Hi folks,
we were searching for explanations for unexpected out-of-memory
situations and have isolated unionfs as a cause.
Background:
embedded ARM-based system, Linux 2.6.24.7 kernel, 32MB memory, no swap
RootFS: jffs2-ro partition overlayed with a second jffs2-rw partition
using unionfs 2.3.3 (same effect with 2.4).
Observed effect:
Starting an application (binary size: 11MB) [1] from the unionfs
partition leads to significant less pageable memory than starting the
same application from a non-unionfs partition.
Test case:
The available free and pageable memory was determined by gradually
filling a tmpfs partition and so forcing the kernel to free up memory by
paging out application code.
Starting the application from unionfs we could fill 10MB of data into
the tmpfs [2] before the oom_killer started to kill processes. The 'RES'
value of the application (as observed with top) remained constant.
Running the same application from a non-unionfs partition the tmpfs
could be filled to 19MB before the oom_killer became active. The 'RES'
of the application was reduced to <1MB during the procedure.
This difference of ~9MB available memory corresponds to the pageable
parts of the application.
Would you have an explanation as to why Linux does not page out the
application when started from unionfs?
Kind regards,
Christian Hitz
[1] The test application was generated with the attached script
(generate.sh). The script generates a big application of which most of
the code is only executed once. So most of this code should be pageable.
About 200000 iterations are needed for an ~10MB application (on ARM).
[2] done with repeated invocations of:
dd if=/def/zero bs=1024 count=1024 of=fill1
--
Christian Hitz
Engineer
BridgeCo AG Direct +41 44 802 33 22
Ringstrasse 14 Fax +41 44 802 33 39
CH-8600 Duebendorf [EMAIL PROTECTED]
Switzerland http://www.bridgeco.net
#!/bin/sh
if [ $# -lt 2 ]; then
echo "usage: $0 num_iterations out_file"
exit
fi
functioncnt=0
echo "#include <stdio.h>" > $2
echo "" >> $2
echo "int global_counter = 0;" >> $2
echo "" >> $2
while [ $functioncnt -lt $1 ]
do
echo "void function_$functioncnt() {" >> $2
echo " int i;" >> $2
echo " for ( i = 0 ; i < 16 ; i++ ) {" >> $2
echo " global_counter++;" >> $2
echo " }" >> $2
echo "" >> $2
echo " for ( ; i >= 0 ; i-- ) {" >> $2
echo " global_counter--;" >> $2
echo " }" >> $2
echo "" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo " global_counter += 2;" >> $2
echo "}" >> $2
echo "" >> $2
functioncnt=`expr $functioncnt + 1`
done
echo "int main(int argc, char *argv[]) {" >> $2
echo " int l_cnt = 0;" >> $2
echo "" >> $2
functioncnt=0
while [ $functioncnt -lt $1 ]
do
echo " function_$functioncnt();" >> $2
functioncnt=`expr $functioncnt + 1`
done
echo "" >> $2
echo " for ( ;; ) {" >> $2
echo " if ( !((l_cnt++)%800000) ) usleep(1);" >> $2
echo " }" >> $2
echo "" >> $2
echo " return 0;" >> $2
echo "}" >> $2
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