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bits_per_key is already configurable. It defines how many bloom bits will be used for every key in the database.

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My change in this patch is to make the Hash code that is used for blooms to be confgurable. In fact,
one can specify a modified HashCode that inspects only parts of the Key to generate the Hash (used
by booms).

Test Plan: none

Differential Revision: https://reviews.facebook.net/D4059
This commit is contained in:
Dhruba Borthakur 2012-07-09 17:31:12 -07:00
parent 80c663882a
commit d11b637f34
1 changed files with 17 additions and 6 deletions
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23
util/bloom.cc
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@ -18,16 +18,27 @@ class BloomFilterPolicy : public FilterPolicy {
private:
size_t bits_per_key_;
size_t k_;
uint32_t (*hash_func_)(const Slice& key);
public:
explicit BloomFilterPolicy(int bits_per_key)
: bits_per_key_(bits_per_key) {
void initialize() {
// We intentionally round down to reduce probing cost a little bit
k_ = static_cast<size_t>(bits_per_key * 0.69); // 0.69 =~ ln(2)
k_ = static_cast<size_t>(bits_per_key_ * 0.69); // 0.69 =~ ln(2)
if (k_ < 1) k_ = 1;
if (k_ > 30) k_ = 30;
}
public:
explicit BloomFilterPolicy(int bits_per_key,
uint32_t (*hash_func)(const Slice& key))
: bits_per_key_(bits_per_key), hash_func_(hash_func) {
initialize();
}
explicit BloomFilterPolicy(int bits_per_key)
: bits_per_key_(bits_per_key) {
hash_func_ = BloomHash;
initialize();
}
virtual const char* Name() const {
return "leveldb.BuiltinBloomFilter";
}
@ -50,7 +61,7 @@ class BloomFilterPolicy : public FilterPolicy {
for (size_t i = 0; i < n; i++) {
// Use double-hashing to generate a sequence of hash values.
// See analysis in [Kirsch,Mitzenmacher 2006].
uint32_t h = BloomHash(keys[i]);
uint32_t h = hash_func_(keys[i]);
const uint32_t delta = (h >> 17) | (h << 15); // Rotate right 17 bits
for (size_t j = 0; j < k_; j++) {
const uint32_t bitpos = h % bits;
@ -76,7 +87,7 @@ class BloomFilterPolicy : public FilterPolicy {
return true;
}
uint32_t h = BloomHash(key);
uint32_t h = hash_func_(key);
const uint32_t delta = (h >> 17) | (h << 15); // Rotate right 17 bits
for (size_t j = 0; j < k; j++) {
const uint32_t bitpos = h % bits;