Linux free memory confusion.

Question

Hi,
I am trying to know the amount of free memory in linux however get more confused.
all hereunder commands run during one minute at the same box:
1)
$ free -m
total used free shared buffers cached
Mem: 24118 23992 126 0 0 14193
-/+ buffers/cache: 9799 14319
Swap: 16383 0 16383
2)
$top
Mem: 24697152k total, 24573876k used, 123276k free, 0k buffers
Swap: 16777208k total, 84k used, 16777124k free, 14539380k cached
3)
$cat /proc/meminfo
MemTotal: 24697152 kB
MemFree: 122956 kB
Buffers: 0 kB
Cached: 14556828 kB
SwapCached: 0 kB
Active: 9905800 kB
Inactive: 14418416 kB
HighTotal: 0 kB
HighFree: 0 kB
LowTotal: 24697152 kB
LowFree: 122956 kB
SwapTotal: 16777208 kB
SwapFree: 16777124 kB
Dirty: 36 kB
Writeback: 0 kB
AnonPages: 9767396 kB
Mapped: 57128 kB
Slab: 179524 kB
PageTables: 32464 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
CommitLimit: 29125784 kB
Committed_AS: 11207060 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 8516 kB
VmallocChunk: 34359729783 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
Hugepagesize: 2048 kB
3)
Script:
---memScriptStart----
TOT=`cat /proc/meminfo | grep MemTotal: | awk '{print $2}'`
USED=`cat /proc/meminfo | grep Active: | awk '{print $2}'`
FREE=$[$TOT - $USED ]
LOG=/tmp/mem_monitor.log
echo > $LOG
SEND=0
if [ "$USED" -gt "0" ]; then
USEDPERC=$[$USED * 100 / $TOT]
echo "Used Percentage : $USEDPERC %"
TOTMB=$[$TOT / 1024 ]
USEDMB=$[$USED / 1024 ]
FREEMB=$[$TOTMB - $USEDMB ]
# echo "Used Percentage : $USEDPERC"
if [ "$USEDPERC" -gt "30" ]; then
SEND=1
STATUS="Warning"
echo "------------------------------------------------------------------" >> $LOG
echo `hostname` >> $LOG
echo "------------------------------------------------------------------" >> $LOG
echo "Total Memory (MB) : $TOTMB" >> $LOG
echo "Used Memory (MB) : $USEDMB" >> $LOG
echo "Free Memory (MB) : $FREEMB" >> $LOG
echo "Used Percentage : $USEDPERC %" >> $LOG
echo "------------------------------------------------------------------" >> $LOG
if [ "$USEDPERC" -gt "95" ]; then
STATUS="Critical"
fi
fi
fi
if [ "$FREEMB" -eq "0" ]; then
SEND=1
STATUS="Fatal"
echo "------------------------------------------------------------------" >> $LOG
echo " No free memory available in " `hostname` >>$LOG
echo "------------------------------------------------------------------" >> $LOG
fi
cat $LOG
---memScriptEnd----
Samle output:
hostname
Total Memory (MB) : 24118
Used Memory (MB) : 10532
Free Memory (MB) : 13586
Used Percentage : 43 %
Thank you,
Edited by: user592759 on May 4, 2011 4:03 AM

Answer 1

Each of the methods you listed will show you memory information, so does also vmstat, which can also be helpful. But memory management is somewhat more complex than total, used and free. What about shared memory /dev/shm, for instance? From what I understand, reporting the free memory will not help you to determine if the computer has enough memory available to perform additional tasks. Your script seems to take the total amount of memory minus the active memory and considers the rest to be free, which is of course different than what the system tools report as free.
The following should describe some of the information available:
http://www.redhat.com/advice/tips/meminfo.html
As long as the server isn’t using swap space, there’s shouldn’t be a concern about physical memory. The memory that’s currency being registered as used can be replaced if another program needs it. To get a "true" reading of memory usage, look at is the -/+ buffers/cache. From an application perspective, the output above shows this server has about 14G of free memory.
http://stackoverflow.com/questions/1407643/how-is-the-linux-calculating-memfree
MemFree in /proc/meminfo is a count of how many pages are free in the buddy allocator. This buddy allocator is the fundamental unit of physical memory allocation in the kernel; however there are a lot of ways pages can be returned to the buddy allocator in time of need - for example, freeing empty SLABs, discarding cache/buffer RAM (even if this means invalidating PTEs in a running process), or as a last resort, swapping things out.
In fact, MemFree is generally controlled to be only 5-10% of total physical RAM, with any extra free RAM being coopted into cache as time goes on. As such, MemFree alone is a very incomplete view of the overall memory situation.