AWS r8i.4xlargevsAWS r8i-flex.16xlarge

r8i.4xlarge pairs 16 vCPUs with 128GB of RAM at $1.1114/hr On-Demand (about $800/mo at 24×7). r8i-flex.16xlarge pairs 64 vCPUs with 512GB at $4.2230/hr (~$3041/mo). r8i.4xlarge is 280% cheaper per hour than r8i-flex.16xlarge ($3.1117/hr gap).

Because both instances are in the **r8i family**, the only thing that changes between them is sizing — same silicon, same architecture (Intel Xeon (x86_64)), same burstable/sustained behavior. The choice is purely about how much capacity you actually need: r8i.4xlarge gives you 16 vCPUs and 128GB of RAM, r8i-flex.16xlarge gives you 64 vCPUs and 512GB. AWS scales pricing close to linearly within a family, so picking the right size is mostly about right-sizing your workload, not getting a better deal per vCPU.

On raw price-per-performance, the two are r8i.4xlarge delivers ~279% more single-thread Sysbench score per dollar (2996 vs 790 points per $1/hr). That's the cleanest signal we have for "which one runs your workload faster per dollar," but it only matters if your workload is single-thread-bound; for parallel workloads the multi-core scores (28004 vs 112300) are what to weigh. Spot pricing flips many of these comparisons — when r8i.4xlarge drops to $0.5594/hr and r8i-flex.16xlarge drops to $0.7435/hr, the cheap-per-hour winner can swing meaningfully.

In practice, pick r8i.4xlarge when your workload is closer to memory-optimized (memory-bound work — in-memory databases, real-time analytics, large caches). Pick r8i-flex.16xlarge when it's closer to memory-optimized (memory-bound work — in-memory databases, real-time analytics, large caches). When neither side is obviously right, the cheaper hourly rate usually wins for fault-tolerant batch workloads, while the higher single-core score usually wins for latency-sensitive web traffic. The regional pricing tables linked from each instance page below show where each is currently cheapest — sometimes a >20% regional gap flips the comparison entirely.