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World’s smallest No SQL Database: Memory

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The first item to go over with the World’s smallest No SQL database is how it uses memory. The codebase is really simple, which is pretty much the point, but can you think about the implications of the current codebase with regards to memory?

All the data is kept in a ConcurrentDictionary<string,byte[]>, and there is an unspoken assumption. Both keys and values are assumed to be small. What happens if that isn’t the case? Well, in .NET, you are going to get memory fragmentation pretty quickly. That means that on 32 bits, you are probably going to be able to store a lot less than the 2 GB of virtual memory that you get from the OS.

We will ignore that as well and move to other aspects of the code. What about how the system actually work in production? There is a lot of work that goes into ensuring that the CPU will be able to access memory efficiently. In particular, most dbs uses some form of an in memory data structure that take advantage of the L1, L2, L3 cache of the CPU. Usually something like a B+Tree. How does the dictionary compare here?

For that matter, what happens when we grow really big? What about when we are paging? How would that work? Can we just go over the keys in the dictionary without having to page through the values as well?


I think that you get my drift by now. And this is literally just me throwing stuff off the top of my head. There are a lot of other things that we need to worry about. For example, if we implement the sensible notion of a buffer pool, we might actually have a buffer that was released by a write but is currently being sent to the client, so we can’t just use buffers, we have to use ref counting as well.

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Published at DZone with permission of Ayende Rahien, DZone MVB. See the original article here.

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