11/9/2023 0 Comments Keyword file linux kernel![]() These backup files are important! You have been warned. There is a strong chance you will eventually make a mistake editing these files. bak extension to the end of that file so you understand it is a backup file later on.Ĭp nf Second, before editing any configuration file, we strongly recommend that you make a copy of that configuration file first. Though that should not matter (and you can use your preferred command-line text editor), this is an important distinction to note nonetheless. ![]() ![]() First, we used the Nano text editor for the examples below. The general idea and process are typically close to what is mentioned below, though.īefore continuing, we need to mention two other things. In these cases, you may have to consult the documentation for the Linux distribution that you are configuring. We make note of this when possible and appropriate below, but if you follow the information in this guide and don't see the configuration file we mentioned, fear not, it's simply called something else for the Linux distribution you are using. For instance, Red Hat and Ubuntu use different applications for networking services in their versions of Linux. Instead, we've focused on some of the more important configuration files for a few different Linux distributions.Įach Linux distribution may use different apps or services for the same function. We couldn't possibly cover every configuration file for each Linux distribution in this mini-book. These configuration options are easy to change as well as migrate to other systems, but these files can be more tedious to document as well. This makes management of the Linux OS both much easier and more difficult. Configurations for applications in Linux are stored in individual configuration files. The Registry is an ultra-fast database that stores configurations for the Windows OS and its applications. The Windows OS does use per-application configuration files as well, but Windows largely depends on something called the Registry. These files are one of the biggest differences between the Linux and Windows operating systems. Linux configuration files are the heart of the Linux operating system. Luckily, our Linux tutorial makes things easier. You see, editing Linux configuration files can be a long, monotonous task, but it's a process that every Linux admin must learn. That was only an attempt to spice up this rather long Linux primer a bit. At least that's what the movies want you to think….ĭid your eyes go cross reading that first line? Don't worry about it. The printf (compiled to _printf_chk) was moved to the very end of the function, after puts and the return to improve branch prediction as mentioned by other answers.Only 733t hax0rs know Nix. Now replace if (i) with: if (_builtin_expect(i, 0)) The instruction order in memory was unchanged: first the printf and then puts and the retq return. */Ĭompile and decompile with GCC 4.8.2 x86_64 Linux: gcc -c -O3 -std=gnu11 main.c * Use time to prevent it from being optimized away. Without _builtin_expect #include "stdio.h" Let's decompile to see what GCC 4.8 does with it Also to hint the decision to make branchy vs. The likely and unlikely macros or C++ ] / ] annotations can hint the compiler's branch layout to favour I-cache locality for the fast path, and minimize taken branches on the fast path. Is it possible to tell the branch predictor how likely it is to follow the branch? (on most ISA,s no).Why did Intel change the static branch prediction mechanism over these years?. ![]() And on modern implementations like x86 since 2013 or so, even that's not a thing anymore: Note that most ISAs don't have a way for the machine code to actually hint the hardware branch predictor, other than static prediction (backward taken / forward not-taken) on some. They don't really care too much about portability as they only target gcc, and they have a very close idea of the assembly they want it to generate. Generally the Linux developers are pretty experienced so I would imagine they would have done that. Like all such performance optimisations you should only do it after extensive profiling to ensure the code really is in a bottleneck, and probably given the micro nature, that it is being run in a tight loop. So long as the prediction is correct most of the time, this will tend to be good for performance. ![]() On the other hand if the prediction is wrong, then it means the processor pipeline needs to be flushed and it can cost several cycles. This can be a big win, if the prediction is correct it means that the jump instruction is basically free and will take zero cycles. They are hint to the compiler to emit instructions that will cause branch prediction to favour the "likely" side of a jump instruction. ![]()
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