# Overview

!!! warning "Puhti and Mahti retirement in 2026"
    Puhti and Mahti will be decommissioned in 2026 and replaced by Roihu, CSC's
    next-generation supercomputer offering enhanced performance and
    capabilities.

    * Puhti computing services have been shut down since 31 July 2026.
    * Mahti computing services will be shut down by 31 August 2026 at 12:00 EEST.
    * Puhti and Mahti storage and login nodes are planned to remain accessible until 15 October 2026 at 12:00 EEST.
    * Users are strongly encouraged to move any required data from these systems by the end of August 2026, as storage access cannot be fully guaranteed after that.

    [Learn more about Roihu :material-arrow-right:](systems-roihu.md)

Puhti, Mahti and Roihu are CSC's supercomputers. Puhti has been available for CSC users
since 2 September 2019 and Mahti has been available since 26 August 2020. LUMI is
one of the pan-European pre-exascale supercomputers, located in CSC's data
center in Kajaani. The CPU partition of LUMI (LUMI-C) has been available since
early 2022, and the full system with the large LUMI-G partition has been available since early 2023.

Puhti contains CPU nodes with a range of memory sizes as well as a large GPU
partition (Puhti AI), while Mahti contains homogeneous CPU nodes and is meant
for larger jobs (minimum 128 CPU-cores). Mahti also contains a GPU partition
since 2021 (Mahti AI) with Nvidia Ampere GPUs. See [specifications](available-systems.md)
for details on the systems and [this page for an outline of differences between LUMI-C and
Mahti](lumi-vs-mahti.md).

Puhti and Mahti are being replaced in by CSC's next national supercomputer
called **Roihu**. [Read more about Roihu here](systems-roihu.md). Roihu has been accessible for CSC users since end of June 2026.

Roihu consists of two main partitions: **Roihu-CPU** and **Roihu-GPU** that have separate login nodes
and software environments.
Roihu-CPU contains AMD EPYC CPUs and is intended for CPU-based computing, while
Roihu-GPU provides GPU-accelerated computing with NVIDIA GH200 Grace Hopper
Superchips. Roihu will also support jobs requiring larger amounts of memory than
has been possible on CSC's previous national supercomputers, and provide
enhanced support for sensitive data workloads.

CSC supercomputers use the Linux operating system and we recommend that you are familiar with
basics of [Linux command line usage](../support/tutorials/env-guide/index.md) before starting.

!!! note
    For an overview of the LUMI supercomputer, see
    [the LUMI documentation](https://docs.lumi-supercomputer.eu/hardware/).

## Accessing CSC supercomputers

To be able to use CSC's supercomputers, you need to have a CSC user account that
belongs to a computing project which has access to the respective supercomputers.
CSC user accounts and projects are managed in the [MyCSC portal](https://my.csc.fi).
Further instructions are provided in the [Accounts section](../accounts/index.md)
of this user guide.

!!! Note
    To access the LUMI supercomputer, you need to [create a LUMI-specific
    project](../accounts/how-to-create-new-project.md#creating-a-lumi-project-and-applying-for-resources).
    For more details on getting started with LUMI, see the [LUMI
    documentation](https://docs.lumi-supercomputer.eu/firststeps/getstarted/).

## Connecting to the supercomputers

--8<-- "ssh-ca.md"

Connect using an SSH client:

```bash
ssh username@puhti.csc.fi
# or
ssh username@mahti.csc.fi
```

Roihu has separate login nodes for the Roihu-CPU and Roihu-GPU partitions:

```bash
ssh username@roihu-cpu.csc.fi
# or
ssh username@roihu-gpu.csc.fi
```

These commands will connect you to one of the login nodes. If you need to
connect to a specific login node, use the commands:

```bash
ssh username@puhti-login[11-12,14-15].csc.fi
# or
ssh username@mahti-login[11-12,14-15].csc.fi
# or
ssh username@roihu-cpu-login[1-4].csc.fi
# or
ssh username@roihu-gpu-login[1-2].csc.fi
```

Where `username` is the username you get from CSC.

For more details, see the [connecting](connecting/index.md) page.

Puhti, Mahti and Roihu can also be accessed via their respective
[web interfaces](webinterface/index.md) available at
[www.puhti.csc.fi](https://www.puhti.csc.fi),
[www.mahti.csc.fi](https://www.mahti.csc.fi) and
[www.roihu.csc.fi](https://www.roihu.csc.fi).

### Scalability

Don't allocate more resources to your job than it can use efficiently. This
needs to be verified for each new code and job type (different input) by a
scaling test. The policy is that the job should be **at least 1.5 times faster**
when you double the resources (cores). [Instructions for performing a scalability
test](../support/tutorials/cmdline-handson.md#scaling-test-for-an-mpi-parallel-job).
Please also consider [other important factors related to performance.](performance.md)

## Projects and quotas

Working in CSC supercomputers is based on projects. The computing and storage
resources are allocated to projects and when you start a batch job, you must
always define the project that the job belongs to.

Projects are managed in the [MyCSC portal](https://my.csc.fi), where you can add
services, resources and users to your CSC projects.

In CSC supercomputers, you can check your currently active projects with the
command:

```text
csc-projects
```

This command shows information for all your CSC projects. You can select just
one project to be reported with the `-p` option. For example:

```bash
[kkayttaj@roihu-login1 ~]$ csc-projects -p project_2012345
-----------------------------------------------------------------
Project: project_2012345        Owner: Kalle Käyttäjä
Title: "Ortotopology modeling"
Start: 2019-08-20 End: 2029-10-23 Status: open
Billing units      Budget        Used      Remain
-------------      ------        ----      ------
CPU BU:             60000          20       59980
GPU BU:              1000          30         970
Storage BU:        300000          40      299960
Cloud BU:             400          10         390
Latest resource grant: 2025-01-31
-----------------------------------------------------------------
Project info updated: 2025-09-02 15:12
```

The command reports the owner of the project, title, start and end dates. In
addition the command prints out the budgeting information for the project: how
many Billing Units of each type have been granted to your project, how many have
been used and how many still remain.

The [disk areas](disk.md) of your projects can be checked with the command:

```text
csc-workspaces
```

## Using CSC supercomputers

* [Systems](available-systems.md): What computational resources are available
* [Usage policy](usage-policy.md): Usage policy of CSC supercomputers
* [Connecting](connecting/index.md): How to connect to  CSC supercomputers
* [Web interfaces](webinterface/index.md): How to connect to CSC supercomputers
  using the web interfaces
* [Disk areas](disk.md): What places are there for storing data on CSC
  supercomputers
* [Object storage](allas-in-roihu.md): Opening connections to Allas and Lumi-O object storage services 
* [Modules](modules.md): How to find the programs you need
* [Applications](../apps/index.md): Application specific instructions.
* [Running jobs](running/getting-started.md): How to run programs on the
  supercomputers
* Installing and compiling your applications:
    * [Installing software](installing.md)
    * [Compiling on Puhti](compiling-puhti.md)
    * [Compiling on Mahti](compiling-mahti.md)
    * [Compiling on Roihu](compiling-roihu.md)
* [Debugging applications](debugging.md): How to debug your applications
* [Performance analysis](performance.md): How to understand the performance of
  your applications
