Modal on Keeping 20,000+ GPUs Healthy Across Clouds

Modal has scaled its multi-cloud GPU worker pool past 20,000 concurrent GPUs and launched over four million cloud instances in the last couple years, and is now sharing the reliability system behind it. The post covers instance type testing and selection, machine image preparation and boot checks, passive and active GPU healthchecking, and observability and support. It reports hyperscaler differences such as Cloud A's 99.6% boot success and Cloud C's H100s running above 90ºC in 2025.