Systems Simulation & Performance Modelling - Member of Technical Staff

About Us

We’re living through a Cambrian explosion of intelligence: new models and new chips, each specialised for different tasks, are arriving all at once. The result is a new era for AI, one of radical heterogeneity.

Callosum is the Intelligent Systems Company. We believe the next generation of AI won't be defined by any single model or chip, but by intelligent systems in which hardware and intelligence co-evolve. We are building the infrastructure that unifies heterogeneous compute across the full stack. This opens a new axis of scaling intelligence: a dynamic system that tailors itself to what each workload actually needs, whether that's speed, cost, precision, or whatever unit comes next.

The last era scaled on a different bet: one bigger model, more of the same chip, more data. That bet is running into structural limits. Frontier models offer extraordinary capability at unsustainable cost, one that today's monolithic infrastructure was never designed to serve.

Our founding principle is that intelligence comes from many specialised systems working together, not from any single component. We build the software orchestration layer that co-evolves models, workflows and silicon into one system, delivering inference tailored to every workload, and demonstrating orders-of-magnitude leaps in capability and cost.

Because our software spans the full stack, our engineering team works directly with heterogeneous accelerators and frontier silicon, including Cerebras, d-Matrix, Intel, NVIDIA, AMD, Normal Computing, Tenstorrent, GreatSky, and Mixx. We are not stopping at today's chips: each new generation of silicon unlocks algorithms that couldn't run before, and we intend to be first to them, every time. If we get it right, it will belong to everyone building on it - not to any single vendor.

In our latest funding round, we raised $100M, led by Atomico with participation from Plural, DCVC and the UK Sovereign AI Fund’s first investment. With this, we are building the infrastructure for the next era of intelligence.

We are engineers and scientists based in London, working across the full depth of the stack. We are curious, intellectually honest, and building what doesn't exist yet. If you thrive on uncharted territory and are energised by the scale of the challenge, we'd love to hear from you.

About the Role

Callosum believes that scaling heterogeneous AI infrastructure requires understanding how emerging hardware will behave as part of larger computational systems. Performance characteristics measured on individual accelerators rarely translate directly to distributed deployments, where networking, memory, and execution strategies introduce new constraints. We are developing simulation infrastructure that allows us to explore these interactions and evaluate future systems before they exist.

This role advances the simulation and performance modelling tools that inform Callosum's hardware and infrastructure decisions. You will extend models that project device-level performance to rack-scale systems, explore alternative hardware configurations and execution strategies, and provide experimental environments for automated optimisation. The work also encompasses profiling, workload tracing, and performance instrumentation, connecting measurements from real systems to accurate and useful models of those we want to build.

What You'll Build

• Extend our simulators to predict heterogeneous compute performance from individual accelerators to multi-rack deployments.

• Model networking, memory, and execution behaviour to evaluate alternative system architectures and configurations.

• Develop profiling, tracing, and workload capture tools that inform and validate simulation models.

• Extend simulation environments for rapid experimentation and automated optimisation of systems and algorithms.

• Characterise emerging accelerator architectures and translate hardware measurements into predictive models of system-level performance.

What Sets You Apart

• Strong experience in systems simulation, performance modelling, or performance-critical systems software.

• Deep understanding of computer architecture, distributed systems, or high-performance computing.

• Familiarity with profiling, tracing, instrumentation, and methods for characterising computational workloads.

• Strong systems programming experience, ideally including C++ or Rust.

• Ability to reason about modelling accuracy, abstraction, and trade-offs between simulation fidelity and computational cost.

What We Offer

• Competitive Salary, determined by skills and experience

• Equity & Ownership

• Private healthcare

• We offer Visa sponsorship and relocation benefits to hire the best in the world

• We work in person at our London office. You'll have the tools, space and setup to do your best work, and if you have specific needs, just tell us

We're committed to building an inclusive workplace where everyone feels welcome, and believe in equal opportunities for all.

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