devutils/ATFQ/book ATFQ git

Part I · How it works

3. How memory works

  1. 3.1RAM and the memory hierarchyEvery level trades size for speed. Rough orders of magnitude: registers under 1 ns, L1 1 ns, L2 4 ns, L3 15 ns, RAM 100 ns, NVMe SSD tens of µs, network to another continent 100 ms.
  2. 3.2NUMA: when some memory is closerOn machines with several CPU sockets, each socket has its own RAM. Reading the other socket's RAM crosses a link between chips: slower, often by half or more.
  3. 3.3ECC and failing RAMRAM can return a wrong bit, mostly from a failing or badly seated module. Without ECC nothing notices: random crashes, corrupted files.