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BlockBasedTableReader: automatically adjust tail prefetch size (#4156)

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Summary:
Right now we use one hard-coded prefetch size to prefetch data from the tail of the SST files. However, this may introduce a waste for some use cases, while not efficient for others.
Introduce a way to adjust this prefetch size by tracking 32 recent times, and pick a value with which the wasted read is less than 10%
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4156

Differential Revision: D8916847

Pulled By: siying

fbshipit-source-id: 8413f9eb3987e0033ed0bd910f83fc2eeaaf5758
main
Siying Dong 6 years ago committed by Facebook Github Bot
parent ab35505e21
commit 8425c8bd4d
8 changed files with 336 additions and 21 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
Show Stats Download Patch File Download Diff File
  1. 88
      db/db_test2.cc
  2. 134
      table/block_based_table_factory.cc
  3. 17
      table/block_based_table_factory.h
  4. 44
      table/block_based_table_reader.cc
  5. 4
      table/block_based_table_reader.h
  6. 44
      table/table_test.cc
  7. 5
      util/file_reader_writer.cc
  8. 21
      util/file_reader_writer.h

88
db/db_test2.cc
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@ -2321,9 +2321,9 @@ TEST_F(DBTest2, RateLimitedCompactionReads) {
options.rate_limiter->GetTotalBytesThrough(Env::IO_LOW);
// Include the explicit prefetch of the footer in direct I/O case.
size_t direct_io_extra = use_direct_io ? 512 * 1024 : 0;
ASSERT_GE(rate_limited_bytes,
static_cast<size_t>(kNumKeysPerFile * kBytesPerKey * kNumL0Files +
direct_io_extra));
ASSERT_GE(
rate_limited_bytes,
static_cast<size_t>(kNumKeysPerFile * kBytesPerKey * kNumL0Files));
ASSERT_LT(
rate_limited_bytes,
static_cast<size_t>(2 * kNumKeysPerFile * kBytesPerKey * kNumL0Files +
@ -2547,6 +2547,88 @@ TEST_F(DBTest2, PinnableSliceAndMmapReads) {
#endif
}
TEST_F(DBTest2, TestBBTTailPrefetch) {
std::atomic<bool> called(false);
size_t expected_lower_bound = 512 * 1024;
size_t expected_higher_bound = 512 * 1024;
rocksdb::SyncPoint::GetInstance()->SetCallBack(
"BlockBasedTable::Open::TailPrefetchLen", [&](void* arg) {
size_t* prefetch_size = static_cast<size_t*>(arg);
EXPECT_LE(expected_lower_bound, *prefetch_size);
EXPECT_GE(expected_higher_bound, *prefetch_size);
called = true;
});
rocksdb::SyncPoint::GetInstance()->EnableProcessing();
Put("1", "1");
Put("9", "1");
Flush();
expected_lower_bound = 0;
expected_higher_bound = 8 * 1024;
Put("1", "1");
Put("9", "1");
Flush();
Put("1", "1");
Put("9", "1");
Flush();
// Full compaction to make sure there is no L0 file after the open.
ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
ASSERT_TRUE(called.load());
called = false;
rocksdb::SyncPoint::GetInstance()->DisableProcessing();
rocksdb::SyncPoint::GetInstance()->ClearAllCallBacks();
std::atomic<bool> first_call(true);
rocksdb::SyncPoint::GetInstance()->SetCallBack(
"BlockBasedTable::Open::TailPrefetchLen", [&](void* arg) {
size_t* prefetch_size = static_cast<size_t*>(arg);
if (first_call) {
EXPECT_EQ(4 * 1024, *prefetch_size);
first_call = false;
} else {
EXPECT_GE(4 * 1024, *prefetch_size);
}
called = true;
});
rocksdb::SyncPoint::GetInstance()->EnableProcessing();
Options options = CurrentOptions();
options.max_file_opening_threads = 1; // one thread
BlockBasedTableOptions table_options;
table_options.cache_index_and_filter_blocks = true;
options.table_factory.reset(NewBlockBasedTableFactory(table_options));
options.max_open_files = -1;
Reopen(options);
Put("1", "1");
Put("9", "1");
Flush();
Put("1", "1");
Put("9", "1");
Flush();
ASSERT_TRUE(called.load());
called = false;
// Parallel loading SST files
options.max_file_opening_threads = 16;
Reopen(options);
ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
ASSERT_TRUE(called.load());
rocksdb::SyncPoint::GetInstance()->DisableProcessing();
rocksdb::SyncPoint::GetInstance()->ClearAllCallBacks();
}
} // namespace rocksdb
int main(int argc, char** argv) {

134
table/block_based_table_factory.cc
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@ -27,10 +27,141 @@
#include "table/block_based_table_builder.h"
#include "table/block_based_table_reader.h"
#include "table/format.h"
#include "util/mutexlock.h"
#include "util/string_util.h"
namespace rocksdb {
void TailPrefetchStats::RecordEffectiveSize(size_t len) {
MutexLock l(&mutex_);
if (num_records_ < kNumTracked) {
num_records_++;
}
records_[next_++] = len;
if (next_ == kNumTracked) {
next_ = 0;
}
}
size_t TailPrefetchStats::GetSuggestedPrefetchSize() {
std::vector<size_t> sorted;
{
MutexLock l(&mutex_);
if (num_records_ == 0) {
return 0;
}
sorted.assign(records_, records_ + num_records_);
}
// Of the historic size, we find the maximum one that satisifis the condtiion
// that if prefetching all, less than 1/8 will be wasted.
std::sort(sorted.begin(), sorted.end());
// Assuming we have 5 data points, and after sorting it looks like this:
//
// +---+
// +---+ | |
// | | | |
// | | | |
// | | | |
// | | | |
// +---+ | | | |
// | | | | | |
// +---+ | | | | | |
// | | | | | | | |
// +---+ | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// +---+ +---+ +---+ +---+ +---+
//
// and we use every of the value as a candidate, and estimate how much we
// wasted, compared to read. For example, when we use the 3rd record
// as candiate. This area is what we read:
// +---+
// +---+ | |
// | | | |
// | | | |
// | | | |
// | | | |
// *** *** *** ***+ *** *** *** *** **
// * | | | | | |
// +---+ | | | | | *
// * | | | | | | | |
// +---+ | | | | | | | *
// * | | | | X | | | | |
// | | | | | | | | | *
// * | | | | | | | | |
// | | | | | | | | | *
// * | | | | | | | | |
// *** *** ***-*** ***--*** ***--*** +****
// which is (size of the record) X (number of records).
//
// While wasted is this area:
// +---+
// +---+ | |
// | | | |
// | | | |
// | | | |
// | | | |
// *** *** *** ****---+ | | | |
// * * | | | | |
// * *-*** *** | | | | |
// * * | | | | | | |
// *--** *** | | | | | | |
// | | | | | X | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// +---+ +---+ +---+ +---+ +---+
//
// Which can be calculated iteratively.
// The difference between wasted using 4st and 3rd record, will
// be following area:
// +---+
// +--+ +-+ ++ +-+ +-+ +---+ | |
// + xxxxxxxxxxxxxxxxxxxxxxxx | | | |
// xxxxxxxxxxxxxxxxxxxxxxxx | | | |
// + xxxxxxxxxxxxxxxxxxxxxxxx | | | |
// | xxxxxxxxxxxxxxxxxxxxxxxx | | | |
// +-+ +-+ +-+ ++ +---+ +--+ | | |
// | | | | | | |
// +---+ ++ | | | | | |
// | | | | | | X | | |
// +---+ ++ | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// | | | | | | | | | |
// +---+ +---+ +---+ +---+ +---+
//
// which will be the size difference between 4st and 3rd record,
// times 3, which is number of records before the 4st.
// Here we assume that all data within the prefetch range will be useful. In
// reality, it may not be the case when a partial block is inside the range,
// or there are data in the middle that is not read. We ignore those cases
// for simplicity.
assert(!sorted.empty());
size_t prev_size = sorted[0];
size_t max_qualified_size = sorted[0];
size_t wasted = 0;
for (size_t i = 1; i < sorted.size(); i++) {
size_t read = sorted[i] * sorted.size();
wasted += (sorted[i] - prev_size) * i;
if (wasted <= read / 8) {
max_qualified_size = sorted[i];
}
prev_size = sorted[i];
}
const size_t kMaxPrefetchSize = 512 * 1024; // Never exceed 512KB
return std::min(kMaxPrefetchSize, max_qualified_size);
}
BlockBasedTableFactory::BlockBasedTableFactory(
const BlockBasedTableOptions& _table_options)
: table_options_(_table_options) {
@ -71,7 +202,8 @@ Status BlockBasedTableFactory::NewTableReader(
table_options_, table_reader_options.internal_comparator, std::move(file),
file_size, table_reader, table_reader_options.prefix_extractor,
prefetch_index_and_filter_in_cache, table_reader_options.skip_filters,
table_reader_options.level, table_reader_options.immortal);
table_reader_options.level, table_reader_options.immortal,
&tail_prefetch_stats_);
}
TableBuilder* BlockBasedTableFactory::NewTableBuilder(

17
table/block_based_table_factory.h
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@ -26,6 +26,22 @@ struct EnvOptions;
using std::unique_ptr;
class BlockBasedTableBuilder;
// A class used to track actual bytes written from the tail in the recent SST
// file opens, and provide a suggestion for following open.
class TailPrefetchStats {
public:
void RecordEffectiveSize(size_t len);
// 0 indicates no information to determine.
size_t GetSuggestedPrefetchSize();
private:
const static size_t kNumTracked = 32;
size_t records_[kNumTracked];
port::Mutex mutex_;
size_t next_ = 0;
size_t num_records_ = 0;
};
class BlockBasedTableFactory : public TableFactory {
public:
explicit BlockBasedTableFactory(
@ -64,6 +80,7 @@ class BlockBasedTableFactory : public TableFactory {
private:
BlockBasedTableOptions table_options_;
mutable TailPrefetchStats tail_prefetch_stats_;
};
extern const std::string kHashIndexPrefixesBlock;

44
table/block_based_table_reader.cc
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@ -731,7 +731,8 @@ Status BlockBasedTable::Open(const ImmutableCFOptions& ioptions,
const SliceTransform* prefix_extractor,
const bool prefetch_index_and_filter_in_cache,
const bool skip_filters, const int level,
const bool immortal_table) {
const bool immortal_table,
TailPrefetchStats* tail_prefetch_stats) {
table_reader->reset();
Footer footer;
@ -741,29 +742,40 @@ Status BlockBasedTable::Open(const ImmutableCFOptions& ioptions,
// prefetch both index and filters, down to all partitions
const bool prefetch_all = prefetch_index_and_filter_in_cache || level == 0;
const bool preload_all = !table_options.cache_index_and_filter_blocks;
// Before read footer, readahead backwards to prefetch data. Do more readahead
// if we're going to read index/filter.
// TODO: This may incorrectly select small readahead in case partitioned
// index/filter is enabled and top-level partition pinning is enabled. That's
// because we need to issue readahead before we read the properties, at which
// point we don't yet know the index type.
const size_t kTailPrefetchSize =
prefetch_all || preload_all ? 512 * 1024 : 4 * 1024;
size_t tail_prefetch_size = 0;
if (tail_prefetch_stats != nullptr) {
// Multiple threads may get a 0 (no history) when running in parallel,
// but it will get cleared after the first of them finishes.
tail_prefetch_size = tail_prefetch_stats->GetSuggestedPrefetchSize();
}
if (tail_prefetch_size == 0) {
// Before read footer, readahead backwards to prefetch data. Do more
// readahead if we're going to read index/filter.
// TODO: This may incorrectly select small readahead in case partitioned
// index/filter is enabled and top-level partition pinning is enabled.
// That's because we need to issue readahead before we read the properties,
// at which point we don't yet know the index type.
tail_prefetch_size = prefetch_all || preload_all ? 512 * 1024 : 4 * 1024;
}
size_t prefetch_off;
size_t prefetch_len;
if (file_size < kTailPrefetchSize) {
if (file_size < tail_prefetch_size) {
prefetch_off = 0;
prefetch_len = static_cast<size_t>(file_size);
} else {
prefetch_off = static_cast<size_t>(file_size - kTailPrefetchSize);
prefetch_len = kTailPrefetchSize;
prefetch_off = static_cast<size_t>(file_size - tail_prefetch_size);
prefetch_len = tail_prefetch_size;
}
TEST_SYNC_POINT_CALLBACK("BlockBasedTable::Open::TailPrefetchLen",
&tail_prefetch_size);
Status s;
// TODO should not have this special logic in the future.
if (!file->use_direct_io()) {
prefetch_buffer.reset(new FilePrefetchBuffer(nullptr, 0, 0, false, true));
s = file->Prefetch(prefetch_off, prefetch_len);
} else {
prefetch_buffer.reset(new FilePrefetchBuffer());
prefetch_buffer.reset(new FilePrefetchBuffer(nullptr, 0, 0, true, true));
s = prefetch_buffer->Prefetch(file.get(), prefetch_off, prefetch_len);
}
s = ReadFooterFromFile(file.get(), prefetch_buffer.get(), file_size, &footer,
@ -1060,6 +1072,12 @@ Status BlockBasedTable::Open(const ImmutableCFOptions& ioptions,
}
if (s.ok()) {
assert(prefetch_buffer.get() != nullptr);
if (tail_prefetch_stats != nullptr) {
assert(prefetch_buffer->min_offset_read() < file_size);
tail_prefetch_stats->RecordEffectiveSize(
file_size - prefetch_buffer->min_offset_read());
}
*table_reader = std::move(new_table);
}

4
table/block_based_table_reader.h
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@ -23,6 +23,7 @@
#include "rocksdb/status.h"
#include "rocksdb/table.h"
#include "table/block.h"
#include "table/block_based_table_factory.h"
#include "table/filter_block.h"
#include "table/format.h"
#include "table/persistent_cache_helper.h"
@ -93,7 +94,8 @@ class BlockBasedTable : public TableReader {
const SliceTransform* prefix_extractor = nullptr,
bool prefetch_index_and_filter_in_cache = true,
bool skip_filters = false, int level = -1,
const bool immortal_table = false);
const bool immortal_table = false,
TailPrefetchStats* tail_prefetch_stats = nullptr);
bool PrefixMayMatch(const Slice& internal_key,
const ReadOptions& read_options,

44
table/table_test.cc
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@ -1075,6 +1075,7 @@ class BlockBasedTableTest
};
class PlainTableTest : public TableTest {};
class TablePropertyTest : public testing::Test {};
class BBTTailPrefetchTest : public TableTest {};
INSTANTIATE_TEST_CASE_P(FormatDef, BlockBasedTableTest,
testing::Values(test::kDefaultFormatVersion));
@ -3594,6 +3595,49 @@ TEST_P(BlockBasedTableTest, BadOptions) {
ASSERT_NOK(rocksdb::DB::Open(options, kDBPath, &db));
}
TEST_F(BBTTailPrefetchTest, TestTailPrefetchStats) {
TailPrefetchStats tpstats;
ASSERT_EQ(0, tpstats.GetSuggestedPrefetchSize());
tpstats.RecordEffectiveSize(size_t{1000});
tpstats.RecordEffectiveSize(size_t{1005});
tpstats.RecordEffectiveSize(size_t{1002});
ASSERT_EQ(1005, tpstats.GetSuggestedPrefetchSize());
// One single super large value shouldn't influence much
tpstats.RecordEffectiveSize(size_t{1002000});
tpstats.RecordEffectiveSize(size_t{999});
ASSERT_LE(1005, tpstats.GetSuggestedPrefetchSize());
ASSERT_GT(1200, tpstats.GetSuggestedPrefetchSize());
// Only history of 32 is kept
for (int i = 0; i < 32; i++) {
tpstats.RecordEffectiveSize(size_t{100});
}
ASSERT_EQ(100, tpstats.GetSuggestedPrefetchSize());
// 16 large values and 16 small values. The result should be closer
// to the small value as the algorithm.
for (int i = 0; i < 16; i++) {
tpstats.RecordEffectiveSize(size_t{1000});
}
tpstats.RecordEffectiveSize(size_t{10});
tpstats.RecordEffectiveSize(size_t{20});
for (int i = 0; i < 6; i++) {
tpstats.RecordEffectiveSize(size_t{100});
}
ASSERT_LE(80, tpstats.GetSuggestedPrefetchSize());
ASSERT_GT(200, tpstats.GetSuggestedPrefetchSize());
}
TEST_F(BBTTailPrefetchTest, FilePrefetchBufferMinOffset) {
TailPrefetchStats tpstats;
FilePrefetchBuffer buffer(nullptr, 0, 0, false, true);
buffer.TryReadFromCache(500, 10, nullptr);
buffer.TryReadFromCache(480, 10, nullptr);
buffer.TryReadFromCache(490, 10, nullptr);
ASSERT_EQ(480, buffer.min_offset_read());
}
} // namespace rocksdb
int main(int argc, char** argv) {

5
util/file_reader_writer.cc
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@ -714,7 +714,10 @@ Status FilePrefetchBuffer::Prefetch(RandomAccessFileReader* reader,
bool FilePrefetchBuffer::TryReadFromCache(uint64_t offset, size_t n,
Slice* result) {
if (offset < buffer_offset_) {
if (track_min_offset_ && offset < min_offset_read_) {
min_offset_read_ = offset;
}
if (!enable_ || offset < buffer_offset_) {
return false;
}

21
util/file_reader_writer.h
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@ -213,21 +213,38 @@ class WritableFileWriter {
// readahead_size will be doubled on every IO, until max_readahead_size.
class FilePrefetchBuffer {
public:
// If `track_min_offset` is true, track minimum offset ever read.
FilePrefetchBuffer(RandomAccessFileReader* file_reader = nullptr,
size_t readadhead_size = 0, size_t max_readahead_size = 0)
size_t readadhead_size = 0, size_t max_readahead_size = 0,
bool enable = true, bool track_min_offset = false)
: buffer_offset_(0),
file_reader_(file_reader),
readahead_size_(readadhead_size),
max_readahead_size_(max_readahead_size) {}
max_readahead_size_(max_readahead_size),
min_offset_read_(port::kMaxSizet),
enable_(enable),
track_min_offset_(track_min_offset) {}
Status Prefetch(RandomAccessFileReader* reader, uint64_t offset, size_t n);
bool TryReadFromCache(uint64_t offset, size_t n, Slice* result);
// The minimum `offset` ever passed to TryReadFromCache(). Only be tracked
// if track_min_offset = true.
size_t min_offset_read() const { return min_offset_read_; }
private:
AlignedBuffer buffer_;
uint64_t buffer_offset_;
RandomAccessFileReader* file_reader_;
size_t readahead_size_;
size_t max_readahead_size_;
// The minimum `offset` ever passed to TryReadFromCache().
size_t min_offset_read_;
// if false, TryReadFromCache() always return false, and we only take stats
// for track_min_offset_ if track_min_offset_ = true
bool enable_;
// If true, track minimum `offset` ever passed to TryReadFromCache(), which
// can be fetched from min_offset_read().
bool track_min_offset_;
};
extern Status NewWritableFile(Env* env, const std::string& fname,

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