Linux – How to detect memory type under Linux

If you can modify the source code, then use rusage data to measure the RSS and record how many TCP connections are in play at the time of the measurement.

If source code cannot be changed, then use the RSS of the network app as reported by top or ps and get the number of network connections at the time of measurement from lsof -i.

Collect this data every minute while your application moves through peak load, and from that data you can come up with a formula that relates number of connections to RAM usage.

Of course there are a lot more things that you could measure, in particular you might want to measure kernel RAM usage, although tcp data structures should be predictable and calculable in advance. In any case, have a look at this question https://serverfault.com/questions/10852/what-limits-the-maximum-number-of-connections-on-a-linux-server for more information on TCP tuning and how to get a clear view of what is happening in the network stack.

What you're asking for is called DMA. You need to write a driver to reserve this memory.

Yes, I realize you said you didn't want the OS to intervene, and a driver becomes part of the OS, but in absence of a driver's reservation, the kernel believes all memory belongs to it. (Unless you tell the kernel to ignore the memory block, per Aaron's answer, that is.)

Chapter 15 (PDF) of "Linux Device Drivers, 3/e" by Rubini, Corbet and Kroah-Hartmann covers DMA and related topics.

If you want an HTML version of this, I found the second-edition version of the chapter elsewhere online. Beware that the 2nd edition is over a decade old now, having come out when kernel 2.4 was new. There's been a lot of work on the memory management subsystem of the kernel since those days, so it may not apply very well any more.