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I am trying to understand the memory usage of a large scale simulation that we are trying to run. When I run it "ps" reports
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
myuser 5252 97.7 0.5 5751412 377392 ? Rs 19:49 1:15 myprogram
We have three arrays in that simulation that each occupy 1.6gb (200 million doubles). Based on the information in
What is RSS and VSZ in Linux memory management
I expected that memory to be listed under RSS, but RSS is only 377MB. Based on the information in the stackoverflow thread I concluded that the memory must be swapped out and looked at "free -m"
total used free shared buffers cached
Mem: 64391 5985 58406 0 463 1295
-/+ buffers/cache: 4226 60164
Swap: 4766 0 4766
and swap is not used at all! Aside from the fact that it is too small anyway. So where is this difference in RSS vs VSZ coming from? Why are the arrays that we allocate part of VSZ and not part of RSS?
I appreciate all help
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RSS is Resident Set Size (physically resident memory - this is currently occupying space in the machine's physical memory), and VSZ is Virtual Memory Size (address space allocated - this has addresses allocated in the process's memory map, but there isn't necessarily any actual memory behind it all right now).
VSZ is short for Virtual Memory Size. It's the total amount of memory a process may hypothetically access. It accounts for the size of the binary itself, any linked libraries, and any stack or heap allocations. When a process is started, VSZ memory becomes RSS memory, over which we'll go now.
rss is the Resident Set Size, which is the actual physical memory the process is using. vms is the Virtual Memory Size which is the virtual memory that process is using.
PS service RSS stands for Resident Set Size and shows how much RAM is utilized at the time the command is output. It also should be noted that it shows the entire stack of physically allocated memory. VSZ - Virtual Memory Size.
Simple answer to your question is that arrays are defined in virtual space thats why memory for array is shown in VSZ only when you will use array it wil become part of RSS. in my view by keeping your thinking simple will give you explanation. VSZ is virtual memory which a process can use while RSS is physical memory actually allocated at the moment. When a virtual memory is actually used OS will allocate the memory which will increase the RSS.
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