问题
So I got into a debate whether multicore CPU allows parallel execution of separate processes.
As far as I understand, each core allows executing different threads but they all have to belong to one process. Or am I wrong?
My reasoning is that, while each core has separate set of registers and L1/L2 cache (depending on hardware), they all have to share other stuff like L3 cache or TLB (I don't have a lot of knowledge about cpu architecture, so feel free to correct me).
I tried searching for an answer, but couldn't find any results (maybe the question is too dumb lol).
Thanks a lot in adance.
回答1:
Multiple threads of multiple processes can be scheduled to run on a single core. Of course, at a given time only one thread runs on the core. The queue of processes to run on the core is managed by the scheduler. A good scheduler will provide to the core a good mix of CPU-bound and I/O-bound processes so that all of the components in the machine have well-balanced load.
So a multi-core CPU allows not only parallel but also concurrent execution of processes. On the other hand, a single core CPU can allow only parallel execution. No concurrency is there in single core machines.
All the resources of a core are given to the thread/process currently running on it (not in Hyper Threading though). The first resource that is in possession of multiple processes at the same time, if I'm not wrong, is Main Memory or RAM. All processes use some part of the RAM even when they are not running on the core. To load the process to the core a Process Control Block (PCB) is loaded from RAM by setting the registers, address spaces and stack to the same state which the process was in, when it was unloaded from the core to give time to another process.
The time quantum for each process varies from a few ms to a few hundred ms. Compared to that a L1/L2 cache access is a few ns and a main memory access is a few hundred ns. The image below should be interesting:
回答2:
Two processes or threads can be run truly concurrently on separate cores provided they don't contend on a shared resource at the electronic level.
The most obvious thing to contend on in an Intel chip is the L3 cache and RAM. If you have two or more Intel chips they're talking to each other over QPI. Whilst this allows a cluster of CPUs each with their own memory controllers to operate in an SMP configuration, it becomes another thing to contend on if threads want data from another chip's memory.
In AMD chips each core has a memory controller, and Hypertransport does the job of synthesizing an SMP configuration. Pleasingly this makes all cores everywhere the pretty much the same, even in multi-chip systems (it's Hypertransport inside and outside the chips).
Both Intel and AMD have done excellent jobs of creating architectures that minimise the memory contention that occurs in a multi-core, symmetric multi-processing system without us having to think too hard about how we write software. If you want the absolute most out of your hardware you can program taking into account the underlying NUMA hardware architecture, and you may (it's really hard) reduce some of the contention that's going on.
Other things that might prevent true concurrent execution is if there's a specialised subsystem serving several cores. For example the UltraSPARC T1 shared a floating point unit between 8 cores. Obviously they can't all use it at once!
FPGAs are often seen as great things for parallelisable computations such as FFTs. However they have limited internal memory, and if the computation starts needing to store more data you have to use external RAM. That immediately limits the degree of parallelism that can be achieved, as different parts of the FPGA start contending for access to the external RAM. In such cases it is doubtful whether an FPGA is the right way to go; an FPGA clocked at 500MHz accessing RAM (which is also very slow, still) with no advanced onboard caching is not going to be as fast as a well design CPU with an advanced cache and multi memory controller subsystems.
来源：https://stackoverflow.com/questions/35635572/can-multiple-os-processes-run-in-parallel-on-multicore-cpu