Avoiding paying the high price
The techniques falling under the second point come into play if there's something we can't avoid doing, but it has a pretty high cost tagged on to it. An example of this is interaction with the operating system or hardware, such as writing data to a file or sending a packet over the network. In this case, we resort to batching, also known in I/O context as buffering—instead of writing or sending a couple of small chunks of data right away, we first gather them and then write or send them together to avoid paying the high cost each time.
On the other hand, we can apply techniques of this type too. In I/O or memory context, this would be the prefetching of data, also known as read-ahead. When reading data from a file, we read more than the user actually requested, hoping that the next portion of data will be needed soon. In the networking context, there are examples of speculative pre-resolving of Domain Name System (DNS) addresses when a user is hovering over a link in browsers or even pre-connecting to such addresses. However, such measures can turn into its counterpart when the prediction fails, and such techniques require very careful tuning!
Related techniques to be mentioned in this context are also avoidance of system calls and avoidance of locking to spare the costs of system call and switching to the kernel context.
We'll see some applications of such techniques in last chapters of the book when we discuss I/O, graphics , and networking.
Another example of when this rule can be used is memory management. General-purpose memory allocators tend to incur rather high costs on single allocations, so the remedy is to preallocate one big buffer at first and then use it for all needs of the program by managing it by ourselves using a custom allocation strategy. If we additionally know how big our objects are going to be, we can just allocate several buffer pools for different object sizes, making the custom allocation strategy rather simple. Preallocating memory at the start used to be a classic measure to improve the performance of memory intensive programs. We'll discuss these technical C++ details in Chapter 3, Deep Dive into C++ and Performance.