14.0 Virtual hosts

To support the use of resources for which there is no full port of the Legion system, Legion supports the notion of "virtual hosts," host objects that run on a fully-supported Legion platform and represent a resource on an unsupported platform (e.g., a Cray T3E). A virtual host object cannot be used to run normal Legion objects (since by definition there is no Legion port for the represented machine). Instead, it is used to run native jobs, such as existing serial and MPI programs, with the standard Legion tools (legion_run, legion_run_multi, and legion_native_mpi_run¹). The virtual nature of the host objects therefore remains transparent. The benefits of incorporating virtual hosts into the Legion system are many: transparent, simplified remote execution on the target machine; resource selection and scheduling of the machine through Legion mechanisms; etc.

To configure a virtual host object, use the following three steps:

1. START A HOST OBJECT

   Start a normal host object (any variety), with the standard legion_starthost command. Instead of starting the host on the desired target machine, however, start it on another machine that can conveniently be used to start jobs on the target machine (e.g., through a queue system, ssh, etc.). This machine is called the physical host. The target machine is called the virtual host.

   For example, start a virtual host object to represent the host t3e.npaci.edu on the physical host gigan.sdsc.edu you would run:

   $ legion_starthost -N /hosts/NPACI-T3E gigan.sdsc.edu \
     /vaults/BootstrapVault

   This gives you a normal host object, except that it is not on its host (Figure 14). It uses the physical host's bootstrap vault.

   Figure 14: Newly created virtual host object

2. SET VIRTUAL ARCHITECTURE FOR THE HOST OBJECT

   When the host object is first created, it is assumed to represent the architecture of the physical machine on which it resides. You must tell Legion that the host object will actually represent a machine of a different architecture. The legion_set_varch command sets a virtual architecture for a host object.

   Continuing the previous example, then, you must set the virtual architecture for host object NPACI-T3E to t3e:

   legion_set_varch /hosts/NPACI-T3E t3e

Figure 15: Virtual host object scripts

3. CONFIGURE VIRTUAL RUN SCRIPTS FOR THE HOST

   When legion_run and other commands make use of a virtual host object to start native jobs, they require a mechanism for starting and managing jobs on the virtual host. To fill this need, there are three scripts that the virtual host object can call on the physical host to make use of the virtual host:

   legion_vrun_run
   legion_vrun_status
   legion_vrun_kill

   These scripts and the virtual host object are located on the physical host (Figure 15). Examples of these scripts are in the following location:

   $LEGION_HPC/src/Tools/VirtualArchitecture

   These versions use simple Unix fork/exec to demonstrate the required interface.

   To configure the host with its required scripts, use the legion_set_vrun command, indicating the path at which the physical host can find the scripts. Continuing the above example:

The virtual host can be used normally for native jobs by registering programs for the (virtual host's) appropriate architecture and running them on the virtual host object. Only native jobs can be run on a virtual host. You can not run remote programs, because virtual hosts have no Legion binaries.

From the user's perspective, virtual and physical host objects are indistinguishable. For example, here we register a program for the T3E and run it on t3e.npaci.edu:

$ legion_register_program a.out /home/andrew/my_program t3e
$ legion_run /home/andrew/my_program

Notice that the program was registered and run from the physical host. In this case, there was no need to specify which host executes my_program, but you can use legion_run's -h flag to specify a virtual host, if necessary.

1. See page 100, page 104, and page 92, respectively, in the Reference Manual.

Table of Contents 1.0 Introduction    1.1 About this manual    1.2 Style conventions    1.3 About Legion 2.0 Installing and running Legion    2.1 Legion structure    2.2 Space requirements    2.3 Software requirements    2.4 RSA and OpenSSL    2.5 Downloading binary files 3.0 Starting a new system    3.1 Before you start    3.2 Set up the environment    3.3 Starting a single host system       3.3.1 Choose a bootstrap host       3.3.2 Set up and configure the system       3.3.3 Start up       3.3.4 Initialize       3.3.5 Set security       3.3.6 Starting up other packages    3.4 Starting a multihost system       3.4.1 Set up       3.4.2 Create a new vault       3.4.3 Create a new host       3.4.4 Adding new users to a secure net       3.4.5 Working in the new net    3.5 Making a set-up script for users 4.0 System shutdown    4.1 Shutting down an insecure system    4.2 Shutting down a secure system 5.0 System restart 6.0 Summary of commands    6.1 Starting a new system    6.2 Start working in a running system    6.3 Shutdown    6.4 Restart 7.0 About Legion security    7.1 Message layer    7.2 MayI layer    7.3 Special implications of security    7.4 Legion and Kerberos    7.5 Session file 8.0 Using security features    8.1 Authentication objects    8.2 Security and context space    8.3 Creating new users    8.4 Logging in users    8.5 Changing user passwords    8.6 Changing implicit parameters and ACL information 9.0 Legion core objects    9.1 Core class objects 10.0 Implementation model    10.1 Implementation caches    10.2 Implementation tools 11.0 Host and vault objects    11.1 About host-vault pairs    11.2 Manipulating host-vault pairing    11.3 Adding a new host       11.3.1 legion_starthost flags       11.3.2 The host object's log    11.4 Adding a new vault       11.4.1 legion_startvault flags    11.5 Backup vaults 12.0 Process control daemon host objects    12.1 Adding a PCD host object       12.1.1 Configure the daemon       12.1.2 Start the daemon and the host object    12.2 PCD host commands       12.2.1 Adding a new account       12.2.2 Removing an account       12.2.3 Viewing available accounts    12.3 How the PCD host object works    12.4 Using a PCD host as your bootstrap host 13.0 Batch queue host objects    13.1 Starting a batch queue host object    13.2 Setting the local queue    13.3 Before running objects on the new host    13.4 Troubleshooting 14.0 Virtual hosts

15.0 Setting up a native MPI host

16.0 Legion domains    16.1 Naming Legion domains    16.2 Domains and binding services    16.3 Joining domains    16.4 Related commands       16.4.1 Listing currently connected domains       16.4.2 Generating cookies       16.4.3 Displaying cookies       16.4.4 Connecting domains 17.0 Resource management    17.1 Scheduling-related commands       17.1.1 Configuring the scheduler       17.1.2 Setting a class's default scheduler       17.1.3 Setting scheduler policy       17.1.4 Adding resources to a collection       17.1.5 Subcollections Getting help Index

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