OS Design: I/O optimizations
In this article I will tell you how the OS manages the I/O devices. We know that I/O devices are slower than CPU or RAM, by this reason the OS developers came up with ideas to add special schedulers, caches and buffers.
Before reading this article I strongly recommend to read this article about processes, threads and dispatcher.
DMA and I/O bus
We all know that CPU, RAM and GPU are connected to a system bus. In hardware the bus is PCI-Express bus that can transfer gygabytes of data by the very small time.
Look the diagram below:
But this connection method was modified when people developed DMA.
DMA (Direct Memory Access) is a sharing data mode, when I/O devices aren’t connected to CPU. I/O devices are connected to the separated I/O bus (SATA). This technology increases speed of transfering the data, because now CPU doesn’t handle I/O devices’ data.
Look at the diagram below:
Caching and buffering
HDD Cache (buffer memory) is a special fast memory that stores the data that is used most often by processes and CPU. This memory chip is located at every HDD/SSD.
Imagine that HDD doesn’t have the cache. We know that HDD doesn’t support parallel writing and reading, and when a lot of processes call HDD to write something, there are a queue of calls. And processes must wait until HDD won’t perform operations ahead the queue and we lose the performance.
Let’s look at the diagram below:
Cache provides quite faster reading operation than if HDD/SSD was without cache.
Schedulers and their algorithms
We know that HDD has a head that writes the data to a magnet disk or reads the data from this disk. These physical operations were too slow and developers came up with interesting solution — schedulers.
Scheduler is a program that manages I/O operations. For example, in the diagram above the scheduler responses for loading the data from HDD to cache and conversely — from cache to HDD.
Let’s look at the default situation: we have one thousand points that are located randomly at the magnet disk and we need to read all these points at the shortest time using HDD’s head.
There are 3 types of scheduling:
- Short-dated scheduling — reading/writing the data from HDD and sending it to the thread
- Medium-dated scheduling — calling to swap the data from HDD to RAM
- Long-dated scheduling — adding the thread to the queue
There are 3 groups of algorithms of reading/writing points:
- FCFS and FCLS — handle the calls in FIFO (First In First Out) or LIFO (Last In First Out) orders. These algorithms aren’t effective and sometimes we may get resource starvation
- SSTF and SJF — handle the calls with shortest turnaround time
- SCAN and LOOK — handle the calls from the beginnig of the magnet disk to ending of this disk and do the same conversely
Modern schedulers use algorithms that include advantages of SSTF/SJF and SCAN/LOOK basic algorithms.
