AWS m8i.8xlargevsAWS m8i-flex.4xlarge

m8i.8xlarge pairs 32 vCPUs with 128GB of RAM at $1.6934/hr On-Demand (about $1219/mo at 24×7). m8i-flex.4xlarge pairs 16 vCPUs with 64GB at $0.8044/hr (~$579/mo). m8i-flex.4xlarge is 52% cheaper per hour than m8i.8xlarge ($0.8890/hr gap).

Because both instances are in the **m8i 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: m8i.8xlarge gives you 32 vCPUs and 128GB of RAM, m8i-flex.4xlarge gives you 16 vCPUs and 64GB. 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 m8i-flex.4xlarge delivers ~111% more single-thread Sysbench score per dollar (1963 vs 4143 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 (56016 vs 28071) are what to weigh. Spot pricing flips many of these comparisons — when m8i.8xlarge drops to $0.6760/hr and m8i-flex.4xlarge drops to $0.3292/hr, the cheap-per-hour winner can swing meaningfully.

In practice, pick m8i.8xlarge when your workload is closer to general-purpose (balanced general-purpose workloads with a 1:4 vCPU-to-memory ratio). Pick m8i-flex.4xlarge when it's closer to general-purpose (balanced general-purpose workloads with a 1:4 vCPU-to-memory ratio). 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.