I need to know how good/poorly my desktop(Loongson & Rasberry Pi 2) is running, so there should at least exits a solution that can see its realtime and history metrics of the the system like cpu, memory, network, disk io etc.

Also tools like Glances, nmon, sar(sysstat) or atop provide a nice realtime metircs, it can't get its history data easily which it more suitable for massive production use, also no web dashborard for a nicely view, so I just pass them. Systems like Icinga, Nagios, Graphite, Zabbix, etc. are so heavy and complicated which are also not convenient for a 2GB memory desktop use.

Below are some tools that may help you for setting up a tiny to small monitoring/metrics system. At the end, I'll pick up the most suitable one for my own.

psdash
A dashboard overview of the system using psutils and Flask.

The installation is quite straightforward with pip packager manager. Unfortunately, As my Debian jessie testing version, I happened to a "PROTOCOL_SSLv3 is not defined" bug that stop the system running, you can modify the python gevent lib to work around(1, 2).   

The web dashboard is qute lighweight and nice, you can get the realtime data from the every-3-second refresh web automatically, the fatal problem is it can't get the history data and charts.

Linux-dash 
Quite similar to psdash, but comes with more technical stack like Node.js, Go, and PHP. It's offical slogon is "A simple, low-overhead web dashboard for GNU / Linux. (~1MB)", besides that, not much eye-catching point.

ezservermonitor-web 
Besides all the feature psdash and Linux-dash, it also has a simple trigger configuration, Say, for the load average metric, it will be displayed with gauges, when is less than 50%, it's green, when is more than 76% util, it turns to red. Also, it has the built-in ping and servers check function, which is handy for your family internal use.

Ezservermonitor also has a console based tools called EZ SERVER MONITOR`SH, without web interface.

Web VMStat 
It’s a small application written in Java and HTML which displays live Linux system statistics. It just takes over vmstat command in a pretty web page with SmoothieCharts and websocketd in realtime.

munin 
The last is usually the best. Yes, it's the only one I want to recommend for you, no matter how many desktops you have, Munin can handle them easily and more importantly, it only taks you few minutes depending on your network quality if you use apt/yum to install it directly, the real out-of-box product. it used RRD as it's backend storage engine. by default, 300s interval which I think it's enough for most of desktop users, with at one year history. 

If you are unsatisfied with its hundreds of plugins, just write a new one or porting from any Unix platform by yourself by any scripts language. This is its killer feature.

After the above comparision, you could choose ones according to your demands.

Want near-realtime(5m or so) and history data with charts? Munin is a best option.

Want realtime data, without history data? psdash, Linux-dash, Web-VmStat are those you're looking for.

Still not satisfied? You may consider using Graphite, Zabbix if you have system admin experience, since both are enterprise level open source product.