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AWS Common Runtime (CRT)

The AWS Common Runtime (CRT) is a C-based library that implements AWS best-practice network / request behaviour — connection pooling, request parallelization, retry with jitter, multi-part upload/download orchestration — and is used by the AWS SDKs across languages to hit service throughput ceilings without each SDK re-implementing the details.

For S3 specifically, the CRT is the mechanism that turns "S3's documented throughput guidance" into executable throughput: the client shapes are baked into a library rather than left as blog recommendations.

(Source: sources/2025-03-14-allthingsdistributed-s3-simplicity-is-table-stakes)

S3 + CRT throughput numbers (2025)

Warfield: "Today, we see individual GPU instances using the CRT to drive hundreds of gigabits per second in and out of S3." Anthropic operates at "tens of terabytes per second" at account level — both figures assume the CRT-backed request pattern.

Why it exists

S3's performance elasticity model is: any customer can use S3's full performance as long as they don't interfere with others. That pushes high-throughput discipline (fan-out, key distribution, parallel multipart, retry shape) onto the client. Leaving those best practices as guidance meant different SDK languages got different performance and customers wrote custom wrappers. The CRT is the fix: one C implementation of the right shape, bound into each language SDK.

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