# How does LUMI-C differ from Mahti?

[LUMI-C](https://docs.lumi-supercomputer.eu/hardware/lumic/) consists of 2048 AMD CPU nodes (2x64 cores each). The node count of [Mahti](systems-mahti.md) is similar, 1404 AMD CPU nodes (2x64 cores each). Although the systems are very alike based on CPU-hardware, core count and performance, there are important differences which are highlighted on this page.

## GPUs and memory

Mahti has only a few (24) GPU-nodes available, while LUMI-C is flanked by [LUMI-G](https://docs.lumi-supercomputer.eu/hardware/lumig/) and [LUMI-D](https://docs.lumi-supercomputer.eu/hardware/lumid/) with a massive GPU capacity. Also, Mahti has 256 GB memory in all CPU nodes, while LUMI-C has also 512 GB and 1024 GB memory nodes (similar to Puhti).

## Accessing and SSH keys

To access LUMI, you need to first [create a LUMI-specific project](../accounts/how-to-create-new-project.md#how-to-create-finnish-lumi-projects). Note that LUMI-projects have a finite duration ([see below](lumi-vs-mahti.md#finite-time-projects)) and cannot be used for running on the national resources and vice versa.

Furthermore, accessing LUMI is only possible using SSH keys uploaded to MyCSC, meaning that you cannot use passwords to connect through SSH. This authentication method is also in use on Mahti (and Puhti) since 14 April 2025. For instructions on how to generate an SSH key pair and uploading the public key to [MyCSC](https://my.csc.fi/), see [Setting up SSH keys](connecting/ssh-keys.md) and the [Get started with LUMI](https://docs.lumi-supercomputer.eu/firststeps/getstarted/) pages.

Similar to Puhti and Mahti, LUMI can, however, also be accessed through a [web interface](https://docs.lumi-supercomputer.eu/runjobs/webui/).

## Finite time projects

Finnish LUMI projects have a finite duration ranging from 3 months to max. 3 years depending on the access mode:

|Access mode  |Length  |Can be extended?               |
|-------------|--------|-------------------------------|
|Regular      |1 year  |No                             |
|Benchmark    |3 months|No                             |
|Extreme scale|1 year  |No                             |
|Development  |1 year  |Yes, twice = max. 3 years total|

For more details on the access modes, see the [LUMI access page on the Services for Research website](https://research.csc.fi/lumi-access). Note that LUMI users from Finland are also eligible to apply for EuroHPC Joint Undertaking (JU) resources. [See more details on European access modes here](https://www.lumi-supercomputer.eu/get-started-2021/users-in-europe/).

## Software installation policy

Similar to Mahti, CSC offers some frequently used applications as pre-installed modules on LUMI. A list of these can be found in [CSC Docs](../apps/by_availability.md#lumi) as well as [LUMI Docs](https://docs.lumi-supercomputer.eu/software/local/csc/).

To facilitate installing own software on LUMI, the [EasyBuild tool](https://docs.lumi-supercomputer.eu/software/installing/easybuild/) is provided along with [installation recipes](https://github.com/Lumi-supercomputer/LUMI-EasyBuild-contrib) (EasyConfig files) using which you can install additional applications to your home or project directories. Additionally, a [container wrapper](https://docs.lumi-supercomputer.eu/software/installing/container-wrapper/) identical to the [Tykky tool](containers/tykky.md) is provided as a means to wrap installations inside an Apptainer/Singularity container. This is recommended especially for Conda and pip environments to alleviate the load on the parallel filesystem.

If you have problems installing your software on LUMI, please [send a ticket to the LUMI user support team](https://lumi-supercomputer.eu/user-support/need-help/)!

## Programming environment and software stacks

The programming environment of LUMI is quite different compared to CSC supercomputers. LUMI comes with three alternative programming environments, namely Cray, GNU and AOCC. Each of the environments have their own compiler suites that become available upon loading the corresponding programming environment module. Moreover, two types of software stacks are offered, the CrayEnv and LUMI stacks. Please refer to the LUMI documentation for a detailed description of the [available collection of compiler suites](https://docs.lumi-supercomputer.eu/development/compiling/prgenv/) and [software stacks](https://docs.lumi-supercomputer.eu/runjobs/lumi_env/softwarestacks/) and how to swap between these.

!!! info "Note"
    Irrespective of the loaded compiler suite, one noticeable difference concerning the LUMI programming environment is that it comes with compiler wrappers that replace commands commonly found on HPC systems such as Mahti. For example, commands for compiling MPI code like `mpicc`, `mpic++` and `mpif90` are unavailable as such. Instead, you should use the wrappers `cc`, `CC` and `ftn`, respectively. See the LUMI documentation for [more details on the available MPI wrappers](https://docs.lumi-supercomputer.eu/development/compiling/prgenv/#compile-an-mpi-program).

## Disk areas and storage

Similar to CSC supercomputers, LUMI uses a [Lustre parallel filesystem](https://docs.lumi-supercomputer.eu/storage/parallel-filesystems/lumip/). However, there's no fast local disk on LUMI similar to the local scratch on Puhti and Mahti. Instead, a fast flash-based Lustre scratch space ([LUMI-F](https://docs.lumi-supercomputer.eu/storage/parallel-filesystems/lumif/)) is available. There is also an object storage similar to Allas, [LUMI-O](https://docs.lumi-supercomputer.eu/storage/lumio/), available. [See the LUMI documentation for more details](https://docs.lumi-supercomputer.eu/storage/).

## Available partitions

LUMI has two types of partitions (queues): three that are allocatable by node (only full nodes can be requested, similar to Mahti) and five that are allocatable by resources (partial nodes can be requested, similar to Puhti). [See more details in the LUMI documentation](https://docs.lumi-supercomputer.eu/runjobs/scheduled-jobs/partitions/), e.g. maximum wall-time/node count and naming of the partitions.

Note that LUMI consortium country projects (e.g. Finnish LUMI projects) use different partitions than EuroHPC JU projects. The latter are prefixed by `ju-` and cannot be used unless you're a member of a project that has been allocated resources by the JU.

## Billing

LUMI does not share the same Billing Units as other CSC services like Mahti. However, the billing model is similar in that there are different units for CPU compute, GPU compute and storage resources, and that granted resources are periodically cut if left unused.

[See the LUMI documentation for more details and precise formulas](https://docs.lumi-supercomputer.eu/runjobs/lumi_env/billing/).

## Sensitive data

LUMI projects are not allowed to handle sensitive (personal) data at the moment!

## Support channels

The main channel for LUMI support is to [contact the LUMI User Support Team (LUST)](https://lumi-supercomputer.eu/user-support/need-help/).
