b_, struct thread, elem);
+
+  return a->priority > b->priority;
+}
+
 /* Sets the current thread's priority to NEW_PRIORITY. */
 void
 thread_set_priority (int new_priority) 
 {
-  if (thread_mlfqs)
-    return;
+  ASSERT (PRI_MIN <= new_priority && new_priority <= PRI_MAX);
+  int old_priority = thread_get_priority ();
+
   thread_current ()->priority = new_priority;
+  if (new_priority < old_priority)
+    thread_yield ();
 }
 
 /* Returns the current thread's priority. */
@@ -408,54 +393,35 @@ thread_get_priority (void)
   return thread_current ()->priority;
 }
 
-/* Updates recent_cpu for a thread. */
-static void
-thread_update_recent_cpu (struct thread *t, void *aux UNUSED)
-{
-  fp32_t curr_recent_cpu = t->recent_cpu;
-  fp32_t dbl_load_avg = fp_mul_int (load_avg, 2);
-  fp32_t recent_cpu_coeff
-      = fp_div (dbl_load_avg, fp_add_int (dbl_load_avg, 1));
-  t->recent_cpu
-      = fp_add_int (fp_mul (recent_cpu_coeff, curr_recent_cpu), t->nice);
-  // recent_cpu was updated, update priority.
-  t->priority = calculate_bsd_priority (t->recent_cpu, t->nice);
-}
-
 /* Sets the current thread's nice value to NICE. */
 void
-thread_set_nice (int nice)
+thread_set_nice (int nice UNUSED) 
 {
-  ASSERT (NICE_MIN <= nice && nice <= NICE_MAX);
-
-  struct thread *t = thread_current ();
-  t->nice = nice;
-  int priority = calculate_bsd_priority (t->recent_cpu, t->nice);
-  struct thread *next_t
-      = list_entry (list_begin (&ready_list), struct thread, elem);
-  if (priority < next_t->priority)
-    thread_yield ();
+  /* Not yet implemented. */
 }
 
 /* Returns the current thread's nice value. */
 int
 thread_get_nice (void) 
 {
-  return thread_current ()->nice;
+  /* Not yet implemented. */
+  return 0;
 }
 
 /* Returns 100 times the system load average. */
 int
 thread_get_load_avg (void) 
 {
-  return fp_round (fp_mul_int (load_avg, 100));
+  /* Not yet implemented. */
+  return 0;
 }
 
 /* Returns 100 times the current thread's recent_cpu value. */
 int
 thread_get_recent_cpu (void) 
 {
-  return fp_round (fp_mul_int (thread_current ()->recent_cpu, 100));
+  /* Not yet implemented. */
+  return 0;
 }
 
 /* Idle thread.  Executes when no other thread is ready to run.
@@ -531,8 +497,7 @@ is_thread (struct thread *t)
 /* Does basic initialization of T as a blocked thread named
    NAME. */
 static void
-init_thread (struct thread *t, const char *name, int nice, int priority,
-             fp32_t recent_cpu)
+init_thread (struct thread *t, const char *name, int priority)
 {
   enum intr_level old_level;
 
@@ -544,10 +509,7 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->status = THREAD_BLOCKED;
   strlcpy (t->name, name, sizeof t->name);
   t->stack = (uint8_t *) t + PGSIZE;
-  t->priority
-      = thread_mlfqs ? calculate_bsd_priority (recent_cpu, nice) : priority;
-  t->nice = nice;
-  t->recent_cpu = recent_cpu;
+  t->priority = priority;
   t->magic = THREAD_MAGIC;
 
   old_level = intr_disable ();
@@ -628,30 +590,8 @@ thread_schedule_tail (struct thread *prev)
     }
 }
 
-/* Calculates BSD priority for a thread */
-static int
-calculate_bsd_priority (fp32_t recent_cpu, int nice)
-{
-  int priority = PRI_MAX - (fp_round (recent_cpu) / 4) - (nice * 2);
-  if (priority < PRI_MIN)
-    return PRI_MIN;
-  if (priority > PRI_MAX)
-    return PRI_MAX;
-  return priority;
-}
-
-/* Returns true if thread a's priority is strictly greater than
-   thread b's priority. */
-static bool
-thread_priority_less (const struct list_elem *a, const struct list_elem *b,
-                      void *aux UNUSED)
-{
-  struct thread *ta = list_entry (a, struct thread, elem);
-  struct thread *tb = list_entry (b, struct thread, elem);
-  return ta->priority > tb->priority;
-}
-
-/* ss's state must have been changed from
+/* Schedules a new process.  At entry, interrupts must be off and
+   the running process's state must have been changed from
    running to some other state.  This function finds another
    thread to run and switches to it.
 
@@ -687,6 +627,17 @@ allocate_tid (void)
   return tid;
 }
 
+/* Returns true iff the priority of the first list element's thread is greater
+   than that of the second list element's thread. */
+bool 
+thread_priority_greater (const struct list_elem *a, const struct list_elem *b,
+                         void *aux UNUSED)
+{
+  struct thread *ta = list_entry (a, struct thread, elem);
+  struct thread *tb = list_entry (b, struct thread, elem);
+  return ta->priority > tb->priority;
+}
+
 /* Offset of `stack' member within `struct thread'.
    Used by switch.S, which can't figure it out on its own. */
 uint32_t thread_stack_ofs = offsetof (struct thread, stack);

src/threads/thread.h

@@ -4,7 +4,6 @@
 #include <debug.h>
 #include <list.h>
 #include <stdint.h>
-#include "threads/fixed-point.h"
 
 /* States in a thread's life cycle. */
 enum thread_status
@@ -25,10 +24,6 @@ typedef int tid_t;
 #define PRI_DEFAULT 31                  /* Default priority. */
 #define PRI_MAX 63                      /* Highest priority. */
 
-#define NICE_MIN -20   /* Lowest niceness. */
-#define NICE_DEFAULT 0 /* Default niceness. */
-#define NICE_MAX 20    /* Highest niceness. */
-
 /* A kernel thread or user process.
 
    Each thread structure is stored in its own 4 kB page.  The
@@ -98,10 +93,6 @@ struct thread
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
-    /* MLFQS items */
-    int nice;                           /* Nice value for this thread */
-    fp32_t recent_cpu;                  /* Amount of time this process received */
-
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
@@ -140,6 +131,8 @@ void thread_yield (void);
 typedef void thread_action_func (struct thread *t, void *aux);
 void thread_foreach (thread_action_func *, void *);
 
+bool priority_more (const struct list_elem *a_, const struct list_elem *b_,
+                    void *aux UNUSED);
 int thread_get_priority (void);
 void thread_set_priority (int);
 
@@ -148,4 +141,7 @@ void thread_set_nice (int);
 int thread_get_recent_cpu (void);
 int thread_get_load_avg (void);
 
+/* Returns true iff the priority of the first list element's thread is greater
+   than that of the second list element's thread. */
+list_less_func thread_priority_greater;
 #endif /* threads/thread.h */