src/ioblksize.h

   1 /* I/O block size definitions for coreutils
   2    Copyright (C) 1989-2013 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  16
  17 /* Include this file _after_ system headers if possible.  */
  18
  19 /* sys/stat.h will already have been included by system.h. */
  20 #include "stat-size.h"
  21
  22
  23 /* As of Jul 2011, 64KiB is determined to be the minimium
  24    blksize to best minimize system call overhead.
  25    This can be tested with this script:
  26
  27    for i in $(seq 0 10); do
  28      bs=$((1024*2**$i))
  29      printf "%7s=" $bs
  30      timeout --foreground -sINT 2 \
  31        dd bs=$bs if=/dev/zero of=/dev/null 2>&1 \
  32          | sed -n 's/.* \([0-9.]* [GM]B\/s\)/\1/p'
  33    done
  34
  35    With the results shown for these systems:
  36    system-1 = 1.7GHz pentium-m with 400MHz DDR2 RAM, arch=i686
  37    system-2 = 2.1GHz i3-2310M with 1333MHz DDR3 RAM, arch=x86_64
  38    system-3 = 3.2GHz i7-970 with 1333MHz DDR3, arch=x86_64
  39
  40    blksize  system-1   system-2   system-3
  41    ---------------------------------------
  42       1024  734 MB/s   1.7 GB/s   2.6 GB/s
  43       2048  1.3 GB/s   3.0 GB/s   4.4 GB/s
  44       4096  2.4 GB/s   5.1 GB/s   6.5 GB/s
  45       8192  3.5 GB/s   7.3 GB/s   8.5 GB/s
  46      16384  3.9 GB/s   9.4 GB/s  10.1 GB/s
  47      32768  5.2 GB/s   9.9 GB/s  11.1 GB/s
  48      65536  5.3 GB/s  11.2 GB/s  12.0 GB/s
  49     131072  5.5 GB/s  11.8 GB/s  12.3 GB/s
  50     262144  5.7 GB/s  11.6 GB/s  12.5 GB/s
  51     524288  5.7 GB/s  11.4 GB/s  12.5 GB/s
  52    1048576  5.8 GB/s  11.4 GB/s  12.6 GB/s
  53
  54    Note that this is to minimize system call overhead.
  55    Other values may be appropriate to minimize file system
  56    or disk overhead.  For example on my current GNU/Linux system
  57    the readahead setting is 128KiB which was read using:
  58
  59    file="."
  60    device=$(df -P --local "$file" | tail -n1 | cut -d' ' -f1)
  61    echo $(( $(blockdev --getra $device) * 512 ))
  62
  63    However there isn't a portable way to get the above.
  64    In the future we could use the above method if available
  65    and default to io_blksize() if not.
  66  */
  67 enum { IO_BUFSIZE = 64*1024 };
  68 static inline size_t
  69 io_blksize (struct stat sb)
  70 {
  71   return MAX (IO_BUFSIZE, ST_BLKSIZE (sb));
  72 }