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| layout | title | author |
|---|---|---|
| salama | Threads are broken | Torsten |
Having just read about rubys threads, i was moved to collect my thoughts on the topic. How this will influence implementation i am not sure yet. But good to get it out on paper as a basis for communication.
Processes
I find it helps to consider why we have threads. Before threads, unix had only processes and ipc, so inter-process-communication.
Processes were a good idea, keeping each programm save from the mistakes of others by restricting access to the processes own memory. Each process had the view of "owning" the machine, being alone on the machine as it were. Each a small turing/ von neumann machine.
But one had to wait for io, the network and so it was difficult, or even impossible to get one process to use the machine to the hilt.
IPC mechnisms were and are sockets, shared memory regions, files, each with their own sets of strengths, weaknesses and api's, all deemed complicated and slow. Each switch encurs a process switch and processes are not lightweight structures.
Thread
And so threads were born as a lightweight mechanisms of getting more things done. Concurrently, because when the one thread is in a kernel call, it is suspended.
Green or fibre
The first threads that people did without kernel support, were quickly found not to solve the problem so well. Because as any thread is calling the kernel, all threads stop. Not really that much won one might think, but wrongly.
Now that Green threads are coming back in fashion as fibres they are used for lightweight concurrency, actor programming and we find that the different viewpoint can help to express some solutions more naturally.
Kernel threads
The real solution, where the kernel knows about threads and does the scheduling, took some while to become standard and makes processes more complicated a fair degree. Luckily we don't code kernels and don't have to worry.
But we do have to deal with the issues that come up. The isse is off course data corruption. I don't even want to go into how to fix this, or the different ways that have been introduced, because the main thrust becomes clear in the next chapter:
Broken model
My main point about threads is that they are one of the worse hacks, especially in a c environemnt. Processes had a good model of a programm with a global memory. The equivalent of threads would have been shared memory with many programs connected. A nightmare. It even breaks that old turing idea and so it is very difficult to reason about what goes on in a multi threaded program, and the only ways this is achieved is by developing a more restrictive model.
In essence the thread memory model is broken. Ideally i would not like to implement it, or if implemented, at least fix it first.
But what is the fix? It is in essence what the process model was, ie each thread has it's own memory.
Thread memory
In OO it is possible to fix the thread model, just because we have no global memory access. In effect the memory model must be inverted: instead of almost all memory being shared by all threads and each thread having a small thread local storage, threads must have mostly thread specific data and a small amount of shared resources.
A thread would thus work as a process used. In essence it can update any data it sees without restrictions. It must exchange data with other threads through specified global objects, that take the role of what ipc used to be.
In an oo system this can be enforced by strict pass-by-value over thread borders.
The itc (inter thread communication) objects are the only ones that need current thread synchronization techniques. The one mechanism that could cover all needs could be a simple lists.
Salama
The original problem of what a program does during a kernel call could be solved by a very small number of kernel threads. Any kernel call would be listed and "c" threads would pick them up to execute them and return the result.
All other threads could be managed as green threads. Threads may not share objects, other than a small number of system provided.