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In this video, I want to tell you a little bit more about analyzing memory statistics.

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First, it helps if you know about the right terminology.

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Cache is an area of memory that is used as temporary storage

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for frequently accessed data for faster access.

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You have cache at different levels.

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From disk, you cache in RAM.

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There's internet cache, for instance, as well, that can be used on a proxy,

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which is at a completely different level.

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Linux buffer memory is used for temporary storage in memory for data while it is moved.

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Most commonly used are the write buffers.

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So when you write a file, the file is committed to memory,

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and then it sits in the buffers until it's time to flush the data to disk.

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Anonymous memory is memory that is allocated by applications.

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Swap is emulated RAM on disk, which can be used for anonymous memory.

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Virtual memory is an address space that is determined by the CPU architecture.

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It has nothing to do with swap.

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The thing is that every Linux application wants to allocate a certain amount of memory.

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That's a reservation.

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That doesn't mean that it is used,

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but it means that it is blocked for that specific application.

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And from the virtual memory, a resident memory is obtained,

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and that is memory that is actually being used.

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Active memory is memory that has recently been used,

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and inactive memory is memory that has not recently been used.

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This is pretty important for the Linux kernel to determine

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which memory pages are a candidate for moving to swap,

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or if the memory pages are in cache.

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The Linux kernel uses this to determine which memory pages can be dropped

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if more memory is needed.

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As I just mentioned, inactive file memory is eligible to be dropped

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in case of memory shortage,

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and inactive anonymous memory is eligible to be moved to swap

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in case of memory shortage.

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Now, if memory shortage is bad,

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then active anonymous memory may be moved to swap as well,

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and that is also bad because your system will be very busy moving data

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in and out of swap, and that won't make it any faster.

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To monitor memory, 3-M is a good start for a generic overview.

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VMstat gives more detailed memory usage information,

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and for the ultimate detail, you want to check out cat-proc meminfo.

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That is where the kernel stores information about memory.

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Let's check it out.

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So here we have 3-M output, where we see the total amount of memory,

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the amount of memory that is used, and the amount of memory that is free.

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And, hey, in case you are thinking, well, between used and free and total,

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there's a difference. What is in this difference?

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Well, that is your buffer cache.

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Total memory is buffer cache plus shared plus free plus used.

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Now, the available parameter considers memory that is currently free,

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but also inactive memory in cache,

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which can be cleared immediately to make space for more memory.

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This system also has 2 GB of swap,

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but the 2 GB of swap is not used at all, and that is what we see here.

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There's no need to use swap, and the Linux kernel doesn't favor

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moving inactive anonymous memory to swap if it doesn't have to.

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I mean, we have enough memory available here.

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Now, VMstat is used with a couple of parameters.

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I'm using VMstat 2.5, which is more than enough.

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The 2 is for a 2-second interval,

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and the 5 means that I want to do it 5 times.

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And right here, I can see my overview of memory,

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and I can see SISO, which is indicating the movement of memory.

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Swap in, swap out.

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So if I have swap that is actively being used,

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I should see a high activity here.

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Now, finally, if you want to know more about the state of your system,

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check out procmeminfo.

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In procmeminfo, you find a lot of parameters.

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The parameters that are most interesting are right here,

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active and inactive memory.

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So active is what really needs to be used

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and needs to be in your RAM, and inactive,

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well, it could be doing something else as well.

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And as you can see on this system,

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we have about 2 gigabytes of inactive memory,

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and that explains a lot about the current state

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of what we see in 3-M.