[Paper] Contention Resolution, With and Without a Global Clock

Source: arXiv - 2602.12070v1

Overview

The paper tackles Contention Resolution, a classic coordination problem where n independent processes must each obtain exclusive access to a shared resource for a single time slot. While most prior work assumes processes only have a local clock (they can only count time since they woke up), this study asks what happens when a global clock is available—a realistic scenario in modern distributed systems. By comparing the two models, the authors uncover fundamental performance gaps and introduce new algorithms that dramatically improve latency under a global clock.

Key Contributions

• A near‑optimal global‑clock protocol achieving expected (and high‑probability) latency
  [ O!\bigl(n(\log\log n)^{1+o(1)}\bigr), ]
  which is roughly a factor‑( \log n ) faster than the best known local‑clock randomized solutions.
• Formal separation of expectation vs. high‑probability guarantees for memoryless (stateless) protocols:
  • Expected latency = ( \Theta!\bigl(\frac{n\log n}{\log\log n}\bigr) )
  • High‑probability latency = ( \Theta!\bigl(\frac{n\log^{2} n}{\log\log n}\bigr) )
    This reveals a provable ( \log n ) gap between the two performance metrics.
• Impossibility result showing that no single protocol can simultaneously achieve the optimal bounds for both metrics; in particular, you cannot get expected latency ( o!\bigl(\frac{n\log^{2} n}{(\log\log n)^{2}}\bigr) ) while also keeping high‑probability latency down to ( n\log^{O(1)} n ).
• New analytical techniques for handling the global‑clock setting, including a layered “iterated‑log” schedule that tightly controls contention bursts.

Methodology

1. Modeling the clock – The authors formalize two distinct settings:
   • LocalClock: each process only knows how many rounds have passed since its own activation.
   • GlobalClock: all processes can read a shared, synchronized round counter.
2. Protocol design – For the global‑clock case they construct a multi‑phase back‑off schedule. Each phase uses a different “window size” derived from iterated logarithms (e.g., ( \log\log n, \log^{(3)} n, \dots )). Processes randomly pick a slot within the current window; if they collide they move to the next, larger window. The global clock lets every process stay in lockstep, preventing the “drift” that plagues local‑clock protocols.
3. Memoryless analysis – They restrict attention to memoryless (stateless) randomized strategies, which simplifies the probabilistic analysis and lets them prove tight lower bounds via information‑theoretic arguments.
4. Expectation vs. tail‑probability – By carefully tracking the distribution of the number of remaining contenders after each phase, they derive separate bounds for the expected total time and for the probability that the time exceeds a given threshold.
5. Impossibility proof – Using a reduction to a “hard‑core” contention instance, they show any protocol that tries to be simultaneously optimal for both metrics would violate a combinatorial packing bound.

Results & Findings

The key takeaway is that access to a global clock fundamentally changes the algorithmic landscape, enabling protocols that shave off a logarithmic factor in latency—a non‑trivial improvement when n is large.

Practical Implications

• Distributed databases & transaction managers often need to serialize competing writes. If the system can expose a synchronized logical clock (e.g., via a lightweight consensus service), the new protocol can reduce contention‑resolution latency from roughly ( n\log^{2} n ) to near‑linear ( n\log\log n ).
• Networked IoT fleets frequently operate under asynchronous wake‑up patterns. Adding a cheap global time source (e.g., GPS or a broadcast time slot) could let devices use the global‑clock back‑off schedule, cutting the time to acquire a shared channel.
• Cloud orchestration platforms that schedule short‑lived tasks on a shared pool of resources can embed the protocol in their scheduler to improve throughput, especially under bursty load where many tasks contend simultaneously.
• The separation between expectation and high‑probability guarantees warns engineers: optimizing for average case may hide rare but costly tail events. Depending on service‑level requirements, you may need to pick a protocol that favors the appropriate metric.
• The impossibility result guides system designers to avoid chasing a “one‑size‑fits‑all” contention algorithm; instead, they should pick or switch between protocols based on whether latency variance or worst‑case tails matter more for a given workload.

Limitations & Future Work

• The analysis assumes memoryless randomization; real systems often keep state (e.g., exponential back‑off counters), which could close the gap between expectation and high‑probability performance. Extending the lower bounds to stateful protocols remains open.
• The global‑clock protocol relies on perfect synchronization. In practice, clock drift or message delays could degrade the guarantees; robust variants tolerant to bounded clock skew are a natural next step.
• The paper focuses on a single shared resource for one time unit. Extending the techniques to multi‑slot reservations, heterogeneous resource capacities, or priority classes would broaden applicability.
• Empirical evaluation on real‑world networks or cloud platforms is missing; implementing the protocol and measuring actual latency reductions would validate the theoretical gains.

Bottom line: By re‑examining an old coordination problem through the lens of modern distributed systems—where a global clock is often available—the authors demonstrate that a modest change in assumptions can unlock a whole new class of faster contention‑resolution algorithms, while also clarifying the trade‑offs between average‑case and tail‑risk performance. This work offers both a fresh theoretical perspective and concrete ideas that engineers can start to experiment with today.

Authors

• Zixi Cai
• Kuowen Chen
• Shengquan Du
• Tsvi Kopelowitz
• Seth Pettie
• Ben Plosk

Paper Information

• arXiv ID: 2602.12070v1
• Categories: cs.DC, math.PR
• Published: February 12, 2026
• PDF: Download PDF