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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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SLABTOP(1)							   User Commands							SLABTOP(1)

NAME
slabtop - display kernel slab cache information in real time SYNOPSIS
slabtop [options] DESCRIPTION
slabtop displays detailed kernel slab cache information in real time. It displays a listing of the top caches sorted by one of the listed sort criteria. It also displays a statistics header filled with slab layer information. OPTIONS
Normal invocation of slabtop does not require any options. The behavior, however, can be fine-tuned by specifying one or more of the fol- lowing flags: -d, --delay=N Refresh the display every n in seconds. By default, slabtop refreshes the display every three seconds. To exit the program, hit q. -s, --sort=S Sort by S, where S is one of the sort criteria. -o, --once Display the output once and then exit. -V, --version Display version information and exit. -h, --help Display usage information and exit. SORT CRITERIA
The following are valid sort criteria used to sort the individual slab caches and thereby determine what are the "top" slab caches to dis- play. The default sort criteria is to sort by the number of objects ("o"). The sort criteria can also be changed while slabtop is running by pressing the associated character. character description header a number of active objects ACTIVE b objects per slab OBJ/SLAB c cache size CACHE SIZE l number of slabs SLABS v number of active slabs N/A n name NAME o number of objects OBJS p pages per slab N/A s object size OBJ SIZE u cache utilization USE COMMANDS
slabtop accepts keyboard commands from the user during use. The following are supported. In the case of letters, both cases are accepted. Each of the valid sort characters are also accepted, to change the sort routine. See the section SORT CRITERIA. <SPACEBAR> Refresh the screen. Q Quit the program. FILES
/proc/slabinfo slab information SEE ALSO
free(1), ps(1), top(1), vmstat(8) NOTES
Currently, slabtop requires a 2.4 or later kernel (specifically, a version 1.1 or later /proc/slabinfo). Kernel 2.2 should be supported in the future. The slabtop statistic header is tracking how many bytes of slabs are being used and it not a measure of physical memory. The 'Slab' field in the /proc/meminfo file is tracking information about used slab physical memory. AUTHORS
Written by Chris Rivera and Robert Love. slabtop was inspired by Martin Bligh's perl script, vmtop. REPORTING BUGS
Please send bug reports to <procps@freelists.org> procps-ng June 2011 SLABTOP(1)
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