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unbiase readamp bitmap
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Summary:
Consider BlockReadAmpBitmap with bytes_per_bit = 32. Suppose bytes [a, b) were used, while bytes [a-32, a)
 and [b+1, b+33) weren't used; more formally, the union of ranges passed to BlockReadAmpBitmap::Mark() contains [a, b) and doesn't intersect with [a-32, a) and [b+1, b+33). Then bits [floor(a/32), ceil(b/32)] will be set, and so the number of useful bytes will be estimated as (ceil(b/32) - floor(a/32)) * 32, which is on average equal to b-a+31.

An extreme example: if we use 1 byte from each block, it'll be counted as 32 bytes from each block.

It's easy to remove this bias by slightly changing the semantics of the bitmap. Currently each bit represents a byte range [i*32, (i+1)*32).

This diff makes each bit represent a single byte: i*32 + X, where X is a random number in [0, 31] generated when bitmap is created. So, e.g., if you read a single byte at random, with probability 31/32 it won't be counted at all, and with probability 1/32 it will be counted as 32 bytes; so, on average it's counted as 1 byte.

*But there is one exception: the last bit will always set with the old way.*

(*) - assuming read_amp_bytes_per_bit = 32.
Closes facebook#2259

Differential Revision: D5035652

Pulled By: lightmark

fbshipit-source-id: bd98b1b9b49fbe61f9e3781d07f624e3cbd92356
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@lightmark @facebook-github-bot
lightmark authored and facebook-github-bot committed May 10, 2017
1 parent a620966 commit 259a00e
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Showing 3 changed files with 106 additions and 108 deletions.
116 changes: 65 additions & 51 deletions db/db_test2.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1644,66 +1644,80 @@ size_t GetEncodedEntrySize(size_t key_size, size_t value_size) {
TEST_F(DBTest2, ReadAmpBitmap) {
Options options = CurrentOptions();
BlockBasedTableOptions bbto;
// Disable delta encoding to make it easier to calculate read amplification
bbto.use_delta_encoding = false;
// Huge block cache to make it easier to calculate read amplification
bbto.block_cache = NewLRUCache(1024 * 1024 * 1024);
bbto.read_amp_bytes_per_bit = 16;
options.table_factory.reset(NewBlockBasedTableFactory(bbto));
options.statistics = rocksdb::CreateDBStatistics();
DestroyAndReopen(options);

const size_t kNumEntries = 10000;
size_t bytes_per_bit[2] = {1, 16};
for (size_t k = 0; k < 2; k++) {
// Disable delta encoding to make it easier to calculate read amplification
bbto.use_delta_encoding = false;
// Huge block cache to make it easier to calculate read amplification
bbto.block_cache = NewLRUCache(1024 * 1024 * 1024);
bbto.read_amp_bytes_per_bit = bytes_per_bit[k];
options.table_factory.reset(NewBlockBasedTableFactory(bbto));
options.statistics = rocksdb::CreateDBStatistics();
DestroyAndReopen(options);

Random rnd(301);
for (size_t i = 0; i < kNumEntries; i++) {
ASSERT_OK(Put(Key(static_cast<int>(i)), RandomString(&rnd, 100)));
}
ASSERT_OK(Flush());
const size_t kNumEntries = 10000;

Close();
Reopen(options);
Random rnd(301);
for (size_t i = 0; i < kNumEntries; i++) {
ASSERT_OK(Put(Key(static_cast<int>(i)), RandomString(&rnd, 100)));
}
ASSERT_OK(Flush());

// Read keys/values randomly and verify that reported read amp error
// is less than 2%
uint64_t total_useful_bytes = 0;
std::set<int> read_keys;
std::string value;
for (size_t i = 0; i < kNumEntries * 5; i++) {
int key_idx = rnd.Next() % kNumEntries;
std::string k = Key(key_idx);
ASSERT_OK(db_->Get(ReadOptions(), k, &value));
Close();
Reopen(options);

// Read keys/values randomly and verify that reported read amp error
// is less than 2%
uint64_t total_useful_bytes = 0;
std::set<int> read_keys;
std::string value;
for (size_t i = 0; i < kNumEntries * 5; i++) {
int key_idx = rnd.Next() % kNumEntries;
std::string key = Key(key_idx);
ASSERT_OK(db_->Get(ReadOptions(), key, &value));

if (read_keys.find(key_idx) == read_keys.end()) {
auto internal_key = InternalKey(key, 0, ValueType::kTypeValue);
total_useful_bytes +=
GetEncodedEntrySize(internal_key.size(), value.size());
read_keys.insert(key_idx);
}

if (read_keys.find(key_idx) == read_keys.end()) {
auto ik = InternalKey(k, 0, ValueType::kTypeValue);
total_useful_bytes += GetEncodedEntrySize(ik.size(), value.size());
read_keys.insert(key_idx);
}
double expected_read_amp =
static_cast<double>(total_useful_bytes) /
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);

double expected_read_amp =
static_cast<double>(total_useful_bytes) /
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
double read_amp =
static_cast<double>(options.statistics->getTickerCount(
READ_AMP_ESTIMATE_USEFUL_BYTES)) /
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);

double read_amp =
static_cast<double>(options.statistics->getTickerCount(
READ_AMP_ESTIMATE_USEFUL_BYTES)) /
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
double error_pct = fabs(expected_read_amp - read_amp) * 100;
// Error between reported read amp and real read amp should be less than
// 2%
EXPECT_LE(error_pct, 2);
}

double error_pct = fabs(expected_read_amp - read_amp) * 100;
// Error between reported read amp and real read amp should be less than 2%
EXPECT_LE(error_pct, 2);
}
// Make sure we read every thing in the DB (which is smaller than our cache)
Iterator* iter = db_->NewIterator(ReadOptions());
for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
ASSERT_EQ(iter->value().ToString(), Get(iter->key().ToString()));
}
delete iter;

// Make sure we read every thing in the DB (which is smaller than our cache)
Iterator* iter = db_->NewIterator(ReadOptions());
for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
ASSERT_EQ(iter->value().ToString(), Get(iter->key().ToString()));
// Read amp is on average 100% since we read all what we loaded in memory
if (k == 0) {
ASSERT_EQ(
options.statistics->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES),
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES));
} else {
ASSERT_NEAR(
options.statistics->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES) *
1.0f /
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES),
1, .01);
}
}
delete iter;

// Read amp is 100% since we read all what we loaded in memory
ASSERT_EQ(options.statistics->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES),
options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES));
}

#ifndef OS_SOLARIS // GetUniqueIdFromFile is not implemented
Expand Down
67 changes: 25 additions & 42 deletions table/block.h
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Original file line number Diff line number Diff line change
Expand Up @@ -29,7 +29,8 @@
#include "rocksdb/statistics.h"
#include "table/block_prefix_index.h"
#include "table/internal_iterator.h"

#include "util/random.h"
#include "util/sync_point.h"
#include "format.h"

namespace rocksdb {
Expand All @@ -46,7 +47,12 @@ class BlockReadAmpBitmap {
public:
explicit BlockReadAmpBitmap(size_t block_size, size_t bytes_per_bit,
Statistics* statistics)
: bitmap_(nullptr), bytes_per_bit_pow_(0), statistics_(statistics) {
: bitmap_(nullptr),
bytes_per_bit_pow_(0),
statistics_(statistics),
rnd_(
Random::GetTLSInstance()->Uniform(static_cast<int>(bytes_per_bit))) {
TEST_SYNC_POINT_CALLBACK("BlockReadAmpBitmap:rnd", &rnd_);
assert(block_size > 0 && bytes_per_bit > 0);

// convert bytes_per_bit to be a power of 2
Expand All @@ -56,62 +62,38 @@ class BlockReadAmpBitmap {

// num_bits_needed = ceil(block_size / bytes_per_bit)
size_t num_bits_needed =
(block_size >> static_cast<size_t>(bytes_per_bit_pow_)) +
(block_size % (static_cast<size_t>(1)
<< static_cast<size_t>(bytes_per_bit_pow_)) !=
0);
((block_size - 1) >> bytes_per_bit_pow_) + 1;
assert(num_bits_needed > 0);

// bitmap_size = ceil(num_bits_needed / kBitsPerEntry)
size_t bitmap_size = (num_bits_needed / kBitsPerEntry) +
(num_bits_needed % kBitsPerEntry != 0);
size_t bitmap_size = (num_bits_needed - 1) / kBitsPerEntry + 1;

// Create bitmap and set all the bits to 0
bitmap_ = new std::atomic<uint32_t>[bitmap_size];
memset(bitmap_, 0, bitmap_size * kBytesPersEntry);

RecordTick(GetStatistics(), READ_AMP_TOTAL_READ_BYTES,
num_bits_needed << bytes_per_bit_pow_);
RecordTick(GetStatistics(), READ_AMP_TOTAL_READ_BYTES, block_size);
}

~BlockReadAmpBitmap() { delete[] bitmap_; }

void Mark(uint32_t start_offset, uint32_t end_offset) {
assert(end_offset >= start_offset);

// Every new bit we set will bump this counter
uint32_t new_useful_bytes = 0;
// Index of first bit in mask (start_offset / bytes_per_bit)
uint32_t start_bit = start_offset >> bytes_per_bit_pow_;
// Index of last bit in mask (end_offset / bytes_per_bit)
uint32_t end_bit = end_offset >> bytes_per_bit_pow_;
// Index of middle bit (unique to this range)
uint32_t mid_bit = start_bit + 1;

// It's guaranteed that ranges sent to Mark() wont overlap, this mean that
// we dont need to set the middle bits, we can simply set only one bit of
// the middle bits, and check this bit if we want to know if the whole
// range is set or not.
if (mid_bit < end_bit) {
if (GetAndSet(mid_bit) == 0) {
new_useful_bytes += (end_bit - mid_bit) << bytes_per_bit_pow_;
} else {
// If the middle bit is set, it's guaranteed that start and end bits
// are also set
return;
}
} else {
// This range dont have a middle bit, the whole range fall in 1 or 2 bits
// Index of first bit in mask
uint32_t start_bit =
(start_offset + (1 << bytes_per_bit_pow_) - rnd_ - 1) >>
bytes_per_bit_pow_;
// Index of last bit in mask + 1
uint32_t exclusive_end_bit =
(end_offset + (1 << bytes_per_bit_pow_) - rnd_) >> bytes_per_bit_pow_;
if (start_bit >= exclusive_end_bit) {
return;
}
assert(exclusive_end_bit > 0);

if (GetAndSet(start_bit) == 0) {
new_useful_bytes += (1 << bytes_per_bit_pow_);
}

if (GetAndSet(end_bit) == 0) {
new_useful_bytes += (1 << bytes_per_bit_pow_);
}

if (new_useful_bytes > 0) {
uint32_t new_useful_bytes = (exclusive_end_bit - start_bit)
<< bytes_per_bit_pow_;
RecordTick(GetStatistics(), READ_AMP_ESTIMATE_USEFUL_BYTES,
new_useful_bytes);
}
Expand Down Expand Up @@ -148,6 +130,7 @@ class BlockReadAmpBitmap {
// this pointer maybe invalid, but the DB will update it to a valid pointer
// by using SetStatistics() before calling Mark()
std::atomic<Statistics*> statistics_;
uint32_t rnd_;
};

class Block {
Expand Down
31 changes: 16 additions & 15 deletions table/block_test.cc
View file
Original file line number Diff line number Diff line change
Expand Up @@ -228,25 +228,30 @@ class BlockReadAmpBitmapSlowAndAccurate {
public:
void Mark(size_t start_offset, size_t end_offset) {
assert(end_offset >= start_offset);

marked_ranges_.emplace(end_offset, start_offset);
}

// Return true if any byte in this range was Marked
bool IsAnyInRangeMarked(size_t start_offset, size_t end_offset) {
bool IsPinMarked(size_t offset) {
auto it = marked_ranges_.lower_bound(
std::make_pair(start_offset, static_cast<size_t>(0)));
std::make_pair(offset, static_cast<size_t>(0)));
if (it == marked_ranges_.end()) {
return false;
}
return start_offset <= it->first && end_offset >= it->second;
return offset <= it->first && offset >= it->second;
}

private:
std::set<std::pair<size_t, size_t>> marked_ranges_;
};

TEST_F(BlockTest, BlockReadAmpBitmap) {
uint32_t pin_offset = 0;
SyncPoint::GetInstance()->SetCallBack(
"BlockReadAmpBitmap:rnd", [&pin_offset](void* arg) {
pin_offset = *(static_cast<uint32_t*>(arg));
});
SyncPoint::GetInstance()->EnableProcessing();
std::vector<size_t> block_sizes = {
1, // 1 byte
32, // 32 bytes
Expand Down Expand Up @@ -279,10 +284,8 @@ TEST_F(BlockTest, BlockReadAmpBitmap) {
if (needed_bits % 32 != 0) {
bitmap_size++;
}
size_t bits_in_bitmap = bitmap_size * 32;

ASSERT_EQ(stats->getTickerCount(READ_AMP_TOTAL_READ_BYTES),
needed_bits * kBytesPerBit);
ASSERT_EQ(stats->getTickerCount(READ_AMP_TOTAL_READ_BYTES), block_size);

// Generate some random entries
std::vector<size_t> random_entry_offsets;
Expand Down Expand Up @@ -316,20 +319,18 @@ TEST_F(BlockTest, BlockReadAmpBitmap) {
current_entry.second);

size_t total_bits = 0;
for (size_t bit_idx = 0; bit_idx < bits_in_bitmap; bit_idx++) {
size_t start_rng = bit_idx * kBytesPerBit;
size_t end_rng = (start_rng + kBytesPerBit) - 1;

total_bits +=
read_amp_slow_and_accurate.IsAnyInRangeMarked(start_rng, end_rng);
for (size_t bit_idx = 0; bit_idx < needed_bits; bit_idx++) {
total_bits += read_amp_slow_and_accurate.IsPinMarked(
bit_idx * kBytesPerBit + pin_offset);
}
size_t expected_estimate_useful = total_bits * kBytesPerBit;
size_t got_estimate_useful =
stats->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES);

stats->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES);
ASSERT_EQ(expected_estimate_useful, got_estimate_useful);
}
}
SyncPoint::GetInstance()->DisableProcessing();
SyncPoint::GetInstance()->ClearAllCallBacks();
}

TEST_F(BlockTest, BlockWithReadAmpBitmap) {
Expand Down

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