Multithreading: What is the point of more threads than cores?

Question

I thought the point of a multi-core computer is that it could run multiple threads simultaneously. In that case, if you have a quad-core machine, what\'s the point of having

Answer 1

If a thread is waiting for a resource (such as loading a value from RAM into a register, disk I/O, network access, launch a new process, query a database, or wait for user input), the processor can work on a different thread, and return to the first thread once the resource is available. This reduces the time the CPU spends idle, as the CPU can perform millions of operations instead of sitting idle.

Consider a thread that needs to read data off a hard drive. In 2014, a typical processor core operates at 2.5 GHz and may be able to execute 4 instructions per cycle. With a cycle time of 0.4 ns, the processor can execute 10 instructions per nanosecond. With typical mechanical hard drive seek times are around 10 milliseconds, the processor is capable of executing 100 million instructions in the time it takes to read a value from the hard drive. There may be significant performance improvements with hard drives with a small cache (4 MB buffer) and hybrid drives with a few GB of storage, as data latency for sequential reads or reads from the hybrid section may be several orders of magnitude faster.

A processor core can switch between threads (cost for pausing and resuming a thread is around 100 clock cycles) while the first thread waits for a high latency input (anything more expensive than registers (1 clock) and RAM (5 nanoseconds)) These include disk I/O, network access (latency of 250ms), reading data off a CD or a slow bus, or a database call. Having more threads than cores means useful work can be done while high-latency tasks are resolved.

The CPU has a thread scheduler that assigns priority to each thread, and allows a thread to sleep, then resume after a predetermined time. It is the thread scheduler's job to reduce thrashing, which would occur if each thread executed just 100 instructions before being put to sleep again. The overhead of switching threads would reduce the total useful throughput of the processor core.

For this reason, you may want to break up your problem in to a reasonable number of threads. If you were writing code to perform matrix multiplication, creating one thread per cell in the output matrix might be excessive, whereas one thread per row or per n rows in the output matrix might reduce the overhead cost of creating, pausing, and resuming threads.

This is also why branch prediction is important. If you have an if statement that requires loading a value from RAM but the body of the if and else statements use values already loaded into registers, the processor may execute one or both branches before the condition has been evaluated. Once the condition returns, the processor will apply the result of the corresponding branch and discard the other. Performing potentially useless work here is probably better than switching to a different thread, which could lead to thrashing.

As we have moved away from high clock-speed single-core processors to multi-core processors, chip design has focused on cramming more cores per die, improving on-chip resource sharing between cores, better branch prediction algorithms, better thread switching overhead, and better thread scheduling.