Get rid of permutation table in dynamic Maglev

src/lib/maglevdyn.c

@@ -17,7 +17,7 @@ unsigned magDataDyn_len(unsigned M, unsigned N)
 	if (M < 3)
 		M = 3;
 	M = primeBelow(M);
-	return sizeof(struct MagDataDynInternal)  + sizeof(unsigned) * (3 + M * N)
+	return sizeof(struct MagDataDynInternal) + sizeof(unsigned) * 3
 		+ sizeof(int) * (M + N);
 }
 
@@ -37,72 +37,71 @@ void magDataDyn_init(unsigned M, unsigned N, void* mem, unsigned len)
 
 	struct MagDataDyn m;
 	magDataDyn_map(&m, mem);
-	for (int i = 0; i < m.N; i++) {
-		unsigned offset = rand() % m.M;
-		unsigned skip = rand() % (m.M - 1) + 1;
-		unsigned* row = m.permutation[i];
-		for (unsigned j = 0; j < m.M; j++) {			
-			row[j] = (offset + j * skip) % m.M;
-		}
-		m.active[i] = -1;
-	}
-	magDataDyn_populate(&m);
-	magDataDyn_free(&m);
+	memset(m.active, 0xff, sizeof(int)*N);
+	memset(m.lookup, 0xff, sizeof(int)*M);
 }
 
 void magDataDyn_map(struct MagDataDyn* m, void* mem)
 {
 	struct MagDataDynInternal* mi = mem;
 	m->M = mi->M;
 	m->N = mi->N;
-	unsigned offset = sizeof(struct MagDataDynInternal);
-	m->lookup = mem + offset;
-	offset += (m->M * sizeof(int));
-	m->permutation = malloc(m->N * sizeof(unsigned*));
-	if (m->permutation == NULL)
-		die("Out of mem\n");
-	unsigned i;
-	for (i = 0; i < m->N; i++) {
-		m->permutation[i] = mem + offset;
-		offset += (m->M * sizeof(unsigned));
-	}
-	m->active = mem + offset;	   
-}
-void magDataDyn_free(struct MagDataDyn* m)
-{
-	free(m->permutation);
+	m->lookup = mem + sizeof(struct MagDataDynInternal);
+	m->active = mem + sizeof(struct MagDataDynInternal) + sizeof(int) * m->M;
 }
 
+struct ActiveTarget {
+	int skip;
+	int c;
+	int idx;
+};
+
+
 void magDataDyn_populate(struct MagDataDyn* d)
 {
-	for (int i = 0; i < d->M; i++) {
-		d->lookup[i] = -1;
-	}
+	struct ActiveTarget* activeTargets = (struct ActiveTarget*) malloc(d->N * sizeof(struct ActiveTarget));
+	if (activeTargets == NULL) die("Out of memory activeTargets");
+	int num_targets = 0;
 
-	// Corner case; no active targets
-	unsigned nActive = 0;
 	for (int i = 0; i < d->N; i++) {
-		if (d->active[i] >= 0) nActive++;
+		int offset = rand();
+		int skip = rand();
+		if (d->active[i] >= 0) {
+			activeTargets[num_targets].idx = i;
+			activeTargets[num_targets].c = offset % d->M;
+			// The old algorithm went "upwards" with random skip values in [1, M-1]
+			// and the next element in the permutation table was given by current+skip % M
+			// But we can compute the next element much faster if we go "downwards",
+			// using another skip' value such that skip + skip' == M
+			// The two methods yield the same permutation sequence.
+			activeTargets[num_targets].skip = d->M - ((skip % (d->M - 1)) + 1);
+			num_targets++;
+		}
 	}
-	if (nActive == 0) return;
+	if (num_targets < 2) { // Corner cases: no active targets or just 1 active target
+		int w = num_targets == 0 ? -1 : activeTargets[0].idx;
+		for (int i = 0; i < d->M; i++) {
+			d->lookup[i] = w;
+		}
+		free(activeTargets);
+		return;
+	}
+
+	int* tmpLookup = (int*) malloc(d->M * sizeof(int));
+	if (tmpLookup == NULL) die ("Out of memory tmpLookup");
+	memset(tmpLookup, 0xff, sizeof(int)*d->M);
 
-	unsigned next[d->N], c = 0;
-	memset(next, 0, sizeof(next));
-	unsigned n = 0;
-	unsigned* row;
-	for (;;) {
-		for (int i = 0; i < d->N; i++) {
-			if (d->active[i] < 0) continue; /* Target not active */
-			row = d->permutation[i];
-			c = row[next[i]];
-			while (d->lookup[c] >= 0) {
-				next[i] = next[i] + 1;
-				c = row[next[i]];
-			}
-			d->lookup[c] = i;
-			next[i] = next[i] + 1;
-			n = n + 1;
-			if (n == d->M) return;
+	int k = 0;
+	for (int n = 0; n < d->M; n++) {
+		int c = activeTargets[k].c;
+		while (tmpLookup[c] >= 0) {
+			c = compute_next_element_in_permutation(c, activeTargets[k].skip, d->M);
 		}
+		tmpLookup[c] = activeTargets[k].idx;
+		activeTargets[k].c = c = compute_next_element_in_permutation(c, activeTargets[k].skip, d->M);
+		k = k < num_targets - 1 ? k+1 : 0;
 	}
+	memcpy(d->lookup, tmpLookup, sizeof(int) * d->M);
+	free(tmpLookup);
+	free(activeTargets);
 }

src/lib/maglevdyn.h

@@ -2,8 +2,7 @@
 
 struct MagDataDyn {
 	unsigned M, N;
-	int *lookup;
-	unsigned** permutation;
+	int* lookup;
 	int* active;
 };
 
@@ -25,9 +24,20 @@ void magDataDyn_init(unsigned M, unsigned N, void* mem, unsigned len);
   Must call magDataDyn_free() to free allocated memory.
  */
 void magDataDyn_map(struct MagDataDyn* m, void* mem);
-void magDataDyn_free(struct MagDataDyn* m);
 
 /*
   Call when the "active" array is updated
  */
 void magDataDyn_populate(struct MagDataDyn* m);
+
+
+// This is equivalent to
+// currentValue - skip >= 0 ? currentValue - skip : currentValue - skip + mod
+// but much faster than any conditional branching. In fact, this is as fast
+// as retrieving the values from a pre-computed table.
+static inline int compute_next_element_in_permutation(int currentValue, int skip, int mod)
+{
+	int v = currentValue - skip;
+	v += mod & ((v >= 0) - 1);
+	return v;
+}

src/lib/test/maglevdyn-test.c

@@ -17,18 +17,18 @@
 
 static void targetAddRemove(unsigned M, unsigned N, unsigned A, float lim);
 static int cmdTest(int argc, char **argv);
+static void magDataDyn_populate_test(struct MagDataDyn* d);
+static void testFastComputationEquevalence(int M);
+static int rand_seed = 0;
 
 int main(int argc, char* argv[])
 {
-	srand(time(NULL));
-
 	if (argc > 1)
 		return cmdTest(argc, argv);
 
-	unsigned int M=1000, N=100, len, i, j;
+	unsigned int M=1000, N=100, len, i;
 	struct MagDataDyn m;
 	void* mem;
-	unsigned* row;
 
 	len = magDataDyn_len(M, N);
 	mem = malloc(len);
@@ -38,25 +38,19 @@ int main(int argc, char* argv[])
 	assert(m.M == primeBelow(M));
 	assert(m.N == N);
 	for (i = 0; i < m.N; i++) {
-		row = m.permutation[i];
-		assert((void*)row - (void*)m.lookup < len);
-		for (j = 0; j < m.M; j++) {
-			assert(row[j] < m.M);
-		}
 		assert(m.active[i] == -1);
 		assert(m.lookup[i] == -1);
 	}
 	m.active[0] = 100;
-	magDataDyn_populate(&m);
+	magDataDyn_populate_test(&m);
 	for (i = 0; i < m.N; i++) {
 		assert(m.lookup[i] == 0);
 	}
 	m.active[1] = 101;
-	magDataDyn_populate(&m);
+	magDataDyn_populate_test(&m);
 	for (i = 0; i < m.N; i++) {
 		assert(m.lookup[i] < 2);
 	}
-	magDataDyn_free(&m);
 	free(mem);
 
 	/*
@@ -67,14 +61,40 @@ int main(int argc, char* argv[])
 	targetAddRemove(109, 20, 10, 24.0); /* perfect = 10% */
 	targetAddRemove(1009, 20, 10, 13.0); /* perfect = 10% */
 	targetAddRemove(10009, 100, 50, 5.0); /* perfect = 2% */
+	testFastComputationEquevalence(37);
 
 	printf("==== maglevdyn-test OK\n");
 	return 0;
 }
 
+static void testFastComputationEquevalence(int M)
+{
+	int permutations[65536];
+	M = primeBelow(M);
+	assert (M < sizeof(permutations)/sizeof(permutations[0]));
+
+	for (int offset = 0; offset < M; offset++ ) {
+		for (int skip = 1; skip < M; skip++) {
+			// Old method with permutation table
+			int value = offset;
+			for (int i = 0; i < M; i++) {
+				permutations[i] = value;
+				value = (value + skip) % M;
+			}
+			// New method with on-the-fly compute:
+			value = offset;
+			int skip_bar = M - skip;
+			for (int i = 0; i < M; i++) {
+				assert (permutations[i] == value);
+				value = compute_next_element_in_permutation(value, skip_bar, M);
+			}
+		}
+	}
+}
 
 static void* create(unsigned M, unsigned N)
 {
+	rand_seed = time(NULL);
 	unsigned len = magDataDyn_len(M, N);
 	void* mem = malloc(len);
 	magDataDyn_init(M, N, mem, len);
@@ -94,15 +114,14 @@ static float addTargets(void* mem, unsigned n)
 			n--;
 		}
 	}
-	magDataDyn_populate(&m);
+	magDataDyn_populate_test(&m);
 
 	// Compute the update impact in percent
 	unsigned ndiff = 0;
 	for (int i = 0; i < m.M; i++) {
 		if (lookup[i] != m.lookup[i])
 			ndiff++;
 	}
-	magDataDyn_free(&m);
 	return 100.0 * (float)ndiff / (float)m.M;
 }
 
@@ -119,15 +138,14 @@ static float removeTargets(void* mem, unsigned n)
 			n--;
 		}
 	}
-	magDataDyn_populate(&m);
+	magDataDyn_populate_test(&m);
 
 	// Compute the update impact in percent
 	unsigned ndiff = 0;
 	for (int i = 0; i < m.M; i++) {
 		if (lookup[i] != m.lookup[i])
 			ndiff++;
 	}
-	magDataDyn_free(&m);
 	return 100.0 * (float)ndiff / (float)m.M;
 }
 
@@ -180,3 +198,8 @@ static int cmdTest(int argc, char **argv)
 }
 
 
+static void magDataDyn_populate_test(struct MagDataDyn* d)
+{
+	srand(rand_seed);
+	magDataDyn_populate(d);
+}

src/nfqlb/cmdFlowLb.c

@@ -411,7 +411,6 @@ STATIC void loadbalancerRelease(struct LoadBalancer* lb)
 		munmap(lb->st, statbuf.st_size);
 		close(lb->fd);
 		free(lb->target);
-		magDataDyn_free(&lb->magd);
 		free(lb);
 	}
 }

src/nfqlb/cmdShm.c

@@ -128,7 +128,6 @@ static int cmdShow(int argc, char **argv)
 			printf(" %d(%d)", magd.active[i], i);
 	}
 	printf("\n");
-	magDataDyn_free(&magd);
 
 	return 0;
 }