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Fix hash order to generate more even distribution (#5247)

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The problem in existing code was the following line

```
start := int(keyCrc%uint32(cardinality)) | 1
```

A given a value of N cardinality the ending result
because of the the bitwise '|' would lead to always
higher affinity to odd sequences.

As can be seen from the test cases that this can
lead to many objects being allocated the same set
of disks or atleast the first disk is an odd disk
always.  This introduces a performance problem
for majority of the objects under concurrent load.

Remove `| 1` to provide a more cleaner distribution
and the new code will be.
```
start := int(keyCrc % uint32(cardinality))
```

Thanks to Krishna Srinivas for pointing out the bitwise
situation here.
This commit is contained in:
Harshavardhana 2017-11-30 12:57:03 -08:00 committed by GitHub
parent 6d7319380c
commit d45a8784fc
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 4 additions and 4 deletions
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2
cmd/xl-v1-utils.go
View File

@ -118,7 +118,7 @@ func hashOrder(key string, cardinality int) []int {
nums := make([]int, cardinality)
keyCrc := crc32.Checksum([]byte(key), crc32.IEEETable)
start := int(keyCrc%uint32(cardinality)) | 1
start := int(keyCrc % uint32(cardinality))
for i := 1; i <= cardinality; i++ {
nums[i-1] = 1 + ((start + i) % cardinality)
}

6
cmd/xl-v1-utils_test.go
View File

@ -110,14 +110,14 @@ func TestHashOrder(t *testing.T) {
}{
// cases which should pass the test.
// passing in valid object name.
{"object", []int{15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}},
{"The Shining Script <v1>.pdf", []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}},
{"object", []int{14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}},
{"The Shining Script <v1>.pdf", []int{16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}},
{"Cost Benefit Analysis (2009-2010).pptx", []int{15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}},
{"117Gn8rfHL2ACARPAhaFd0AGzic9pUbIA/5OCn5A", []int{3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2}},
{"SHØRT", []int{11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}},
{"There are far too many object names, and far too few bucket names!", []int{15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}},
{"a/b/c/", []int{3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2}},
{"/a/b/c", []int{7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6}},
{"/a/b/c", []int{6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5}},
{string([]byte{0xff, 0xfe, 0xfd}), []int{15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}},
}