Questions[memory](ubuntu)

When running top, I get:

top - 09:10:52 up 18:37,  1 user,  load average: 1.56, 1.33, 1.15
Tasks:  39 total,   1 running,  38 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.1%us,  0.6%sy,  1.0%ni, 92.5%id,  5.8%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   4194304k total,  3045460k used,  1148844k free,        0k buffers
Swap:        0k total,        0k used,        0k free,  2256128k cached

See the part: 2256128k cached? What exactly does that mean?

I am comparing two VPS from two provider, this one always has a large amount there (70% of the used memory) and the other VPS provider always shows 0.

In Ubuntu I have faced the segmentation fault error many times. What is a segmentation fault and when does it occur?

On Windows, I could change how much RAM Java could use by typing something like -Xincgc -Xmx2048M in the Java Runtime Parameters. How can I do that in Ubuntu?

I have used top to see the memory usage at the moment. But I would like to monitor the memory usage over a period of time. E.g start monitoring and then execute a few commands, and final stop the monitoring and see how much memory that have been used during the period.

How can I do this on Ubuntu Server?

I guess I could start a cronjob every 5th second or so, and invoke a command that log the current memory usage in a textfile. But what command should I use to get the current memory usage in a format that is easy to log to a text file?

I was wondering how do you know where the largest files in my system are stored.

For example---

Disk Space Used: 1GB Java: 500MB Java Percentage: 50% maybe represented in a pie chart. Maybe?

I know this maybe a feature overkill. I sometimes forget having stored things and wonder why my disk is so full.

So basically a command that will allow me to run on the file system and provide me with information on disk space used.

Please and thank you.