Getting Started

Practical guide to use HermitCore

A lightweight unikernel for a scalable and predictable runtime behavior

The project HermitCore is a novel unikernel targeting at a scalable and predictable runtime behavior for HPC and cloud environments. HermitCore could be used a classical unikernel within a virtual machine. In addition, it extends the multi-kernel approach (like McKernel) with unikernel features to provide better programmability and scalability for hierarchical systems. By starting HermitCore applications as multi-kernel, cores will be isolated from the Linux system and the applications run bare-metal on these cores. This approach achieves a lower OS jitter and a better scalability. HermitCore applications and the Linux system can communicate via an IP interface (e.g., inter-kernel communication) or via the message passing interface iRCCE, which was initially designed for the Single-Chip Cloud Computer (SCC).

HermitCore provides a comprehensive toolchain, which is based on the GCC and supports C/C++, Fortran, Go, Pthreads, and OpenMP. For an improvement of the performance, Intel’s OpenMP runtime is used instead of GCC’ per default. Therefore, Intel’s C-Compiler icc could be used as well to build HermitCore applications. Both runtimes are part of HermitCore’s source code.

In addition to the multi-kernel approach described above, HermitCore can be used as classical standalone unikernel as well. In this case HermitCore run a single-kernel exclusive on the hardware or within a virtual machine. This reduces the demand on resources and improves the boot time, which is an excellent behavior for cloud computing.

HermitCore is the result of a research project at RWTH Aachen University and is currently an experimental approach, i.e., not production ready. Please use it carefully.

The first paper, which sketchs the design of HermitCore, was presented at the International Workshop on Runtime and Operating Systems for Supercomputers (ROSS 2016). The slides summarize the design and present the first performance results. At the 30th International Conference on Architecture of Computing Systems the migration from a pure multi-kernel to a kernel, which supports a multi-kernel and also a unikernel mode, was presented.


HermitCore is being developed on GitHub. Create your own fork, send us a pull request, and chat with us on Slack.


The build process works currently only on x86-based Linux systems. To build the HermitCore kernel and applications you need:

  • CMake
  • Netwide Assember (NASM)
  • recent host compiler such as GCC
  • HermitCore cross-toolchain, i.e. Binutils, GCC, newlib, pthreads

HermitCore cross-toolchain

We provide prebuilt packages (currently Debian-based only) of the HermitCore toolchain, which can be installed as follows:

$ echo "deb [trusted=yes] vivid main" | sudo tee -a /etc/apt/sources.list
$ sudo apt-get -qq update
$ sudo apt-get install binutils-hermit newlib-hermit pthread-embedded-hermit gcc-hermit libhermit

For non-Debian based systems, a docker image with the complete toolchain is provided and can be installed as follows:

$ docker pull rwthos/hermitcore

The following commad starts within the new docker container a shell and mounts from the host system the directory ~/src to /src:

$ docker run -i -t -v ~/src:/src rwthos/hermitcore:latest

Within the shell the cross-toolchain can be used to build HermitCore applications.

If you want to build the toolchain yourself, have a look at the repository hermit-toolchain, which contains scripts to build the whole toolchain.

Depending on how you want to use HermitCore, you might need additional packages such as:

  • QEMU (apt-get install qemu-system-x86)

Building HermitCore

Preliminary work

To build HermitCore from source (without compiler), the repository with its submodules has to be cloned.

$ git clone
$ cd HermitCore
$ git submodule init
$ git submodule update

We require a fairly recent version of CMake (3.7) which is not yet present in most Linux distributions. We therefore provide a helper script that fetches the required CMake binaries from the upstream project and stores them locally, so you only need to download it once.

$ . cmake/
-- Downloading CMake
--2017-03-28 16:13:37--
Loaded CA certificate '/etc/ssl/certs/ca-certificates.crt'
Connecting to||:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 30681434 (29M) [application/x-gzip]
Saving to: ‘cmake-3.7.2-Linux-x86_64.tar.gz’

cmake-3.7.2-Linux-x86_64.tar.gz         100%[===================>]  29,26M  3,74MB/s    in 12s     

2017-03-28 16:13:50 (2,48 MB/s) - ‘cmake-3.7.2-Linux-x86_64.tar.gz’ saved [30681434/30681434]

-- Unpacking CMake
-- Local CMake v3.7.2 installed to cmake/cmake-3.7.2-Linux-x86_64
-- Next time you source this script, no download will be necessary

So before you build HermitCore you have to source the script everytime you open a new terminal.

Building the library operating systems and its examples

To build HermitCore go to the directory with the source code, create a build directory, and call in the new dirctory cmake followed by make.

$ mkdir build
$ cd build
$ cmake ..
$ make
$ sudo make install

If your toolchain is not located in /opt/hermit/bin then you have to supply its location to the cmake command above like so:

$ cmake -DTOOLCHAIN_BIN_DIR=/home/user/hermit/bin ..

Assuming that binaries like x86_64-hermit-gcc and friends are located in that directory. To install your new version in the same directory, you have to set the installation path and to install HermitCore as follows:

$ cmake -DTOOLCHAIN_BIN_DIR=/home/user/hermit/bin -DCMAKE_INSTALL_PREFIX=/home/user/hermit ..
$ make
$ make install

Note: If you use the cross compiler outside of this repository, the compiler uses per default the library operating systems located by the toolchain (e.g. /opt/hermit/x86_64-hermit/lib/libhermit.a).


Part of HermitCore is a small helper tool, which is called proxy. This tool helps to start HermitCore applications within a virtual machine or bare-metal on a NUMA node. In principle it is a bridge to the Linux system. If the proxy is register as loader to the Linux system, HermitCore applications can be started like common Linux applications. The proxy can be registered with following command.

$ sudo -c sh 'echo ":hermit:M:7:\\x42::/opt/hermit/bin/proxy:" > /proc/sys/fs/binfmt_misc/register'
$ # dirct call of a HermitCore appliaction
$ /opt/hermit/x86_64-hermit/extra/tests/hello

Otherwise the proxy must be started directly and get the path to HermitCore application as argument. Afterwards, the proxy start the HermitCore applications within a VM ore bare-metal on a NUMA node.

$ # using QEMU
$ HERMIT_ISLE=qemu /opt/hermit/bin/proxy /opt/hermit/x86_64-hermit/extra/tests/hello


As classical standalone unikernel within a virtual machine

HermitCore applications can be directly started as standalone kernel within a virtual machine. In this case, iRCCE is not supported.

$ cd build
$ make install DESTDIR=~/hermit-build
$ cd ~/hermit-build/opt/hermit
$ # using QEMU
$ HERMIT_ISLE=qemu bin/proxy x86_64-hermit/extra/tests/hello
$ # using uHyve
$ HERMIT_ISLE=uhyve bin/proxy x86_64-hermit/extra/tests/hello

With HERMIT_ISLE=qemu, the application will be started within a QEMU VM. Please note that the loader requires QEMU and uses per default KVM. Furthermore, it expects that the executable is called qemu-system-x86_64.

With HERMIT_ISLE=uhyve, the application will be started within a thin hypervisor powered by Linux’s KVM API and therefore requires KVM support. uhyve has a considerably smaller startup time than QEMU, but lacks some features such as GDB debugging. In principle, it is an extension of ukvm.

In this context, the environment variable HERMIT_CPUS specifies the number of cpus (and no longer a range of core ids). Furthermore, the variable HERMIT_MEM defines the memory size of the virtual machine. The suffix of M or G can be used to specify a value in megabytes or gigabytes respectively. Per default, the loader initializes a system with one core and 2 GiB RAM. For instance, the following command starts the stream benchmark in a virtual machine, which has 4 cores and 6GB memory.

$ HERMIT_ISLE=qemu HERMIT_CPUS=4 HERMIT_MEM=6G bin/proxy x86_64-hermit/extra/benchmarks/stream

To enable an ethernet device for uhyve, we have to setup a tap device on the host system. For instance, the following command establish the tap device tap100 on Linux:

$ sudo ip tuntap add tap100 mode tap
$ sudo ip addr add broadcast dev tap100
$ sudo ip link set dev tap100 up
$ sudo bash -c 'echo 1 > /proc/sys/net/ipv4/conf/tap100/proxy_arp'

Per default, uhyve’s network interface uses IP address, for the gateway and as network mask. The default configuration could be overloaded by the environment variable HERMIT_IP, HERMIT_GATEWAY and HERMIT_MASk. To enable the device, HERMIT_NETIF must be set to the name of the tap device. For instance, the following command starts an HermitCore application within uhyve and enable the network support:

$ HERMIT_ISLE=uhyve HERMIT_IP="" HERMIT_GATEWAY="" HERMIT_MASk="" HERMIT_NETIF=tap100 bin/proxy x86_64-hermit/extra/tests/hello

If qemu is used as hyervisor, the virtual machine emulates an RTL8139 ethernet interface and opens at least one TCP/IP ports. It is used for the communication between HermitCore application and its proxy. With the environment variable HERMIT_PORT, the default port (18766) can be changed for the communication.

As multi-kernel within a virtual machine

Boot the test image of a minimal Linux system within a VM. For this, go to the build directory and boot the image by our makefiles.

$ cd build
$ make qemu
$ # or 'make qemu-dep' to build HermitCore dependencies before

Within the QEMU session you can start HermitCore application just the same as traditional Linux programs:

(QEMU) $ /hermit/x86_64-hermit/extra/tests/hello
smpboot: CPU 1 is now offline
Hello World!!!
argv[0] = /hermit/x86_64-hermit/extra/tests/hello
Receive signal with number 30
Hostname: hermit.localdomain
x86: Booting SMP configuration:
smpboot: Booting Node 0 Processor 1 APIC 0x1

Per default, the virtual machine has 10 cores, 2 NUMA nodes, and 8 GiB RAM. Inside the VM runs a small Linux system, which already includes the patches for HermitCore. Per NUMA node (= HermitCore isle) there is a directory called isleX under /sys/hermit , where X represents the NUMA node ID.

The demo applications are located in the directories /hermit/x86_64-hermit/extra/{tests,benchmarks}. A HermitCore loader is already registered. By starting a HermitCore application, a proxy will be executed on the Linux system, while the HermitCore binary will be started on isle 0 with cpu 1. To change the default behavior, the environment variable HERMIT_ISLE is used to specify the (memory) location of the isle, while the environment variable HERMIT_CPUS is used to specify the cores.

For instance, HERMIT_ISLE=1 HERMIT_CPUS="3-5" /hermit/x86_64-hermit/extra/tests/hello starts a HelloWorld demo on the HermitCore isle 1, which uses the cores 3 to 5. The output messages are forwarded to the Linux proxy and printed on the Linux system.

HermitCore’s kernel messages of isleX are available via cat /sys/hermit/isleX/log.

There is a virtual IP device for the communication between the HermitCore isles and the Linux system (see output of ifconfig). Per default, the Linux system has the IP address The HermitCore isles starts with the IP address for isle 0 and is increased by one for every isle.

More HermitCore applications are available at /hermit/usr/{tests,benchmarks} which is a shared directory between the host and QEMU.

As multi-kernel on a real machine

Note: to launch HermitCore applications, root privileges are required.

A modified Linux kernel has to be installed. Afterwards switch to the branch hermit for a relative new vanilla kernel or to centos, which is compatible to the current CentOS 7 kernel. Configure the kernel with make menuconfig for your system. Be sure, that the option CONFIG_HERMIT_CORE in Processor type and features is enabled.

$ git clone
$ cd linux
$ # see comments above
$ git checkout hermit
$ make menuconfig
$ make

Install the Linux kernel and its initial ramdisk on your system (see descriptions of your Linux distribution). We recommend to disable Linux NO_HZ feature by setting the kernel parameter nohz=off.

Install HermitCore to your system (by default to /opt/hermit):

$ cd build
$ sudo make install
$ ls -l /opt/hermit

After a reboot of the system, register the HermitCore loader at your system with following command:

$ sudo -c sh 'echo ":hermit:M:7:\\x42::/opt/hermit/bin/proxy:" > /proc/sys/fs/binfmt_misc/register'

The IP device between HermitCore and Linux currently does not support IPv6. Consequently, disable it (might be slightly different on your distribution):

$ echo 'net.ipv6.conf.mmnif.disable_ipv6 = 1' | sudo tee /etc/sysctl.conf

Per default, the IP device uses a static IP address range. Linux has to use, where HermitCore isles start with (isle 0). The interface is mmnif.

Please configure your network accordingly. For CentOS, you have to create the file /etc/sysconfig/network-scripts/ifcfg-mmnif:


You can now start applications the same way as from within a virtual machine (see description above).

Building your own HermitCore applications

You can take usr/tests as a starting point to build your own applications. All that is required is that you include [...]/HermitCore/cmake/HermitCore-Application.cmake in your application’s CMakeLists.txt. It doesn’t have to reside inside the HermitCore repository. Other than that, it should behave like normal CMake.


We provide profiling support via the XRay profiler. See usr/xray/ for more information on how to use it.


If the application is started via make qemu, debugging via GDB is enabled by default on port 1234. When run via proxy (HERMIT_ISLE=qemu), set HERMIT_DEBUG=1.

$ gdb x86_64-hermit/extra/tests/hello
(gdb) target extended-remote :1234
Remote debugging using :1234
0xffffffff8100b542 in ?? ()



You can configure the -mtune=name compiler flag by adding -DMTUNE=name to the cmake command when configuring the project.

Please note, if the applications is started within a VM, the hypervisor has to support the specified architecture name.

If QEMU is started by our proxy and the environment variable HERMIT_KVM is set to 0, the virtual machine will be not accelerated by KVM. In this case, the -mtune flag should be avoided.

TCP connections

With the environment variable HERMIT_APP_PORT, an additional port can be open to establish an TCP/IP connection with your application.

Dumping the kernel log

By setting the environment variable HERMIT_VERBOSE to 1, the proxy prints at termination the kernel log messages onto the screen.

Network tracing

By setting the environment variable HERMIT_CAPTURE_NET to 1 and HERMIT_ISLE to qemu, QEMU captures the network traffic and creates the trace file qemu-vlan0.pcap. For instance with Wireshark you are able to analyze the file.


If HERMIT_MONITOR is set to 1 and HERMIT_ISLE to qemu, QEMU establishes a monitor which is available via telnet at port 18767. With the environment variable HERMIT_PORT, the default port (18766) can be changed for the communication between the HermitCore application and its proxy. The connection to the system monitor is automatically set to HERMIT_PORT+1, i.e., the default port is 18767.