kcbenchrate - Man Page

kcbenchrate [options]

Kcbenchrate compiles a Linux kernel on each CPU core in parallel to test a system's performance or stability.

Note: The optimal number of workers ('-w') that delivers the best result depends on the machine being benched. See the section "On the Default Number of Workers" below for details.

To get comparable results from different machines you need to use the exact same operating system on all of them. There are multiple reasons for this recommendation, but one of the main reasons is: the Linux version this benchmark downloads and compiles depends on the operating system's default compiler.

If you choose to ignore this recommendation at least make sure to hard code the Linux version to compile ('-s 5.4'), as for example compiling 5.7 will take longer than 5.4 or 4.19 and thus lead to results one cannot compare. Also, make sure the compiler used on the systems you want to compare is from similar, as for example gcc10 will try harder to optimize the code than gcc8 or gcc9 and thus take more time for its work.

Kcbench is accompanied by kcbenchrate. Both are quite similar, but work slightly different:

• kcbench tries to build one kernel as fast as possible. This approach is called 'speed run' and let's make start multiple compilers jobs in parallel by using 'make -j #'. That way kcbench will use a lot of CPU cores most of the time, except during those few phases where the Linux kernel build process is singled threaded and thus utilizes just one CPU core. That for example is the case when vmlinux is linked.
• kcbenchrate tries to keep all CPU cores busy constantly by starting workers on all of them, which each builds one kernel with just one job ('make -j 1'). This approach is called 'rate run'. It takes a lot longer to generate a result than kcbench; it also needs a lot more storage space, but will utilize the machine and its processors better.

The optimal number of workers (-w) in most cases will be identical to the number of CPU cores in the tested machine, that's why this is the default. But some systems might be a bit faster if they are oversubscribed a little. Others might be quicker if you only utilize the real CPU cores and let the cores idle which are only available due to SMT (Simultaneous Multi-Threading, also called Hyper-threading/HT by Intel).

For details and some results that show unexpected effects see the kcbench man page in the section 'ON THE DEFAULT NUMBER OF JOBS'.

Ideally kcbenchrate would do what kcbench does and try a few settings to narrow down the optimal setting. As this would take quite a while this exercise is left to the user. Impatient users should consider finding the optimal number of jobs with kcbench and then try to start kernbenchrate with as many workers, as it might be a good setting for it as well. You can also try to experiment with the number of jobs used per worker (-j), maybe some machines perform best if you start worker on every second core, but use 2 jobs per worker.

The compilation is unlikely to fail, as long as you are using a settled GCC version to natively compile the source of a current Linux kernel for popular architectures like ARM, ARM64/Aarch64, or x86_64. For other cases there is a bigger risk that compilation will fail due to factors outside of what kcbench/kcbenchrate control. They nevertheless try to catch a few common problems and warn, but they can not catch them all, as there are to many factors involved:

• Brand new compiler generations are sometimes stricter than their predecessors and thus might fail to compile even the latest Linux kernel version. You might need to use a pre-release version of the next Linux kernel release to make it work or simply need to wait until the compiler or kernel developers solve the problem.
• Distributions enable different compiler features that might have an impact on the kernel compilation. For example gcc9 was capable of compiling Linux 4.19 on many distributions, but started to fail on Ubuntu 19.10 due to a feature that got enabled in its GCC. Try compiling a newer Linux kernel version in this case.
• Cross compilation increases the risk of running into compile problems in general, as there are many compilers and architectures our there. That for example is why compiling the Linux kernel for an unpopular architecture is more likely to fail due to bugs in the compiler or the Linux kernel sources that nobody had noticed before when the compiler or kernel was released. This is even more likely to happen if you start kcbench/kcbenchrate with '-m/--allmodconfig' to build a more complex kernel.

Running benchmarks is very tricky. Here are a few of the aspects you should keep mind when doing so:

• Do not compare results from two different archs (like ARM64 and x86_64); kcbench/kcbenchrate compile different code in that case, as they will compile a native kernel on each of those archs. This can be avoided by cross compiling for a third arch that is not related to any of the archs compared (say RISC-V when comparing ARM64 and x86_64).
• Unless you want to bench compilers do not compare results from different compiler generations, as they will apply different optimizations techniques. For example to not compare results from GCC7 and GCC9, as the later optimizes harder and thus will take more time generating the code. That's also why the Linux version compiled by default depends on the machine's compiler: you sometimes can't compile older kernels with the latest compilers anyway, as new compiler generations often uncover bugs in the Linux kernel source that need get fixed for compiling to succeed. For example, when GCC10 was close to release it was incapable of compile the then latest Linux version 5.5 in an allmodconfig configuration due to a bug in the Linux kernel sources.
• Compiling a Linux kernel scales very well and thus can utilize processors quite well. But be aware that some parts of the Linux compile process will only use one thread (and thus one CPU core), for example when linking vmlinuz; the other cores will idle meanwhile. The effect on the result will grow with the number of CPU cores.

If you want to work against that consider using '-m' to build an allmodconfig configuration with modules; comping a newer, more complex Linux kernel version can also help. But the best way to avoid this effect is by running kcbenchrate.

• kcbench/kcbenchrate by default set CCACHE_DISABLE=1 when calling 'make' to avoid interference from ccache.

To let kcbenchrate decide everything automatically simply run:

$ kcbenchrate

By default the line you are looking for is this:

4 workers completed 8 kernels so far (avrg: 1100.75 s/run) with a rate of 13.08 kernels/hour.

On this quad-core processor four workers each compiled two kernels. On average, it took each worker 1100.77 seconds to compile one kernel image. With a speed like this the machine can compile 13.08 kernels per hour (3600/1100.75*4).

• kcbenchrate lacks something similar to 'kcbench --detailedresults'
• kcbenchrate takes the results verbatim and does not validate them for saneness. Thus, if for example there is some hiccup in the system that heavily slows down one worker temporary kcbenchrate will neither notice nor tell you.

kcbench(1), time(1)

Thorsten Leemhuis <linux [AT] leemhuis [DOT] info>

kcbench(1).