Threads in x86 assembler (using the GNU assember: as)

Question

Whilst learning the "assembler language" (in linux on a x86 architecture using the GNU as assembler), one of the aha moments was the possibility of using system calls. These system calls come in very handy and are sometimes even necessary as your program runs in user-space.
However system calls are rather expensive in terms of performance as they require an interrupt (and of course a system call) which means that a context switch must be made from your current active program in user-space to the system running in kernel-space.

The point I want to make is this: I'm currently implementing a compiler (for a university project) and one of the extra features I wanted to add is the support for multi-threaded code in order to enhance the performance of the compiled program. Because some of the multi-threaded code will be automatically generated by the compiler itself, this will almost guarantee that there will be really tiny bits of multi-threaded code in it as well. In order to gain a performance win, I must be sure that using threads will make this happen.

My fear however is that, in order to use threading, I must make system calls and the necessary interrupts. The tiny little (auto-generated) threads will therefore be highly affected by the time it takes to make these system calls, which could even lead to a performance loss...

my question is therefore twofold (with an extra bonus question underneath it):

• Is it possible to write assembler code which can run multiple threads simultaneously on multiple cores at once, without the need of system calls?
• Will I get a performance gain if I have really tiny threads (tiny as in the total execution time of the thread), performance loss, or isn't it worth the effort at all?

My guess is that multithreaded assembler code is not possible without system calls. Even if this is the case, do you have a suggestion (or even better: some real code) for implementing threads as efficient as possible?

Many thanks!

Answer 1

Implement user-mode threading.

Historically, threading models are generalised as N:M, which is to say N user-mode threads running on M kernel-model threads. Modern useage is 1:1, but it wasn't always like that and it doesn't have to be like that.

You are free to maintain in a single kernel thread an arbitrary number of user-mode threads. It's just that it's your responsibility to switch between them sufficiently often that it all looks concurrent. Your threads are of course co-operative rather than pre-emptive; you basically scatted yield() calls throughout your own code to ensure regular switching occurs.

Answer 2

First you should learn how to use threds in C. On GNU/Linux you will probably want to use Posix threads or GLib threads. Then you can simply call the C from assembly code.

Here are some pointers:

• Posix threads: link text
• A tutorial where you will learn how to call C functions from assembly: link text

Answer 3

System calls are not that slow now, with syscall or sysenter instead of int. Still, there will only be an overhead when you create or destroy the threads. Once they are running, there are no system calls. User mode threads will not really help you, since they only run on one core.

Answer 4

If you want to gain performance, you'll have to leverage kernel threads. Only the kernel can help you get code running simultaneously on more than one CPU core. Unless your program is I/O bound (or performing other blocking operations), performing user-mode cooperative multithreading (also known as fibers) is not going to gain you any performance. You'll just be performing extra context switches, but the one CPU that your real thread is running will still be running at 100% either way.

System calls have gotten faster. Modern CPUs have support for the sysenter instruction, which is significantly faster than the old int instruction. See also this article for how Linux does system calls in the fastest way possible.

Make sure that the automatically-generated multithreading has the threads run for long enough that you gain performance. Don't try to parallelize short pieces of code, you'll just waste time spawning and joining threads. Also be wary of memory effects (although these are harder to measure and predict) -- if multiple threads are accessing independent data sets, they will run much faster than if they were accessing the same data repeatedly due to the cache coherency problem.

Answer 5

The short answer is that you can't. When you write assembly code it runs sequentially (or with branches) on one and only one logical (i.e. hardware) thread. If you want some of the code to execute on another logical thread (whether on the same core, on a different core on the same CPU or even on a different CPU), you need to have the OS set up the other thread's instruction pointer (CS:EIP) to point to the code you want to run. This implies using system calls to get the OS to do what you want.

User threads won't give you the threading support that you want, because they all run on the same hardware thread.

Edit: Incorporating Ira Baxter's answer with Parlanse. If you ensure that your program has a thread running in each logical thread to begin with, then you can build your own scheduler without relying on the OS. Either way, you need a scheduler to handle hopping from one thread to another. Between calls to the scheduler, there are no special assembly instructions to handle multi-threading. The scheduler itself can't rely on any special assembly, but rather on conventions between parts of the scheduler in each thread.

Either way, whether or not you use the OS, you still have to rely on some scheduler to handle cross-thread execution.

Answer 6

"Doctor, doctor, it hurts when I do this". Doctor: "Don't do that".

The short answer is you can do multithreaded programming without calling expensive OS task management primitives. Simply ignore the OS for thread scheduling operations. This means you have to write your own thread scheduler, and simply never pass control back to the OS. (And you have to be cleverer somehow about your thread overhead than the pretty smart OS guys). We chose this approach precisely because windows process/thread/ fiber calls were all too expensive to support computation grains of a few hundred instructions.

Our PARLANSE programming langauge is a parallel programming language: See http://www.semdesigns.com/Products/Parlanse/index.html

PARLANSE runs under Windows, offers parallel "grains" as the abstract parallelism construct, and schedules such grains by a combination of a highly tuned hand-written scheduler and scheduling code generated by the PARLANSE compiler that takes into account the context of grain to minimimze scheduling overhead. For instance, the compiler ensures that the registers of a grain contain no information at the point where scheduling (e.g., "wait") might be required, and thus the scheduler code only has to save the PC and SP. In fact, quite often the scheduler code doesnt get control at all; a forked grain simply stores the forking PC and SP, switches to compiler-preallocated stack and jumps to the grain code. Completion of the grain will restart the forker.

Normally there's an interlock to synchronize grains, implemented by the compiler using native LOCK DEC instructions that implement what amounts to counting semaphores. Applications can fork logically millions of grains; the scheduler limits parent grains from generating more work if the work queues are long enough so more work won't be helpful. The scheduler implements work-stealing to allow work-starved CPUs to grab ready grains form neighboring CPU work queues. This has been implemented to handle up to 32 CPUs; but we're a bit worried that the x86 vendors may actually swamp use with more than that in the next few years!

PARLANSE is a mature langauge; we've been using it since 1997, and have implemented a several-million line parallel application in it.