r12916@catbus: nickm | 2007-05-24 12:43:45 -0400

 Add math functions to round values to the nearest power of 2.  Make mempools more careful about making sure that the size of their chunks is a little less than a power of 2, not a little more.


svn:r10304

4 files changed, 64 insertions, 4 deletions

diff --git a/src/common/compat.h b/src/common/compat.h
index 085af036d..cd3643740 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -96,6 +96,7 @@ extern INLINE double U64_TO_DBL(uint64_t x) {
 #if defined(__GNUC__) && __GNUC__ >= 3
 #define ATTR_NORETURN __attribute__((noreturn))
 #define ATTR_PURE __attribute__((pure))
+#define ATTR_CONST __attribute__((const))
 #define ATTR_MALLOC __attribute__((malloc))
 #define ATTR_NORETURN __attribute__((noreturn))
 #define ATTR_NONNULL(x) __attribute__((nonnull x))
@@ -108,6 +109,7 @@ extern INLINE double U64_TO_DBL(uint64_t x) {
 #else
 #define ATTR_NORETURN
 #define ATTR_PURE
+#define ATTR_CONST
 #define ATTR_MALLOC
 #define ATTR_NORETURN
 #define ATTR_NONNULL(x)
diff --git a/src/common/mempool.c b/src/common/mempool.c
index 2b1775c1b..665a803af 100644
--- a/src/common/mempool.c
+++ b/src/common/mempool.c
@@ -361,18 +361,24 @@ mp_pool_new(size_t item_size, size_t chunk_capacity)
   /* Now we figure out how many items fit in each chunk.  We need to fit at
    * least 2 items per chunk. No chunk can be more than MAX_CHUNK bytes long,
    * or less than MIN_CHUNK. */
-  /* XXXX020 Try a bit harder here: we want to be a bit less than a power of
-     2, not a bit over. */
   if (chunk_capacity > MAX_CHUNK)
     chunk_capacity = MAX_CHUNK;
-  if (chunk_capacity < alloc_size * 2 + CHUNK_OVERHEAD)
-    chunk_capacity = alloc_size * 2 + CHUNK_OVERHEAD;
+  /* Try to be around a power of 2 in size, since that's what allocators like
+   * handing out. 512K-1 byte is a lot better than 512K+1 byte. */
+  chunk_capacity = (size_t) round_to_power_of_2(chunk_capacity);
+  while (chunk_capacity < alloc_size * 2 + CHUNK_OVERHEAD)
+    chunk_capacity *= 2;
   if (chunk_capacity < MIN_CHUNK)
     chunk_capacity = MIN_CHUNK;
 
   pool->new_chunk_capacity = (chunk_capacity-CHUNK_OVERHEAD) / alloc_size;
   pool->item_alloc_size = alloc_size;
 
+  log_debug(LD_MM, "Capacity is %lu, item size is %lu, alloc size is %lu",
+            (unsigned long)pool->new_chunk_capacity,
+            (unsigned long)pool->item_alloc_size,
+            (unsigned long)(pool->new_chunk_capacity*pool->item_alloc_size));
+
   return pool;
 }
 
diff --git a/src/common/util.c b/src/common/util.c
index d4a30a145..7ca08363d 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -214,6 +214,54 @@ _tor_free(void *mem)
 }
 
 /* =====
+ * Math
+ * ===== */
+
+/** Returns floor(log2(u64)).  If u64 is 0, (incorrectly) returns 0. */
+int
+tor_log2(uint64_t u64)
+{
+  int r = 0;
+  if (u64 >= (U64_LITERAL(1)<<32)) {
+    u64 >>= 32;
+    r = 32;
+  }
+  if (u64 >= (U64_LITERAL(1)<<16)) {
+    u64 >>= 16;
+    r += 16;
+  }
+  if (u64 >= (U64_LITERAL(1)<<8)) {
+    u64 >>= 8;
+    r += 8;
+  }
+  if (u64 >= (U64_LITERAL(1)<<4)) {
+    u64 >>= 4;
+    r += 4;
+  }
+  if (u64 >= (U64_LITERAL(1)<<2)) {
+    u64 >>= 2;
+    r += 2;
+  }
+  if (u64 >= (U64_LITERAL(1)<<1)) {
+    u64 >>= 1;
+    r += 1;
+  }
+  return r;
+}
+
+/** Return the power of 2 closest to <b>u64</b>. */
+uint64_t
+round_to_power_of_2(uint64_t u64)
+{
+  int lg2 = tor_log2(u64);
+  uint64_t low = U64_LITERAL(1) << lg2, high = U64_LITERAL(1) << (lg2+1);
+  if (high - u64 < u64 - low)
+    return high;
+  else
+    return low;
+}
+
+/* =====
  * String manipulation
  * ===== */
 
diff --git a/src/common/util.h b/src/common/util.h
index 6b32708c5..f04160935 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -142,6 +142,10 @@ extern int dmalloc_free(const char *file, const int line, void *pnt,
 /** Macro: true if two values have different boolean values. */
 #define bool_neq(a,b) (!(a)!=!(b))
 
+/* Math functions */
+int tor_log2(uint64_t u64) ATTR_CONST;
+uint64_t round_to_power_of_2(uint64_t u64);
+
 /* String manipulation */
 
 /** Allowable characters in a hexadecimal string. */