Seeing through hardware counters: a journey to threefold performance increase

In one of our previous blogposts, A Microscope on Microservices we outlined three broad domains of observability (or “levels of magnification,” as we referred to them) — Fleet-wide, Microservice and Instance. We described the tools and techniques we use to gain insight within each domain. There is, however, a class of problems that requires an even stronger level of magnification going deeper down the stack to introspect CPU microarchitecture. In this blogpost we describe one such problem and the tools we used to solve it.

It started off as a routine migration. At Netflix, we periodically reevaluate our workloads to optimize utilization of available capacity. We decided to move one of our Java microservices — let’s call it GS2 — to a larger AWS instance size, from m5.4xl (16 vCPUs) to m5.12xl (48 vCPUs). The workload of GS2 is computationally heavy where CPU is the limiting resource. While we understand it’s virtually impossible to achieve a linear increase in throughput as the number of vCPUs grow, a near-linear increase is attainable. Consolidating on the larger instances reduces the amortized cost of background tasks, freeing up additional resources for serving requests and potentially offsetting the sub-linear scaling. Thus, we expected to roughly triple throughput per instance from this migration, as 12xl instances have three times the number of vCPUs compared to 4xl instances. A quick canary test was free of errors and showed lower latency, which is expected given that our standard canary setup routes an equal amount of traffic to both the baseline running on 4xl and the canary on 12xl. As GS2 relies on AWS EC2 Auto Scaling to target-track CPU utilization, we thought we just had to redeploy the service on the larger instance type and wait for the ASG (Auto Scaling Group) to settle on the CPU target. Unfortunately, the initial results were far from our expectations:

The first graph above represents average per-node throughput overlaid with average CPU utilization, while the second graph shows average request latency. We can see that as we reached roughly the same CPU target of 55%, the throughput increased only by ~25% on average, falling far short of our desired goal. What’s worse, average latency degraded by more than 50%, with both CPU and latency patterns becoming more “choppy.” GS2 is a stateless service that receives traffic through a flavor of round-robin load balancer, so all nodes should receive nearly equal amounts of traffic. Indeed, the RPS (Requests Per Second) data shows very little variation in throughput between nodes:

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