爲何個人Linux系統中空閒內存如此之少？

"How come there is only so few free memory on my Linux PC?"

Come to same question? No matter how much you put RAM in your motherboard, you quickly notice the free RAM is reduced so fast. Free RAM miscalculation? No!

Before answering this, first check the memory summary located on the upper side of top's display (you may need to press 'm' to unhide it). There, you will find two fields: buffers and cached. "Buffers" represent how much portion of RAM is dedicated to cache disk block. "Cached" is similar like "Buffers", only this time it caches pages from file reading. For thorough understanding of those terms, refer to Linux kernel book like Linux Kernel Development by Robert M. Love.

It is enough to understand that both "buffers" and "Cached" repre- sent the size of system cache. They dynamically grow or shrink as requested by internal Linux kernel mechanism.

Besides consumed by cache, the RAM itself is also occupied by application data and code. So, to conclude, free RAM size here means RAM area that isn't occupied by cache nor application data/code. Generally, you can consider cache area as another "free" RAM since it will be shrunk gradually if the application demands more memory.

On the task point of view, you might wonder which field truly represent memory consumption. VIRT field? certainly not! Recall that this field represent everything inside task address space, including the related shared libraries. After reading top source code and proc.txt (inside Documentation/filesystem folder of kernel source's tree), I conclude that RSS field is the best field describing task's memory consumption. I said "best" because you should consider it as approximation and isn't 100% accurate on all time.