Disseminated, Distributed OS for Hardware Resource Disaggregation
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LegoOS is a disseminated, distributed operating system built for hardware resource disaggregation. LegoOS is a research operating system being built from scratch and released by researchers from Purdue University. LegoOS splits traditional operating system functionalities into loosely-coupled monitors, and run those monitors directly on hardware device. You can find more details from our OSDI'18 paper.

[Paper] [Keynote] [Tech Notes]

Table of Contents


Codebase Organization

Several terms in this repository are used differently from the paper description. Some of them might be used interchangeably here.

Paper Term Code Term
Resource Monitor Resource Manager
Global Resource Manager Global Resource Monitor
ExCache pcache
p-local zerofill

Now let's first get familiar with the codebase. If you have played with Linux kernel, welcome home. We reused most of Linux code to ease our own porting of InfiniBand drivers. The consequence is now LegoOS supports almost all essential Linux kernel functionalities. Overall, LegoOS has a similar directory organization:

  • arch/ is for low-level ISA-specific hooks
  • drivers/ has acpi, infiniband, pci, and tty drivers
  • init/, kernel/, lib/, and mm/ are shared essential core kernel utilities
  • linux-modules/ are Linux kernel modules for storage manager and global resource monitors

This code repository has many major subsystems (e.g., managers, monitors, networking). The following table describes where you can find the corresponding code:

Major Subsystems Directory
Processor Manager managers/processor/
Memory Manager managers/memory/
Storage Manager linux-modules/storage/
Managers' Network Stack net/
Global Process Monitor linux-modules/monitor/gpm/
Global Memory Monitor linux-modules/monitor/gmm/
Monitors' Network Stack linux-modules/fit/

As for the Processor Manager (PM), it has the following subsystems:

PM Internal Purpose Directory
pcache Virtual Cache Management managers/processor/pcache/
strace Syscall Tracer managers/processor/strace/
fs Filesystem State Layer managers/processor/fs/
mmap Virtual Memory State Layer managers/processor/mmap/
replication Memory Replication managers/processor/replication.c
fork Process Creation Notification managers/processor/fork.c
exec Execute Notification managers/processor/exec.c
misc misc all others

As for the Memory Manager (MM), it has the following subsystems:

MM Internal Purpose Directory
pcache Handle pcache Events managers/memory/handle_pcache/
loader Program Loader managers/memory/loader/
pgcache Page Cache managers/memory/pgcache/
replication Handle Memory Replication managers/memory/replica/
vm Virtual Memory managers/memory/vm/
fs Filesystem Operations managers/memory/m2s_read_write.c
misc misc all others

Storage manager and global resource monitors are not LegoOS's main focus at this stage, each of them has one simple task just as their name suggested.

Platform Requirement

LegoOS has been tested only in the following hardware setting:

Hardware Vendor and Model
Server Dell PowerEdge R730
CPU Intel Xeon E5-2620 v3
InfiniBand NIC Mellanox MCX354A-TCBT ConnectX-3 VPI

And the following toolchains:

Software Version
CentOS 7.2
GCC 4.8.5 20150623 (Red Hat 4.8.5-16)
GNU assembler (x86_64-redhat-linux)
GNU ld 20130226
GNU libc 2.17
GRUB2 2.02

Of all the above hardware and software requirments, the CPU and the Infiniband NIC are the hard requirements. Currently, LegoOS can only run on Intel x86 CPUs. As for the Infiniband NIC card, LegoOS has ported an mlx4_ib driver, which probably can run on other Mellanox cards, but we have not tested other than the one we used. As long as you have the CPU and the Infiniband NIC, we think you can run LegoOS on top your platform. You need at least two machines, connected by Infiniband switch (back-to-back connection is not supported now).

We understand that one key for an OS to be successful is let people be able to try it out. We are deeply sorry that we can not provide further technical support if you are using a different platform.

Configure and Compile

The README is still raw and scratchy, it might not be complete and it might also seems confusing. The whole tutorial can be improved only if there are people trying out LegoOS and give us feedback. If you have any issues, please don't hesitate to contact us (Github Issue is preferred). We really appreciate your input here.

CAVEAT: Configure, compile, and run a LegoOS kernel is similar to test a new Linux kernel. You need to have root access to the machine. The whole process may involve multiple machine power cycles. Before you proceed, make sure you have some methods (e.g., IPMI) to monitor and reboot remote physical machine. It is possible to just use virtual machines, but with a constrained setting (described below). If you running into any issues, please don’t hesitate to contact us!

For processor and memory manager, LegoOS uses the Linux Kconfig way. If are not familiar with it, or encounter any issues while configuring LegoOS, we recommend you refer to online Kconfig tutorials.

For storage and global resource managers, which are built as Linux kernel modules, LegoOS uses a header file to manually typeset all configurations. We will describe the details below.

Each manager or monitor should be configured and complied at its own machine's directory. To be able to run LegoOS, you need at least two physical machines.

Configure Processor or Memory Manager

The default setting of LegoOS won't require any knowledge of Kconfig, all you need to do is changing the generated .config file. If you want to hack those Kconfig files, we recommend you read the documentation from Linux kernel and some other online resources.

And note that this is just the general configuration steps. If you want to configure for specific settings, such as running with only one processor and one memory manager, please refer to the following sections for more detailed steps.

  1. make defconfig: After this doing, a .config file will be created locally.

  2. Configure Processor Manager: Open .config, find and delete the following line:

    # CONFIG_COMP_PROCESSOR is not set
  3. Configure Memory Manager: Open .config, find and delete the following line:

    # CONFIG_COMP_MEMORY is not set
  4. Step 2) and Step 3) are exclusive, you only need to configure one type of manager. After you finished one of them, type make. If you did step 2), you will see the following lines promoted, type Y and Enter. You can type Enter for all Kconfig options. Except the ones such as setting up default home memory ID, which will be covered by Network section below. For now, just set the ID to a random number (e.g., 0). All default settings works well.

    [LegoOS git:(master)] $ make
    scripts/kconfig/conf  --silentoldconfig Kconfig
    * Restart config...
    * Lego Processor Component Configurations
    Configure Lego as processor component (COMP_PROCESSOR) [N/y/?] (NEW) y
      Enable Process Checkpoint (CHECKPOINT) [N/y/?] (NEW)

After doing above steps, the LegoOS kernel will be ready at arch/x86/boot/bzImage.

Configure Processor ExCache Size

There is one more knob for processor managers: ExCache Size. We reused the old way of reserving DRAM from Linux: the memmap=nn[KMG]$ss[KMG]. Due to implementation issues, the semantic in LegoOS is not very straightforward. Basically, the actual ExCache size is half the size you specified at memmap.

Assume you want to reserve contiguous DRAM start from physical address 4GB:

  • To have a 512MB ExCache, use memmap=1G$4G
  • To have a 1GB ExCache, use memmap=2G$4G

For example, this is how it looks like in a CentOS 7, /boot/grub2/grub.cfg file if 1GB ExCache is configured:

menuentry 'CentOS Linux (4.0.0-lego+) 7 (Core)' ... {
        linux16 /vmlinuz-4.0.0-lego+ memmap=2G\$4G
        initrd16 ...

[Fat note: 1) Other bootloaders may have different semantics, and make sure you modified the right boot menuentry. 2) In grub2, make sure you are using linux16/initrd16 instead of linux/initrd. The latter pair will load kernel into a physical address where LegoOS does not support.]

At LegoOS boot time, the ExCache configuration will be printed at the very beginning, and it has to be something like memmap=X$X without any \ in the middle. Processor manager will complain if memmap goes wrong. An example output is here.

Configure Linux Modules

Storage manager, global resource monitors, and their network stack are linux kernel modules. They can only run on Linux-3.11.1. Because their network stack is only supported at this kernel version.

Once you have switched Linux-3.11.1, just go to linux-modules/ and type make, which will compile all the following modules (and their config files):

Module Config File
Storage Manager linux-modules/storage/CONFIG_LEGO_STORAGE.h
Global Resource Monitors linux-modules/monitor/include/monitor_config.h
FIT linux-modules/fit/fit_config.h

Configure Network

At current stage, setup InfiniBand connection is still a little bit complicated, and it involves hardcoded information. Unlike Ethernet, InfiniBand can not just connect to each other. It needs Ethernet to exchange some initial information first. The initial information includes: Local IDentifier (LID) and Queue Pair Number (QPN). Unfortunately, we currently do not have decent Ethernet drivers and socket code that could run everywhere. Thus, instead of using Ethernet to exchange LID and QPN, we manually hardcode them into the source code, and let InfiniBand layer use this hardcoded information directly. Do note that the hardcoded information is about remote machines, which the local machine is trying to connect to.

Also, make sure you have the InfiniBand NIC descibed in Platform Requirement. They must be connected through a InfiniBand switch.


LegoOS uses a customized network stack named FIT, which is built based on LITE. For more information of LITE, please refer to this paper. Here are some general concepts about FIT in LegoOS:

  • FIT is a layer on top of kernel InfiniBand verbs
  • FIT uses one polling thread to handle CQE, and this will not be the performance bottleneck
  • FIT builds multiple QPs between each pair of machine
  • Users of FIT share underlying QPs, multiplexed by FIT
  • LegoOS mostly just uses the ibapi_send_reply API


This subsection tries to explain several Kconfig options related to QP. You don't need to tune any configurations of this subsection for a default run. If a default setting does not work, please create a Github issue with detailed error message (especially dmesg from linux kernel modules).

The number of QPs between each pair of machine is controlled by: CONFIG_FIT_NR_QPS_PER_PAIR. The default is 12, which is the number of CPU cores (one NUMA socket) we have in our platform.

The QPN information is controlled by: CONFIG_FIT_FIRST_QPN, default to 80. This is the QPN of the first QP created by FIT layer.

For example, assume you use both above default settings, then FIT layer will have 12 QPs, and the first QP's QPN is 80. Since FIT is the only user who will create QPs, the 12 QPs will have consecutive QPNs in the range of [80, 91].

Now, the trick here is, we configure all LegoOS manager's FIT layer to use the same configuration, then each manager knows exactly what others' QPN information would be, which is [CONFIG_FIT_FIRST_QPN, CONFIG_FIT_FIRST_QPN + CONFIG_FIT_NR_QPS_PER_PAIR - 1].

And this solves the hardcoded QPN issue.


This subsection tries to explain how LID should be hardcoded. This process involes two steps: 1) get LID information from iblinkinfo, 2) build the LID table at net/lego/fit_machine.c.

InfiniBand LID can be obtained by running iblinkinfo at Linux. A snippet output from our platform would be:


CA: wuklab00 mlx4_0:
      0xe41d2d0300309251      8    1[  ] ==( 4X          10.0 Gbps Active/  LinkUp)==>      22    1[  ] "MF0;wuklab-ibsw:IS5035/U1" ( )
CA: wuklab01 mlx4_0:
      0xe41d2d0300309301     27    1[  ] ==( 4X          10.0 Gbps Active/  LinkUp)==>      22    2[  ] "MF0;wuklab-ibsw:IS5035/U1" ( )
CA: wuklab02 mlx4_0:
      0xe41d2d03003092d1     24    1[  ] ==( 4X          10.0 Gbps Active/  LinkUp)==>      22    3[  ] "MF0;wuklab-ibsw:IS5035/U1" ( )


From the above snippet, we learn a mapping between hostname and LID (hostname is not a must have, it is just like a domain name for IP address):

  • wuklab00 - LID 8
  • wuklab01 - LID 27
  • wuklab02 - LID 24

Now we have the LID information, let us hardcode them into a table at both net/lego/fit_machine.c and linux-modules/fit/fit_machine.c:

static struct fit_machine_info WUKLAB_CLUSTER[] = {
[0]     = {     .hostname =     "wuklab00",     .lid =  8,       },
[1]     = {     .hostname =     "wuklab01",     .lid =  27,      },
[2]     = {     .hostname =     "wuklab02",     .lid =  24,      },

Please make sure to fill the correct LID numbers. Any typos here will lead to an unsuccessful connection after early boot and it's hard to debug.

Node ID and Number of Machines

Now we've built the necessary information, it's time to think about the real connection. Currently, LegoOS does not support hotplug a hardware component at runtime (it is important and doable, but requires some extra pure engineering effort). Thus, you need to configure the node ID and number of connected machines at compile time.

They are described by these two configurations:


For one run, all LegoOS instance must have the same CONFIG_FIT_NR_NODES. And each LegoOS instance must have its unique CONFIG_FIT_LOCAL_ID. The detailed configuration will be described at 1P-1M and 1P-1M-1S sections.

After setting up above configurations, you also need to manually change the lego_cluster_hostnames array at net/fit/fit_machine.c. The array specifies the machines used in one run, and the array must be built based the ID sequence.

Configure Output

Setup printk()

LegoOS output debug messages (printk()) to two sources: 1) serial port, 2) VGA terminal. Mostly only the output to serial port is useful, because this can be saved and later being examined. The output to VGA is useful when we run LegoOS with virtual machine (VM), so we are able to know what's going on (pretty old school, right?).

They are controlled by the following options in Kconfig:

# TTY Layer Configurations
# CONFIG_TTY_VT is not set
# CONFIG_TTY_SERIAL_BAUD9600 is not set

To enable VGA output, enable CONFIG_TTY_VT.

To enable serial output, enable CONFIG_TTY_SERIAL.

  • Two ports are supported: ttyS0 and ttyS1, they map to CONFIG_TTY_SERIAL_TTYS0 and CONFIG_TTY_SERIAL_TTYS1, respectively. Only one of them should be enabled at one time.
  • Two baud rate are supported: 9600 and 115200, they map to CONFIG_TTY_SERIAL_BAUD9600 and CONFIG_TTY_SERIAL_BAUD115200, respectively. Only one of them can be enabled at one time.
  • For example, if the other end of serial cable is a Linux host that uses /dev/ttyS1, 115200, then the serial config at LegoOS side should use the combination of CONFIG_TTY_SERIAL_TTYS1 and CONFIG_TTY_SERIAL_BAUD115200.

Setup Serial Connection

Option 1: Virtual Machine

If LegoOS is running within a virtual machine, you will be able to configure your hypervisor to save the serial output from LegoOS to a local host's file. In this setting, each port maps to one specific file, and baud rate does not matter. For virsh + qemu environment, you can add the following script to VM's description file. Please refer to other hypervisors' manual if you are not using virsh + qemu.

<serial type='file'>
  <source path='/root/LegoOS-ttyS0'/>
  <target port='0'/>
<serial type='file'>
  <source path='/root/LegoOS-ttyS1'/>
  <target port='1'/>

(Choose any pathname you see fit)

Option 2: Physical Machine

If LegoOS is running directly on a physical machine, you will need another machine to catch the serial output. These two servers can either be 1) directly connected by serial cable, or 2) connected through a serial switch.

In a direct serial connection setting, each LegoOS machine will need one peer physical machine to catch its output. This essentially increases the machine usage by 2x. Based on our own experience, we highly recommend you setup a serial switch.

Install and Run

LegoOS's processor and memory manager pretend as a Linux kernel by having all the necessary magic numbers at bzimage header. Thus, GRUB2 will treat LegoOS kernel as a normal Linux kernel. By doing so, LegoOS can leverage all existing boot options available.

Once you have successfully compiled the processor or memory manager, you can install the image simply by typing make install. After this, you will be able to find the LegoOS kernel image installed at /boot directory. For example:

[LegoOS git:(master)] $ ll /boot/vmlinuz-4.0.0-lego+
-rw-r--r--. 1 root root 1941056 Sep 27 17:41 /boot/vmlinuz-4.0.0-lego+

LegoOS pretends as a Linux-4.0.0 to fool glibc-2.17, which somehow requires a pretty high version Linux kernel. To run LegoOS, you need to reboot machine, and then boot into LegoOS kernel.


This section describes the case where we run LegoOS with only one processor manager and one memory manager, or 1P-1M setting. This is the simplest setting in LegoOS. This setting piggybacks a statically-linked user program binary into LegoOS image, thus we don't need another storage manager. The limitation is that only a simple user-program can be staticlly-linked and piggybacked (e.g., programs at usr/), because the difficulties of compiling a large program and kernel image size limitation.

This setting requires a special Kconfig option: CONFIG_USE_RAMFS, at both processor and memory. And this setting requires two physical machines (or virtual machines running on different physical host).

  1. Network setting:
    • Set CONFIG_FIT_LOCAL_ID and CONFIG_FIT_NR_NODES properly at both processor and memory manager. For example, processor can use CONFIG_FIT_LOCAL_ID=0, CONFIG_FIT_NR_NODES=2, and memory can use CONFIG_FIT_LOCAL_ID=1, CONFIG_FIT_NR_NODES=2.
    • At processor manager, set the CONFIG_DEFAULT_MEM_NODE equals to the node ID of the memory manager. The CONFIG_DEFAULT_STORAGE_NODE will not have any effect. For example, use CONFIG_DEFAULT_MEM_NODE=1.
    • At memory manager, no need to setup default memory/storage node
  2. At both processor and memory manager, open .config, find and enable CONFIG_USE_RAMFS option.
  3. At memory manager, open .config, find CONFIG_RAMFS_OBJECT_FILE, and set it to the pathname to your test user program. The user program has to be statically-complied. To start, you can set as follows:

In 1P-1M setting, the above user program set at memory manager (usr/general.o here) will be executed automatically when processor and memory manager connected. Current LegoOS's ramfs option is limited to include only one user program.

Sample .config

We provid two .config samples for 1P-1M setting. In these samples, we are using usr/general.o and ttyS1 115200. VGA terminal output is also enabled. You can find processor manager's output log here (recorded while running LegoOS processor manager within VM).

  • Processor
    • make defconfig
    • cp Documentation/configs/1P-1M-Processor .config
    • make
  • Memory
    • make defconfig
    • cp Documentation/configs/1P-1M-Memory .config
    • make


This section describes the case where we run LegoOS with one processor manager, one memory manager, and one storage manager, or 1P-1M-1S setting. This setting emulates the effect of breaking one monolithic server and connect the CPU, memory, and disk by network. This setting requires three physical machines, and there is no need for global resource managers (Note about VM: you will be able to run processor manager and memory manager within VM, but storage can not. Because VM setting produces unstable QPN).


  1. Network setting

    • Set node ID properly, for all processor, memory, and storage managers
    • At storage manager, modify linux-modules/fit/fit_config.h
    • At both processor and memory manager
      • set CONFIG_DEFAULT_MEM_NODE equals to the node ID of the memory manager
      • set CONFIG_DEFAULT_STORAGE_NODE equals to the node ID of the storage manager
  2. Make sure CONFIG_USE_RAMFS is not configured at both processor and memory manager.

  3. At processor manager, open managers/processor/core.c file, and find the function procmgmt(), type the name and arguments of the user program that you wish to run. The user program is at the storage node, you have to use the absolute pathname from the storage node. For example, to run TensorFlow:

    static int procmgmt(void *unused)
          init_filename = "/usr/bin/python";
          argv_init[0] = init_filename;
          argv_init[1] = "/root/cifar10_main.py";

Our current way of running user program is very raw. Basically we manually specify the user program and arguments during compile time. This limits us to be able to test only one program during each run. LegoOS can not work with /bin/bash, but we have tried to use the basic fork()+wait() way to serialize testing.


After you have successfully configured and compiled the LegoOS images, you need to install the processor and memory managers and reboot these two machines. The following steps assume you have just finished compiling and tries to reboot:

  1. Install processor manager into /boot of its own machine.
  2. Install memory manager into /boot of its own machine.
  3. Reboot only processor and memory machines into vmlinux-4.0.0-lego.
  4. When both processor and memory machines hit fit: Please wait for enough MAD..., then at storage manager, do: insmod fit.ko. This may take around a minute.
  5. At storage manager, after the above command return, do: insmod storage.ko.
  6. You should be able to see many messages printed out after the above command return.

Sample .config

We provid two .config and fit_config.h samples for the 1P-1M-1S setting. To start, you can follow the below steps:

  • Processor
    • make defconfig
    • cp Documentation/configs/1P-1M-1S-Processor .config
    • make
  • Memory
    • make defconfig
    • cp Documentation/configs/1P-1M-1S-Memory .config
    • make
  • Storage
    • cp Documentation/configs-1P-1M-1S-fit_config.h linux-modules/fit/fit_config.h
    • cd linux-modules
    • make

1P-1M-1S perfectly emulates the effect of disaggregating a single monolithic server. Unlike 1P-1M setting, this setting can run any user program, either dynamically-linked or statically-linked, as long as there is no missing syscall. But please be careful and patient while setting things up, any mistakes may lead to an unsuccessful run. Sorry for the inconvenience.

Multiple Managers

To be able to run multiple managers, you will need at least five physical machines. Because now LegoOS will need global resource monitors (multiple monitors can co-exist). And due to our early implementation decisions, storage manager and global resource monitors can not run on one physical server. For example, in a 1P-2M-1S setting, you will need: one server for processor manager, two servers for memory managers, one for storage manager, and one for global resource monitors.

We will provide detailed tutorial on this soon.

Virtual Machine

In general, you will be able run both processor and memory manager on VM without any issue. But we can not run storage manager within a VM. The reason is our network setting. We need to know peer's QP number (QPN) beforehand. While the QPN generated by a Linux which is running inside a VM, is not stable.

Overall, 1P-1M can be tested with VM. With 1P-1M-1S setting, the processor and memory manager can run inside VM, while storage manager has to run on physical machine.

VM Setup

It is recommended to have multiple CPU cores and several GB memory for each VM. The reason is LegoOS need at least two kernel threads which are pinned to cores to do network communication. For processor, if Victim Cache is configured, one more victim flush thread will be created.

For example, a simple basic configuration: 8 vCPUs, and 8GB memory.


In order to run LegoOS on a VM, we need to export IB device from host to VM. And this VM must have exclusive access to this IB device. Please refer to Mellanox tutorials on this topic.