user2360915 user2360915 - 5 months ago 23
Linux Question

Linux amount of swap displayed by "free" is different from "smem"

I am trying to analyze from where the amount of swap is from, and looking at smem display I get a completely different amount of swap usage.

Free shows the following :

[root@server1 ~/smem-1.3]# free -k
total used free shared buffers cached
Mem: 24554040 24197360 356680 0 510200 14443128
-/+ buffers/cache: 9244032 15310008
Swap: 20980880 2473120 18507760


And smem shows :

PID User Command Swap USS PSS RSS
...
18829 oracle oracle_1 (LOCAL=NO) 0 3.9M 98.3M 10.1G
18813 oracle oracle_1 (LOCAL=NO) 0 3.9M 98.6M 10.1G
18809 oracle oracle_1 (LOCAL=NO) 0 4.1M 99.2M 10.0G
28657 oracle ora_lms0_1 56.0K 54.1M 100.3M 4.2G
29589 oracle ora_lms1_1 964.0K 69.7M 118.9M 4.5G
29886 oracle ora_dbw1_1 5.7M 20.8M 130.9M 10.2G
29857 oracle ora_dbw0_1 4.2M 22.6M 133.0M 10.3G
11075 ccm_user /usr/java/jre1.6/bin/java - 197.8M 133.9M 135.9M 140.7M
21688 bsuser /usr/local/java/bin/java -c 30.7M 145.1M 147.2M 152.1M
29930 oracle ora_lck0_1 2.3M 58.6M 169.8M 1.0G
29901 oracle ora_smon_1 0 78.0M 195.6M 4.3G
15604 oracle /var/oragrid/jdk/jre//bin/j 65.4M 253.9M 254.3M 262.2M
-------------------------------------------------------------------------------
359 10 678.8M 2.5G 13.5G 1.2T


Why free shows me "2.4G" and smem only shows me 679M? One of them is showing some wrong result.

I need to find out where are the remaining 1.8G, or prove that free is showing wrong results.

Last but not least, the kernel is 2.6.18.

Answer Source

Well, the main issue is RSS(resident set size) and PSS(proportional set size). From http://www.selenic.com/smem/ as it says - "PSS instead measures each application's "fair share" of each shared area to give a realistic measure". On the otherhand, RSS overestimates by calculating shared memory area of multiple applications as their own. And this is why, you see the difference. In simple word, smem can differentiate between applications shared memory and rather than treating shared area as every applications own!