CPU and core
Physical core
Number of physical cores = cpu number (number of CPUs loaded on the machine) * number of cores per cpu
Virtual core
The so-called 4-core 8 thread, 4 core refers to the physical core. Through Hyper-Threading Technology, two virtual cores are simulated with one physical core, two threads per core, for a total of eight threads.
It appears to the operating system to be 8 cores, but is actually 4 physical cores.
Hyper-threading technology enables a single physical core to implement thread-level parallel computing, but not as good as two physical cores.
Single core cpu and multicore cpu
Are all a cpu, the difference is the number of cores on each cpu
Multi-core CPU is an alternative to multiple single-core CPUs. Multi-core CPUs reduce the size and power consumption.
A core can only execute one thread at the same time
Process and thread
understanding
A process is the smallest unit allocated by the operating system for resources (including cpu, memory, disk IO, etc.).
Thread is the basic unit of cpu scheduling and allocation
We open WeChat, the browser is a process
A process may have multiple subtasks, such as WeChat to accept messages and send messages. These subtasks are threads.
Resources are allocated to processes, and threads share process resources.
Compared
Thread switching
The cpu allocates a time slice (that is, the time allocated to the thread) to the thread, and switches to another thread after the time slice is executed.
The state of the thread is saved before switching, and the current state is known when the next time slice is given to this thread.
From the state of saving thread A to switching to thread B, the process of reloading the state of thread B is called context switching.
Up and down switching will consume a lot of cpu time.
Thread overhead
Context switch consumption
Thread creation and extinction overhead
Threads need to be saved to maintain the thread local stack, which will consume memory
Serial, concurrent and parallel
Serial
Multiple tasks, one execution after execution and another.
Metaphor: After dinner, watch the game.
Concurrent
Multiple threads run on a single core, one thread runs at the same time, the system keeps switching threads, and it looks like it is running at the same time. In fact, the threads keep switching.
Metaphor: I will go to the food hall to eat, and then I will go to the living room to watch the game.
parallel
Each thread is assigned to a separate core, and the threads run simultaneously.
Metaphor: Watching the game while eating.
Multi-core thread number selection
Compute-intensive
Programs are mainly complex logic judgments and complex operations.
The utilization of cpu is high, there is no need to open too many threads, and opening too many threads will waste resources because of switching context when the thread switches.
IO intensive
The program is mainly IO operations, such as disk IO (read file) and network IO (network request).
Because IO operations will block threads, cpu utilization is not high, you can open more threads, you can switch to other ready threads when blocking, improve cpu utilization.
to sum up
One way to improve performance: increase hardware level, processing speed or number of cores.
Another way: According to the scene, the number of threads is reasonably set, and the CPU utilization is improved on the software.
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