ClickHouse/dbms/src/Storages/tests/seek_speed_test.cpp

#include <IO/ReadBufferFromFile.h>
#include <IO/WriteBufferFromFile.h>
#include <Common/Stopwatch.h>
#include <Poco/File.h>
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>

/** We test the hypothesis that skipping unnecessary parts of seek-forward never degrades overall read speed.
  * Before the measurements, it is desirable to discard disk cache: `echo 3 > /proc/sys/vm/drop_caches`.
  *
  * Result: yes, even frequent relatively short seek forward does not worsen anything on all tested parameters
  * - 1MiB of data, 16 0 0 16 vs 16 16 32 16
  * - 1GiB of data, 1048576 0 0 vs 1048576 512 1024 vs 1048576 1048576 1048576
  * - 1GiB of data, 1024 0 0 vs 1024 512 1024
  */

int main(int argc, const char ** argv)
{
    if (argc < 5 || argc > 6)
    {
        std::cerr << "Usage:\n"
            << argv[0] << " file bytes_in_block min_skip_bytes max_skip_bytes [buffer_size]" << std::endl;
        return 0;
    }

    int block = atoi(argv[2]);
    int min_skip = atoi(argv[3]);
    int max_skip = atoi(argv[4]);
    size_t buf_size = argc <= 5 ? DBMS_DEFAULT_BUFFER_SIZE : static_cast<size_t>(atoi(argv[5]));

    UInt64 size = Poco::File(argv[1]).getSize();
    UInt64 pos = 0;
    DB::ReadBufferFromFile in(argv[1], buf_size);
    char * buf = new char[block];
    int checksum = 0;
    UInt64 bytes_read = 0;

    Stopwatch watch;

    while (!in.eof())
    {
        UInt64 len = static_cast<UInt64>(rand() % (max_skip - min_skip + 1) + min_skip);
        len = std::min(len, size - pos);
        off_t seek_res = in.seek(len, SEEK_CUR);
        pos += len;
        if (seek_res != static_cast<off_t>(pos))
        {
            std::cerr << "Unexpected seek return value: " << seek_res << "; expeted " << pos << ", seeking by " << len << std::endl;
            return 1;
        }
        len = std::min(static_cast<UInt64>(block), size - pos);
        in.read(&buf[0], len);
        checksum += buf[0] + buf[block - 1];
        pos += len;
        bytes_read += len;
    }
    watch.stop();

    std::cout << checksum << std::endl;    /// don't optimize

    std::cout << "Read " << bytes_read << " out of " << size << " bytes in "
        << std::setprecision(4) << watch.elapsedSeconds() << " seconds ("
        << bytes_read / watch.elapsedSeconds() / 1000000 << " MB/sec.)" << std::endl;

    return 0;
}