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Top Forums UNIX for Advanced & Expert Users Memory fragmentation in a Linux settop box Post 302908333 by MadeInGermany on Monday 7th of July 2014 07:56:38 AM
Old 07-07-2014
I have made the observation that the Linux kernel has performance hick-ups if there is continuous I/O.
This has to do with buffering all and everything, according to the dogma "every unused byte of memory is wasted memory". If allocation happens too fast, freeing buffers can take a long time.
But the term "memory fragmentation" does not fit here.
All commercial Unix kernels have configurable limits for buffers and caches; and behave smoothly in such a situation (and little slower in others).
In the very past I had a SuSE PAE kernel that even invoked OOM-killer at continuous I/O. The following program, invoked every 5 minutes, helped:
Code:
#!/bin/bash
dropcaches=/proc/sys/vm/drop_caches
meminfo=/proc/meminfo
[ -e $dropcaches ] || exit
lowfree=`
while read key val kb
do
 if [ "$key" = "LowFree:" ]; then
  [ $val -lt 102400 ] && echo $val
  break
 fi
done < $meminfo
`
[ -z "$lowfree" ] && exit
#enable shortly
echo 0 > $dropcaches
sleep 1
#disable buffercache, leave only dentry cache
sync
echo 1 > $dropcaches

---------- Post updated at 06:56 AM ---------- Previous update was at 03:14 AM ----------

In the case of a 64-bit kernel only the following should be called every 5 or 10 minutes:
Code:
#!/bin/bash
dropcaches=/proc/sys/vm/drop_caches
echo 1 > $dropcaches

In addition try to write smaller portions of data to the disk, in /etc/sysctl.conf:
Code:
vm.dirty_background_ratio = 5
vm.dirty_ratio = 15


Last edited by MadeInGermany; 09-14-2014 at 12:28 PM..
 

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FREE(1) 							   User Commands							   FREE(1)

NAME
free - Display amount of free and used memory in the system SYNOPSIS
free [options] DESCRIPTION
free displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the ker- nel. The information is gathered by parsing /proc/meminfo. The displayed columns are: total Total installed memory (MemTotal and SwapTotal in /proc/meminfo) used Used memory (calculated as total - free) free Unused memory (MemFree and SwapFree in /proc/meminfo) shared Memory used (mostly) by tmpfs (Shmem in /proc/meminfo, available on kernels 2.6.32, displayed as zero if not available) buffers Memory used by kernel buffers (Buffers in /proc/meminfo) cached Memory used by the page cache (calculated as Cached - Shmem in /proc/meminfo - the Cached value is actually the sum of page cache and tmpfs memory) OPTIONS
-b, --bytes Display the amount of memory in bytes. -k, --kilo Display the amount of memory in kilobytes. This is the default. -m, --mega Display the amount of memory in megabytes. -g, --giga Display the amount of memory in gigabytes. --tera Display the amount of memory in terabytes. -h, --human Show all output fields automatically scaled to shortest three digit unit and display the units of print out. Following units are used. B = bytes K = kilos M = megas G = gigas T = teras If unit is missing, and you have petabyte of RAM or swap, the number is in terabytes and columns might not be aligned with header. -c, --count count Display the result count times. Requires the -s option. -l, --lohi Show detailed low and high memory statistics. -o, --old Display the output in old format, the only difference being this option will disable the display of the "buffer adjusted" line. -s, --seconds seconds Continuously display the result delay seconds apart. You may actually specify any floating point number for delay, usleep(3) is used for microsecond resolution delay times. --si Use power of 1000 not 1024. -t, --total Display a line showing the column totals. --help Print help. -V, --version Display version information. FILES
/proc/meminfo memory information SEE ALSO
ps(1), slabtop(1), top(1), vmstat(8). AUTHORS
Written by Brian Edmonds. REPORTING BUGS
Please send bug reports to <procps@freelists.org> procps-ng September 2011 FREE(1)
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