Change crush_ln to provide 32 more digits.

32 more digits(more precision) were given by crush_ln, so we dont
 need to do left shift in bucket_straw2_choose now.

This change will slightly improve the quality of distribution.

expect                  169.664
osd     weight  count   adjusted
0       1       179     179
1       1.75    291     166
2       3.062   507     165
3       5.359   937     174
4       9.379   1581    168
5       16.41   2771    168
6       28.72   4847    168
7       50.27   8690    172
8       87.96   14924   169
9       153.9   26069   169
10      269.4   45706   169
11      471.4   79754   169
12      825     139847  169
13      1444    245707  170
14      2527    428190  169
std dev 12.4895
     vs 12.6005 (expected)

Signed-off-by: Xiaoxi Chen <[email protected]>

src/crush/crush_ln_table.h

@@ -24,6 +24,9 @@
 #define CEPH_CRUSH_LN_H
 
 
+// RH_LH_tbl[2*k] = 2^48/(1.0+k/128.0)
+// RH_LH_tbl[2*k+1] = 2^48*log2(1.0+k/128.0)
+
 static int64_t __RH_LH_tbl[128*2+2] = {
   0x0001000000000000ll, 0x0000000000000000ll, 0x0000fe03f80fe040ll, 0x000002dfca16dde1ll,
   0x0000fc0fc0fc0fc1ll, 0x000005b9e5a170b4ll, 0x0000fa232cf25214ll, 0x0000088e68ea899all,

src/crush/mapper.c

@@ -253,12 +253,12 @@ static int bucket_straw_choose(struct crush_bucket_straw *bucket,
 	return bucket->h.items[high];
 }
 
-// compute 2^12*log2(input+1)
-unsigned crush_ln(unsigned xin)
+// compute 2^44*log2(input+1)
+uint64_t crush_ln(unsigned xin)
 {
-    unsigned x=xin, x1, result;
+    unsigned x=xin, x1;
     int iexpon, index1, index2;
-    uint64_t RH, LH, LL, xl64;
+    uint64_t RH, LH, LL, xl64, result;
 
     x++;
 
@@ -277,16 +277,17 @@ unsigned crush_ln(unsigned xin)
     xl64 >>= 48;
     x1 = xl64;
 
-    result = iexpon << 12;
+    result = iexpon;
+    result <<= (12 + 32);
 
     index2 = x1 & 0xff;
     // LL ~ 2^48*log2(1.0+index2/2^15)
     LL = __LL_tbl[index2];
 
     LH = LH + LL;
 
-    LH >>= (48-12);
-    result += (unsigned)LH;
+    LH >>= (48-12 - 32);
+    result += LH;
 
     return result;
 }
@@ -319,15 +320,15 @@ static int bucket_straw2_choose(struct crush_bucket_straw2 *bucket,
 		 *
 		 * the natural log lookup table maps [0,0xffff]
 		 * (corresponding to real numbers [1/0x10000, 1] to
-		 * [0, 0xffff] (corresponding to real numbers
+		 * [0, 0xffffffffffff] (corresponding to real numbers
 		 * [-11.090355,0]).
 		 */
-		ln = crush_ln(u) - 0x10000;
+		ln = crush_ln(u) - 0x1000000000000ll;
 
 		/*
 		 * divide by 16.16 fixed-point weight
 		 */
-		draw = (ln << 32) / bucket->item_weights[i];
+		draw = ln / bucket->item_weights[i];
 
 		if (i == 0 || draw > high_draw) {
 			high = i;