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sortbin
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Avoid unnecessary I/O during merging

This commit is contained in:
Joris van Rantwijk 2022-06-26 01:14:19 +02:00
parent 24290acc9c
commit b091d8b1eb
1 changed files with 268 additions and 202 deletions
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444
src/sortbin.cpp
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@ -32,6 +32,7 @@
#include <algorithm>
#include <iterator>
#include <memory>
#include <numeric>
#include <stdexcept>
#include <string>
#include <system_error>
@ -87,15 +88,8 @@ struct SortStrategy
{
/** Strategy for a single merge pass. */
struct MergePass {
/** Number of records per input block into this merge pass. */
uint64_t records_per_input_block;
/** Total number of input blocks into this merge pass. */
uint64_t num_input_blocks;
/** Number of input blocks to merge into each output block. */
unsigned int branch_factor;
/** Number of records in each input block into this pass. */
std::vector<uint64_t> records_per_block;
};
/** Number of records per block during the initial sort pass. */
@ -104,6 +98,9 @@ struct SortStrategy
/** Number of blocks for the initial sort pass. */
uint64_t num_sort_blocks;
/** Number of blocks that are processed during the first merge pass. */
uint64_t num_sort_blocks_first_merge;
/** List of merge passes. */
std::vector<MergePass> merge_pass;
};
@ -411,10 +408,9 @@ public:
/**
* Read binary records from an input file with buffering.
*
* The input stream reads from a sequence of discontinuous, equally spaced
* blocks in the input file. All blocks have the same size, except for
* the last block which may be shorter if it runs to the end of the file.
* Each block contains a flat array of binary records.
* The input stream reads from a sequence of discontinuous blocks in
* the input file. Each block contains a flat array of binary records.
* An explicit list of these blocks is passed to the stream constructor.
*
* The input stream starts in the "empty" state.
* The first call to "next_block()" enables reading records from the
@ -426,6 +422,8 @@ class RecordInputStream
{
// TODO : double-buffering with delayed I/O via background thread
public:
typedef std::vector<std::tuple<uint64_t, uint64_t>> BlockList;
/**
* Construct a record input stream.
*
@ -433,12 +431,8 @@ public:
*
* @param input_file Input file where records read from.
* @param record_size Record size in bytes.
* @param start_offset Offset in input file of first input section.
* @param block_size Size of each input block in bytes.
* Must be a multiple of "record_size".
* The last input block may be shorter if it runs
* to the end of the file.
* @param block_stride Distance between start of blocks in bytes.
* @param blocks Vector of input blocks specified as tuple
* (file_offset, number_of_records).
* @param buffer_size Buffer size in bytes.
* Must be a multiple of "record_size".
* Note: Each RecordInputStream creates two buffers
@ -447,24 +441,18 @@ public:
RecordInputStream(
BinaryFile& input_file,
unsigned int record_size,
uint64_t start_offset,
uint64_t block_size,
uint64_t block_stride,
BlockList&& blocks,
size_t buffer_size)
: m_input_file(input_file),
m_record_size(record_size),
m_block_offset(start_offset),
m_block_size(block_size),
m_block_stride(block_stride),
m_next_block(0),
m_block_remaining(0),
m_file_offset(0),
m_bufpos(NULL),
m_bufend(NULL),
m_blocks(blocks),
m_buffer(buffer_size)
{
assert(start_offset <= input_file.size());
assert(block_size % record_size == 0);
assert(block_size <= block_stride);
assert(buffer_size % record_size == 0);
assert(buffer_size > record_size);
}
@ -515,16 +503,16 @@ public:
void next_block()
{
assert(m_bufpos == m_bufend);
assert(m_next_block < m_blocks.size());
uint64_t num_records;
std::tie(m_file_offset, num_records) = m_blocks[m_next_block];
m_block_remaining = num_records * m_record_size;
m_next_block++;
uint64_t file_size = m_input_file.size();
assert(m_block_offset <= file_size);
m_file_offset = m_block_offset;
m_block_remaining =
std::min(m_block_size, file_size - m_block_offset);
m_block_offset += std::min(m_block_stride,
file_size - m_block_offset);
assert(m_file_offset <= file_size);
assert(m_block_remaining <= file_size - m_file_offset);
refill_buffer();
}
@ -547,13 +535,12 @@ private:
BinaryFile& m_input_file;
const unsigned int m_record_size;
uint64_t m_block_offset;
uint64_t m_block_size;
uint64_t m_block_stride;
size_t m_next_block;
uint64_t m_block_remaining;
uint64_t m_file_offset;
unsigned char * m_bufpos;
unsigned char * m_bufend;
BlockList m_blocks;
std::vector<unsigned char> m_buffer;
};
@ -1171,16 +1158,20 @@ void single_pass(
* the last block in the file which may be smaller.
*
* @param input_file Input file.
* @param output_file Output file for this pass.
* @param output_file1 Output file for this pass.
* @param output_file2 Second output file for this pass.
* @param records_per_block Number of records per sort block.
* @param num_blocks Number of sort blocks.
* @param num_blocks_file1 Number of blocks for the first output file.
* @param ctx Reference to context structure.
*/
void sort_pass(
BinaryFile& input_file,
BinaryFile& output_file,
BinaryFile& output_file1,
BinaryFile& output_file2,
uint64_t records_per_block,
uint64_t num_blocks,
uint64_t num_blocks_file1,
const SortContext& ctx)
{
unsigned int record_size = ctx.record_size;
@ -1225,6 +1216,8 @@ void sort_pass(
block_num_records);
// Write block.
BinaryFile& output_file =
(block_index < num_blocks_file1) ? output_file1 : output_file2;
output_file.write(
buffer.data(),
first_record_idx * record_size,
@ -1232,72 +1225,11 @@ void sort_pass(
}
timer.stop();
log(ctx, "initial sort pass finished\n");
log(ctx, "end initial sort pass\n");
log(ctx, " t = %.3f seconds\n", timer.value());
}
/**
* Merge 2 sorted blocks of records into a single sorted block.
*
* @param instream1 Input stream containing block 1.
* @param instream2 Input stream containing block 2.
* @param output_stream Output stream for the merged block.
* @param record_size Record size in bytes.
*/
void merge_2_blocks(
RecordInputStream& instream1,
RecordInputStream& instream2,
RecordOutputStream& output_stream,
size_t record_size)
{
// Input blocks should not be empty.
assert(!instream1.empty());
assert(!instream2.empty());
const unsigned char * rec1 = instream1.record();
const unsigned char * rec2 = instream2.record();
// Merge until one stream runs empty.
while (true) {
// Choose which record should go first.
if (record_compare(rec1, rec2, record_size) < 0) {
// Push record from stream 1 and load next record.
output_stream.put(rec1);
instream1.next_record();
if (instream1.empty()) {
rec1 = NULL;
break;
}
rec1 = instream1.record();
} else {
// Push record from stream 2 and load next record.
output_stream.put(rec2);
instream2.next_record();
if (instream2.empty()) {
rec2 = NULL;
break;
}
rec2 = instream2.record();
}
}
// At most one of the streams still has records left.
// Copy those records to the output.
while (!instream1.empty()) {
output_stream.put(instream1.record());
instream1.next_record();
}
while (!instream2.empty()) {
output_stream.put(instream2.record());
instream2.next_record();
}
}
/**
* Merge sorted blocks of records into a single sorted block.
*
@ -1308,7 +1240,7 @@ void merge_2_blocks(
* May be less than the length of input_streams.
* @param filter_dupl True to eliminate duplicate records.
*/
void merge_n_blocks(
void merge_blocks(
std::vector<std::unique_ptr<RecordInputStream>>& input_streams,
RecordOutputStream& output_stream,
size_t record_size,
@ -1419,25 +1351,22 @@ void merge_n_blocks(
void merge_pass(
BinaryFile& input_file,
BinaryFile& output_file,
uint64_t records_per_block,
uint64_t num_blocks,
unsigned int branch_factor,
const SortStrategy::MergePass& merge_pass,
bool filter_dupl,
const SortContext& ctx)
{
assert(branch_factor > 1);
assert(branch_factor <= num_blocks);
size_t num_blocks = merge_pass.records_per_block.size();
// Only filter duplicates when the output is a single block.
assert((!filter_dupl) || (branch_factor == num_blocks));
assert((!filter_dupl) || num_blocks <= ctx.branch_factor);
Timer timer;
timer.start();
// Calculate number of buffers:
// 2 buffers per input stream + more buffers for output stream.
size_t num_output_buffers = 2 + (branch_factor - 1) / 2;
size_t num_buffers = 2 * branch_factor + num_output_buffers;
size_t num_output_buffers = 2 + (ctx.branch_factor - 1) / 2;
size_t num_buffers = 2 * ctx.branch_factor + num_output_buffers;
// Calculate buffer size.
// Must be a multiple of the record size and the transfer alignment.
@ -1446,21 +1375,23 @@ void merge_pass(
// TODO : double-buffering with I/O in separate thread
// Prepare a list of blocks for each input stream.
std::vector<RecordInputStream::BlockList> stream_blocks(ctx.branch_factor);
uint64_t file_offset = 0;
for (size_t p = 0; p < num_blocks; p++) {
uint64_t num_records = merge_pass.records_per_block[p];
unsigned int streamidx = p % ctx.branch_factor;
stream_blocks[streamidx].emplace_back(file_offset, num_records);
file_offset += num_records * ctx.record_size;
}
// Initialize input streams.
std::vector<std::unique_ptr<RecordInputStream>> input_streams;
for (unsigned int i = 0; i < branch_factor; i++) {
uint64_t block_size = records_per_block * ctx.record_size;
uint64_t start_offset = i * block_size;
uint64_t block_stride = branch_factor * block_size;
if (start_offset >= input_file.size()) {
break;
}
for (unsigned int i = 0; i < ctx.branch_factor; i++) {
input_streams.emplace_back(new RecordInputStream(
input_file,
ctx.record_size,
start_offset,
block_size,
block_stride,
std::move(stream_blocks[i]),
buffer_size));
}
@ -1475,54 +1406,35 @@ void merge_pass(
// Every group consists of "branch_factor" blocks, except the last
// group which may contain fewer blocks.
// Each group produces one output block.
uint64_t block_index = 0;
size_t block_index = 0;
while (block_index < num_blocks) {
// Determine how many blocks will be merged in this group.
unsigned int this_branch_factor = branch_factor;
if (branch_factor > num_blocks - block_index) {
this_branch_factor = num_blocks - block_index;
// If this is the first merge pass, the last group may have
// fewer than "branch_factor" blocks.
unsigned int blocks_this_group = ctx.branch_factor;
if (blocks_this_group > num_blocks - block_index) {
blocks_this_group = num_blocks - block_index;
}
// Merging a single block with itself would be dumb.
// And our strategy planner is not that dumb.
assert(blocks_this_group > 1);
// Skip to the next section of each active input stream.
for (unsigned int i = 0; i < this_branch_factor; i++) {
for (unsigned int i = 0; i < blocks_this_group; i++) {
input_streams[i]->next_block();
}
if (this_branch_factor == 1) {
// Last group contains just 1 block.
// Copy it to the output.
assert(!filter_dupl);
RecordInputStream * instream = input_streams[0].get();
while (!instream->empty()) {
output_stream.put(instream->record());
instream->next_record();
}
} else if (this_branch_factor == 2 && !filter_dupl) {
// Special case for merging 2 blocks.
merge_2_blocks(
*input_streams[0],
*input_streams[1],
output_stream,
ctx.record_size);
} else {
// Merge more than 2 blocks or filter duplicates.
merge_n_blocks(
// Merge the blocks.
merge_blocks(
input_streams,
output_stream,
ctx.record_size,
this_branch_factor,
blocks_this_group,
filter_dupl);
}
// Skip to the start of the next block group.
block_index += this_branch_factor;
block_index += blocks_this_group;
}
// Flush output stream buffers.
@ -1562,33 +1474,167 @@ SortStrategy plan_multi_pass_strategy(
uint64_t num_records = file_size / ctx.record_size;
uint64_t num_sort_blocks = 1 + (num_records - 1) / records_per_sort_block;
// A list of blocks is constructed in memory for each merge pass.
// There are several of these lists. Together they will consume
// about 40 bytes per block. If we get an insanely large input file
// with a small memory limit, this metadata could by itself consume
// too much memory.
// Let's do a sanity check to ensure that the metadata uses less than
// 25% of the memory limit.
if (num_sort_blocks >= ctx.memory_size / 4 / 40) {
throw std::logic_error(
"Not enough memory to manage the list of blocks");
}
// Plan the merge passes.
//
// In prinicple, every pass merges groups of "branch_factor" input blocks
// into one output block per group, thus reducing the number of remaining
// blocks by a factor "branch_factor".
//
// However, this gets more complicated if the merge tree is not perfectly
// balanced, which happens if the number of sort blocks is not a power
// of "branch_factor". In that case, the first merge pass will have
// to make things right by handling only a subset of the data.
//
// The first merge pass processes a subset of the sort blocks.
// It merges groups of "branch_factor" sort blocks into one output block
// per group. The last group in this pass may contain fewer than
// "branch_factor" sort blocks. After the first merge pass, the number
// of remaining blocks is an exact power of "branch_factor".
// The remaining blocks are in general not all the same size.
//
// After the first merge pass, each subsequent pass (if any) merges
// groups of exactly "branch_factor" blocks into one output block per
// group. These blocks are in general not all the same size.
//
// Example:
//
// branch_factor = 3
// num_sort_blocks = 12
//
// Sorted blocks:
// [S00] [S01] [S02] [S03] [S04] [S05] [S06] [S07] [S08] [S09] [S10] [S11]
//
// There are 3 merge passes.
// The first merge pass handles only sort blocks S00 - S04.
// The next two merge passes handle groups of exactly 3 blocks.
//
// [S00] [S01] [S02] [S03] [S04] [S05] [S06] [S07] [S08] [S09] [S10] [S11]
// | | | | |
// +-----+-----+ +--+--+
// | |
// [S00-S02] [S03-S04] [S05] [S06] [S07] [S08] [S09] [S10] [S11]
// | | | | | | | | |
// +-----------+--------+ +-----+-----+ +-----+-----+
// | | |
// [S00-S05] [S06-S08] [S09-S11]
// | | |
// +--------------------+-----------------+
// |
// [S00-S11]
//
// Note that a subset of sort blocks enter into the first merge pass
// while the remaining sort blocks go directly into the second merge pass.
// (It is also possible that all sort blocks go into the first pass,
// if the merge tree is perfectly balanced.)
//
// Determine the number of full merge passes (2nd pass and later).
unsigned int num_merge_pass = 0;
uint64_t num_merge_blocks = 1;
while (num_merge_blocks * ctx.branch_factor < num_sort_blocks) {
num_merge_blocks *= ctx.branch_factor;
num_merge_pass++;
}
// Add a first merge pass.
num_merge_pass++;
// Determine the number of merge groups in the first merge pass.
// The last group may have fewer than "branch_factor" input blocks.
uint64_t num_merge_ops_first_pass =
(num_sort_blocks - num_merge_blocks + (ctx.branch_factor - 1) - 1)
/ (ctx.branch_factor - 1);
// Determine the number of sort blocks to process in the first merge pass.
uint64_t num_sort_blocks_first_merge =
num_sort_blocks - num_merge_blocks + num_merge_ops_first_pass;
assert(num_sort_blocks_first_merge <= num_sort_blocks);
SortStrategy strategy;
strategy.records_per_sort_block = records_per_sort_block;
strategy.num_sort_blocks = num_sort_blocks;
strategy.num_sort_blocks_first_merge = num_sort_blocks_first_merge;
// Plan the merge passes.
// Start with the result of the initial sort pass.
uint64_t records_per_block = records_per_sort_block;
uint64_t num_blocks = num_sort_blocks;
// Keep merging until there is only one block left.
while (num_blocks > 1) {
// Calculate the number of blocks out of this merge pass.
uint64_t num_merged_blocks = 1 + (num_blocks - 1) / ctx.branch_factor;
// Choose the smallest branch factor that produces this nr of blocks.
unsigned int branch_factor = 1 + (num_blocks - 1) / num_merged_blocks;
SortStrategy::MergePass merge_pass;
merge_pass.records_per_input_block = records_per_block;
merge_pass.num_input_blocks = num_blocks;
merge_pass.branch_factor = branch_factor;
strategy.merge_pass.push_back(merge_pass);
// Result of this merge pass will go into the next pass.
records_per_block *= branch_factor;
num_blocks = num_merged_blocks;
// Plan the details of each merge pass.
//
// The first merge pass handles a subset of the sort blocks.
// All of these sort blocks are the same size, except possibly the
// last block if it runs to the end of the file.
{
strategy.merge_pass.emplace_back();
SortStrategy::MergePass& merge_pass = strategy.merge_pass.back();
for (size_t i = 0; i < num_sort_blocks_first_merge; i++) {
uint64_t records_this_block =
std::min(records_per_sort_block,
num_records - i * records_per_sort_block);
merge_pass.records_per_block.push_back(records_this_block);
}
}
// Plan the rest of the passes.
for (unsigned int mp = 1; mp < num_merge_pass; mp++) {
strategy.merge_pass.emplace_back();
SortStrategy::MergePass& merge_pass = strategy.merge_pass.back();
SortStrategy::MergePass& prev_pass = *(strategy.merge_pass.end() - 2);
uint64_t records_this_pass = 0;
// Output from the previous pass is input into this pass.
uint64_t records_this_block = 0;
for (size_t i = 0; i < prev_pass.records_per_block.size(); i++) {
records_this_block += prev_pass.records_per_block[i];
if ((i + 1) % ctx.branch_factor == 0) {
merge_pass.records_per_block.push_back(records_this_block);
records_this_pass += records_this_block;
records_this_block = 0;
}
}
// The last group of the first pass may merge fewer than
// "branch_factor" blocks.
if (records_this_block > 0) {
merge_pass.records_per_block.push_back(records_this_block);
records_this_pass += records_this_block;
}
if (mp == 1) {
// The second pass handles sort blocks that were skipped
// during the first pass.
for (size_t i = num_sort_blocks_first_merge;
i < num_sort_blocks;
i++) {
records_this_block = std::min(
records_per_sort_block,
num_records - i * records_per_sort_block);
merge_pass.records_per_block.push_back(records_this_block);
records_this_pass += records_this_block;
}
}
// Check that number of input blocks is divisible by branch_factor.
assert(merge_pass.records_per_block.size() % ctx.branch_factor == 0);
// Check that all records are accounted for.
assert(records_this_pass == num_records);
}
// Double-check that the last pass produces a single output block.
SortStrategy::MergePass& last_pass = strategy.merge_pass.back();
assert(last_pass.records_per_block.size() <= ctx.branch_factor);
return strategy;
}
@ -1608,6 +1654,10 @@ void sortbin(
{
log(ctx, "using memory_size = %" PRIu64 " bytes\n", ctx.memory_size);
if (ctx.branch_factor < 2) {
throw std::logic_error("Invalid branch factor (must be >= 2)");
}
// We want file I/O to occur on 4096-byte boundaries.
// To ensure this, we want to do I/O on multiples of 4096 records.
// To ensure this is possible, we need room for ~ 32k records per branch.
@ -1659,30 +1709,48 @@ void sortbin(
// The merge passes alternate between tempfile-to-outputfile and
// outputfile-to-tempfile.
// The final merge pass will be tempfile-to-outputfile.
// Depending on the number of merge passes, the initial sort pass
// will either be inputfile-to-tempfile or inputfile-to-outputfile.
BinaryFile * output_or_temp_file[2] = { &output_file, &temp_file };
BinaryFile * sort_output_file =
// The final merge pass will be tempfile-to-outputfile.
// Depending on the number of merge passes, the first merge pass
// reads either from the output file or from the tempfile.
//
// The sort pass feeds blocks into the first merge pass,
// but may also feed blocks into the second merge pass if the
// merge tree is unbalanced.
{
BinaryFile * sort_output_first_merge_pass =
output_or_temp_file[num_merge_pass % 2];
BinaryFile * sort_output_second_merge_pass =
output_or_temp_file[(num_merge_pass - 1) % 2];
// Execute the initial sort pass.
sort_pass(
input_file,
*sort_output_file,
*sort_output_first_merge_pass,
*sort_output_second_merge_pass,
strategy.records_per_sort_block,
strategy.num_sort_blocks,
strategy.num_sort_blocks_first_merge,
ctx);
}
// Execute the merge passes.
for (unsigned int mp = 0; mp < num_merge_pass; mp++) {
const SortStrategy::MergePass& mpass = strategy.merge_pass[mp];
size_t num_blocks = mpass.records_per_block.size();
uint64_t num_records = std::accumulate(
mpass.records_per_block.begin(),
mpass.records_per_block.end(),
uint64_t(0));
log(ctx,
"running merge pass %u / %u:"
"%" PRIu64 " blocks, branch factor %u\n",
" %zu blocks, %" PRIu64 " records\n",
mp,
num_merge_pass,
strategy.merge_pass[mp].num_input_blocks,
strategy.merge_pass[mp].branch_factor);
num_blocks,
num_records);
// Filter duplicate records only on the last pass.
bool filter_dupl = ctx.flag_unique && (mp + 1 == num_merge_pass);
@ -1697,9 +1765,7 @@ void sortbin(
merge_pass(
*pass_input_file,
*pass_output_file,
strategy.merge_pass[mp].records_per_input_block,
strategy.merge_pass[mp].num_input_blocks,
strategy.merge_pass[mp].branch_factor,
strategy.merge_pass[mp],
filter_dupl,
ctx);
}