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/* I/O block size definitions for coreutils
Copyright (C) 1989-2012 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
/* Include this file _after_ system headers if possible. */
/* sys/stat.h will already have been included by system.h. */
#include "stat-size.h"
/* As of Jul 2011, 64KiB is determined to be the minimium
blksize to best minimize system call overhead.
This can be tested with this script:
for i in $(seq 0 10); do
bs=$((1024*2**$i))
printf "%7s=" $bs
timeout --foreground -sINT 2 \
dd bs=$bs if=/dev/zero of=/dev/null 2>&1 \
| sed -n 's/.* \([0-9.]* [GM]B\/s\)/\1/p'
done
With the results shown for these systems:
system-1 = 1.7GHz pentium-m with 400MHz DDR2 RAM, arch=i686
system-2 = 2.1GHz i3-2310M with 1333MHz DDR3 RAM, arch=x86_64
system-3 = 3.2GHz i7-970 with 1333MHz DDR3, arch=x86_64
blksize system-1 system-2 system-3
---------------------------------------
1024 734 MB/s 1.7 GB/s 2.6 GB/s
2048 1.3 GB/s 3.0 GB/s 4.4 GB/s
4096 2.4 GB/s 5.1 GB/s 6.5 GB/s
8192 3.5 GB/s 7.3 GB/s 8.5 GB/s
16384 3.9 GB/s 9.4 GB/s 10.1 GB/s
32768 5.2 GB/s 9.9 GB/s 11.1 GB/s
65536 5.3 GB/s 11.2 GB/s 12.0 GB/s
131072 5.5 GB/s 11.8 GB/s 12.3 GB/s
262144 5.7 GB/s 11.6 GB/s 12.5 GB/s
524288 5.7 GB/s 11.4 GB/s 12.5 GB/s
1048576 5.8 GB/s 11.4 GB/s 12.6 GB/s
Note that this is to minimize system call overhead.
Other values may be appropriate to minimize file system
or disk overhead. For example on my current GNU/Linux system
the readahead setting is 128KiB which was read using:
file="."
device=$(df -P --local "$file" | tail -n1 | cut -d' ' -f1)
echo $(( $(blockdev --getra $device) * 512 ))
However there isn't a portable way to get the above.
In the future we could use the above method if available
and default to io_blksize() if not.
*/
enum { IO_BUFSIZE = 64*1024 };
static inline size_t
io_blksize (struct stat sb)
{
return MAX (IO_BUFSIZE, ST_BLKSIZE (sb));
}
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