From Virtual to Physical
In the previous article, we introduced a way to classify the memory a process reclaimed. We used 4 quadrants using two axis: private/shared and anonymous/file-backed. We also evoked the complexity of the sharing mechanism and the fact that all memory is basically reclaimed to the kernel.
Everything we talked about was virtual. It was all about reservation of memory addresses, but a reserved address is not always immediately mapped to physical memory by the kernel. Most of the time, the kernel delays the actual allocation of physical memory until the time of the first access (or the time of the first write in some cases)… and even then, this is done with the granularity of a page (commonly 4KiB). Moreover, some pages may be swapped out after being allocated, that means they get written to disk in order to allow other pages to be put in RAM.
As a consequence, knowing the actual size of physical memory used by a process (known as resident memory of the process) is really a hard game… and the sole component of the system that actually knows about it is the kernel (it’s even one of its jobs). Fortunately, the kernel exposes some interfaces that will let you retrieve some statistics about the system or a specific process. This article enters into the depth of the tools provided by the Linux ecosystem to analyze the memory pattern of processes.
At Intersec we chose the C programming language because it gives us a full control on what we’re doing, and achieves a high level of performances. For many people, performance is just about using as few CPU instructions as possible. However, on modern hardware it’s much more complicated than just CPU. Algorithms have to deal with memory, CPU, disk and network I/Os… Each of them adds to the cost of the algorithm and each of them must be properly understood in order to guarantee both the performance and the reliability of the algorithm.
The impact of CPU (and as a consequence, the algorithmic complexity) on performances is well understood, as are disk and network latencies. However the memory seems much less understood. As our experience with our customers shows, even the output of widely used tools, such as
top, are cryptic to most system administrators.
This post is the first in a series of five about memory. We will deal with topics such as the definition of memory, how it is managed, how to read the output of tools… This series will address subjects that will be of interest for both developers and system administrators. While most rules should apply to most modern operating systems, we’ll talk more specifically about Linux and the C programming language.
At Intersec, technology matters…Because it’s the core of our business, we aim to provide our clients with the most innovative and disruptive technological solutions.
“It appears that perfection is reached not when there is nothing more to add but when there is nothing left to take away.”
Antoine de Saint–Exupéry
This blogs is driven by our Intersec R&D team. We hope to share insights about any technology related subjects, be it programming, code optimization, development tools, best practice etc…
May our experience be useful to others to always thrive for the best.