How Efficiency Can Compromise System Resilience

What’s the “right” level of CPU utilization for a server? If you look at a monitoring dashboard from a well-designed and well-run service, what CPU utilization should we hope to see, averaged over a day or two?

It’s a very general question, and it’s not clear it should have a single answer. That said, for a long time, I generally believed that higher is always better: we should aim for as close to 100% utilization as we can. Why? Well, anything less than 100% represents unused hardware capacity, which means we’re wasting resources. If a service isn’t maxing out its CPU, we could move it onto a smaller instance or run some additional work on that node.

This simplistic intuition, it turns out, is rarely quite right.

Suppose we achieve that ideal and our service is running close to 100% utilization. What happens now if we go unexpectedly viral and receive an unexpected burst of traffic? Or if we want to deploy a new feature which takes a little bit of extra CPU on each request?

If we are at 100% utilization, and then something happens to increase load, then we’re in trouble! Running at 100% utilization leaves us no room to absorb incremental load. We will either degrade in some way, have to scramble to add emergency capacity, or both.

This toy example is an instance of a very general phenomenon: Improvements to efficiency often trade off against resiliency, and the further we optimize a system, the worse this tradeoff tends to become.

Past some point, making a system more efficient will mean making it less resilient, and, conversely, building in robustness tends to make a system less efficient (at least in the short run). This isn’t to say there are no win/wins; sometimes it is possible to move the Pareto frontier outwards; fixing “dumb” performance bugs sometimes has this effect. However past some level of effort, you will be forced to make tradeoffs.

By “resilience,” here, it’s important to note that I mean something much broader than just “reliability” or “ability to stay up”; I mean a much more general notion of “ability to absorb or respond to change” – change of all sorts, including both bugs or failures, but also change in product needs, change in the market, change in the organization or team composition, whatever.

Examples of this tradeoff

This tradeoff occurs in domains beyond capacity planning. It applies at almost every level of almost any technical system or organization. Here’s some other places where I’ve observed it:

Redundancy

It’s extremely common to run multiple instances of a service, with a load balancer set up such that if any instance fails, load will be transparently routed to the others. In sophisticated organizations, this pattern may be applied at the level of the entire datacenter, with an architecture that allows entire datacenters to fail and their load to be routed to others.

In order for this to work, each instance must have enough spare capacity to absorb the incremental load from a failed-over instance. In steady-state, in the absence of failure, that capacity must be sitting idle, or, at best, serving lower-priority work that can be dropped at a moment’s notice. The more redundancy we want, the more capacity we must hold idle in steady state.

Optimization

Sophisticated performance optimizations often work by exploiting specific properties or structures of the problem domain. By baking invariants into data structures and code organization, you can often win a huge amount of performance. However, the more deeply you rely on specific assumptions, the harder they become to change, which makes heavily-optimized code often much harder to evolve or add features to.

As a concrete example, in my reflection on Sorbet, I talk about how we decided that type inference would be local-only and single-pass, and baked that assumption into our code and data structures. This choice resulted in substantial efficiency gains, but made the system in some sense more brittle: There are many features that some competing type systems have which would be impossible or prohibitively hard to implement in the Sorbet codebase, because of these assumptions. I remain confident this was the correct choice for that project, but the tradeoffs are worth acknowledging.

Hillel Wayne, refers to a similar property as “clever code”, which he defines as

code which exploits knowledge about the problem.

He, too, talks about how clever code (in this sense) tends to be efficient but sometimes fragile.

Serialization formats

It’s hard to find a more efficient serialization format than “just copy your in-memory structs directly to disk”; virtually no code is required and there’s close to zero serialization cost to save or load data.

However, this results in a brittle system – even adding a new field requires rewriting data or some other special handling. And sharing data between machines with different endians or word sizes becomes a challenge.

On the flip side, writing all of your data as JSON or some similar generic container creates endless flexibility for adding new fields or for new modules to communicate with each other without impacting existing code, but at the cost of substantial overhead on the wire and in CPU work serializing and deserializing data streams.

Distributed systems

One of my favorite systems papers ever is the COST paper, which examines a number of big-data platforms, and observes that many of them have the desirable property of scaling (near-)linearly with available hardware, but do so at the cost of being ludicrously less efficient than a tuned single-threaded implementation.

I’ve found this to be a common tradeoff. Distributed computation frameworks are flexible and resilient in the sense of being able to handle near-arbitrary workloads by scaling up. They can handle someone deploying inefficient code by scaling out, and handle hardware failures transparently. Need to process more data? Just add more hardware (often transparently, using some sort of autoscaling).

On the flip side, carefully-coded single-node solutions will tend to be faster (sometimes 10-100x faster!), but are much more brittle: If the dataset no longer fits on one node, or if you need to perform a 10x more expensive analysis, or if a new engineer on the team unwittingly commits slow code inside a tight inner loop, the entire system may fall over or fail to perform its job.

Small teams vs larger organizations

Small teams – including “teams” of one person – can be incredibly productive and efficient. The smaller the team, the less communication overhead, and the easier it is to keep rich shared context inside every engineer’s head. There’s less need to write documentation, less need to communicate about changes, less time spent onboarding and training new members, and so on. Small teams can get much further using strategies like “think carefully and try hard” than large teams, and often need to rely less on tooling like linters and careful defensive abstraction design.

Under the right circumstances, it’s sometimes possible for a small team with a careful design and experienced engineers to roughly match the raw output of teams 10x their size – an enormous improvement in efficiency!

Small teams, however, are much more brittle and less resilient to changed in the organization, technology landscape, or business needs of the project. If one person departs from a 4-person team, you’re down by 25% of your bandwidth – and worse, the team has little practice hiring and onboarding new members, and large amounts of knowledge and documentation live only in the remaining members’ heads.

Similarly, change in business focus or new product launch requires many more features or other development from the team’s system, it’s relatively easy to exceed the team’s capacity to support it, and growing the team quickly will be challenge for all the same reasons.

Automation versus human processes

In general, it’s more efficient – cheaper, faster, and often more reliable – for a machine to perform a task than for a human to perform it manually.

Humans, however, are endlessly adaptable, while machines (both physical machines and software systems) are much more brittle and set in their ways. A system with humans in the loop has many more options for responding on the fly to changes in circumstances or unexpected events.

Even if we keep all the same humans around but augment them with automation to accelerate parts of their job, we risk automation dependency, in which the humans become overly reliant on the automation and trust inappropriately, or where their ability to function without the automation atrophies such that they are no longer able to appropriately step in “on manual” when needed.

Slack

Many of these specific observations are really observations about slack (not the software product).

Systems with healthy amounts of slack will – at least in the short term, on a simplistic analysis – by definition be inefficient: That slack is time or resources which are going “idle” and which could instead be producing output.

Taking a broader view, though, that slack is key to resiliency: having “give” in the system lets the system cope with small disruptions or mishaps; developers or operators can use that slack to step in and handle unexpected load or resolve underlying issues before they become catastrophic or externally visible.

A system with no slack is efficient as long it works, but brittle and will break down quickly in the presence of any changes to its usual operating modes.

Conclusions

I’ve tried to touch on a number of concrete examples where efficiency and reliability are at odds with each other, and trade off or at least create pressure in opposing directions. Hopefully I’ve convinced you that this is a broad phenomenon, and that even in cases where there may not be a strict tradeoff yet, the two values will tend to oppose each other and suggest different decisions.

Unfortunately, this observation alone rarely tells us what to do with any particular system. If we’re looking at some system and a first analysis suggests it’s making inefficient use of inputs, we can’t tell without looking more closely whether it’s a den of dysfunction and bad design choices, or whether that superficial inefficiency is supporting vast reserves of redundancy and flexibility and slack that will enable it to weather any change that may come. We need to look closer, and we almost always need domain expertise in the particular team and particular problem.

Furthermore, sometimes there are free lunches. Some design choices or decisions do shift the Pareto front outwards, and not just move us along it. They may be rare in well-optimized systems, but we can’t ignore their possibility. And many systems just haven’t been that well optimized yet!

In addition, the optimal point in the design space varies depending on the system. Sometimes reliability or resiliency is critically important, and we are right to tolerate massive first-order inefficiency. Sometimes, however, extreme efficiency is the correct goal: perhaps our margins are thin enough for it to be our only choice, or perhaps we are sufficiently confident in the stability of our domain and of the demands upon our system that we feel confident we will not face overly-drastic change of any sort.

So, mostly, all I can do is call for us, as engineers and designers and observers of systems, to work with an awareness of this tradeoff and its implications. When we accuse a system of being wasteful and inefficient, it’s worth pausing to ask what that “waste” may be buying. When we set out to optimize a system, pause to understand where the joints and flexibility in the current system are, and which ones are vital, and to do our best to preserve those. When we set metrics or goals for a system or a team or an organization that ask for efficiency, let us be aware that, absent countervailing pressures, we are probably also asking for the system to become more brittle and fragile, too.