Blacksmith: How a Serverless CI Platform Achieves Profitability Through Multitenancy

Blacksmith, a YC startup, built a serverless cloud platform for CI workloads. Initially relying on simulations to project margins, they discovered the economic benefits of multitenancy. By leveraging the spiky nature of CI workloads – short-lived jobs (5-40 minutes) and geographically diverse customers – Blacksmith efficiently utilizes its server resources. Their fleet of hundreds of bare-metal gaming CPUs, virtualized with Firecracker, creates and destroys microVMs on demand. More customers mean more random activity, smoothing overall operation, lowering the cost to serve each job, and boosting profit margins. The article details profitability at various utilization levels, and the impact of time zones and geography on resource utilization, ultimately demonstrating the economic power of their multitenant model.