What uses the memory on raspberry pi? - raspberry-pi

Question

On my pi after start there is no free memory, but i can not found, waht uses it:
pi#node1 ~ $ cat /proc/cpuinfo
processor : 0
model name : ARMv6-compatible processor rev 7 (v6l)
BogoMIPS : 2.00
Features : half thumb fastmult vfp edsp java tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xb76
CPU revision : 7
Hardware : BCM2708
Revision : 0013
Serial : 00000000bf2e5e5c
pi#node1 ~ $ uname -a
Linux node1 4.0.7+ #801 PREEMPT Tue Jun 30 18:15:24 BST 2015 armv6l GNU/Linux
pi#node1 ~ $ head -n1 /etc/issue
Raspbian GNU/Linux 7 \n \l
pi#node1 ~ $ grep MemTotal /proc/meminfo
MemTotal: 493868 kB
pi#node1 ~ $ grep "model name" /proc/cpuinfo
model name : ARMv6-compatible processor rev 7 (v6l)
pi#node1 ~ $ ps -eo pmem,pcpu,vsize,pid,cmd | sort -k 1 -nr | head -5
0.6 0.2 6244 2377 -bash
0.3 0.0 6748 2458 sort -k 1 -nr
0.3 0.0 4140 2457 ps -eo pmem,pcpu,vsize,pid,cmd
0.2 0.1 9484 2376 sshd: pi#pts/0
0.2 0.1 5600 2236 /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 104:107
pi#node1 ~ $ free
total used free shared buffers cached
Mem: 493868 478364 15504 0 500 4956
-/+ buffers/cache: 472908 20960
Swap: 102396 116 102280
I am not a linux expert, but if I understand it right, there is just 15Mb free memory, but no task uses more than 0.6%. Than why is not there more free?

Answer 1

Memory is not exclusively allocated by Processes.
The bootloader and the init ram filesystem is stored in memory.
The kernel (could be very big) is loaded into memory.
The kernel reserve memory for it's processes. ps shows 0.0% for these system processes.
Driver allocate buffer memory
The graphics card needs memory
If you have not configured your swap space on a harddrive or SD card, it uses memory.
The network system allocates memory for unix sockets and shared memory.
100 processes with 0.1 % are 10%.
And, if you start a process and stop it not all of it memory will be released.
Try it. Show the memory usage with free. Start a process that need some memory. Stop the process and use free again. I would bet that there is more memory usage than before.
Edit
Here is an example of a pi with less memory usage. I have no problems running java on it. I have a WLAN Dongle and a original NOIR CAM installed.
I installed Raspbian Wheezy. I used a kernel that I compiled from sources:
> uname -a
Linux raspberrypi 3.18.14+ #2 PREEMPT Sun May 31 20:19:04 UTC 2015 armv6l GNU/Linux
> head -n1 /etc/issue
Raspbian GNU/Linux 7 \n \l
On this pi I can run java -version in an acceptable period of time.
time java -version
java version "1.8.0"
Java(TM) SE Runtime Environment (build 1.8.0-b132)
Java HotSpot(TM) Client VM (build 25.0-b70, mixed mode)
real 0m1.012s
user 0m0.800s
sys 0m0.190s
Here is my memory footprint
> free
total used free shared buffers cached
Mem: 380816 138304 242512 0 8916 96728
-/+ buffers/cache: 32660 348156
Swap: 102396 0 102396