In the third post of the memory series, we briefly explained locality and why it is an important principle to keep in mind while developing a memory-intensive program. This new post is going to be more concrete and explains what actually happens behind the scene in a very simple example. This post is a follow-up to a recent interview with a (brilliant) candidate1. As a subsidiary question, we resented him with the following two structure definitions:

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Introduction Here we are! We spent 4 articles explaining what memory is, how to deal with it and what are the kind of problems you can expect from it. Even the best developers write bugs. A commonly accepted estimation seems to be around of few tens of bugs per thousand of lines of code, which is definitely quite huge. As a consequence, even if you proficiently mastered all the concepts covered by our articles, you’ll still probably have a few memory-related bugs.

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malloc() not the one-size-fits-all allocator malloc() is extremely convenient because it is generic. It does not make any assumptions about the context of the allocation and the deallocation. Such allocators may just follow each other, or be separated by a whole job execution. They may take place in the same thread, or not… Since it is generic, each allocation is different from each other, meaning that long term allocations share the same pool as short term ones.

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Developer point of view In the previous articles we dealt with memory classification and analysis from an outer point of view. We saw that memory can be allocated in different ways with various properties. In the remaining articles of the series we will take a developer point of view. At Intersec we write all of our software in C, which means that we are constantly dealing with memory management. We want our developers to have a solid knowledge of the various existing memory pools.

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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.

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Introduction 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.

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