When to use full system FS vs syscall emulation SE

Question

Since syscall emulation is much easier to setup, I'm wondering what are the advantages of using the full system emulation when running an userland program.

Or in other words, what interesting aspects are modeled in the full system but not syscall emulation mode, and when are they significant?

It is mentioned in the docs at: http://gem5.org/Splash_benchmarks that full system is

Realistic: you're getting the actual Linux thread scheduler to schedule your threads

Is this the only advantage, or are there any other advantage for users that are optimizing their applications or investigating micro-architecture?

I also suspect that the MMU simulation is another important feature that is only modeled properly in full system mode, and could affect program performance.

Answer 1

This is the superficial understanding that I've gathered so far.

Advantages of SE:

• sometimes easier to setup benchmarks, if all syscalls you need are implemented (see also, see also), and if you have just the right cross compiler, which of course no one has documented properly which one that is.
• SE runs Dhrystone about 2x https://github.com/cirosantilli/linux-kernel-module-cheat/tree/00d282d912173b72c63c0a2cc893a97d45498da5#user-mode-vs-full-system-benchmark That benchmark makes no syscalls.

Disadvantages of SE:

• in practice, harder to setup benchmarks, because it is too fragile / has too many restrictions.

  If your content does not work immediately out of the box, it is easier to just create or download a full system image and go for that instead, which is much more reliable.

  Here is a sample minimal working Ubuntu setup if you are still interested: How to compile and run an executable in gem5 syscall emulation mode with se.py?

• less representative, since no actual OS is running

• no dynamic linking for ARM as of June 2018: How to run a dynamically linked executable syscall emulation mode se.py in gem5?

• multithreading requires more hacks through https://github.com/gem5/m5threads

So my recommendation is: just use FS mode. It is just simpler to setup, more representative, and the performance hit is acceptable for most.