Horcrux: Automatic JavaScript Parallelism for Resource-Efficient Web Computation

https://www.usenix.org/conference/osdi21/presentation/mardani

• Modern web browsing

  • Performance matters

    • User: 53% of visits are abandoned if a mobile site takes longer than 3s to load

    • Content Providers: if your site makes $100,000/day, 1s improvement in page load increases revenue by $7,000 daily

• The problem: computation delays

  • Evaluation setup:

    • Performance metrics: Page Load Time (PLT), Speed Index (SI)

  • Developed Region, Emerging Region

• Why are computation delays significant

  • 80% more JavaScript over the last 5 years

  • Mobile Devices

    • More cores != better performance

    • Reason: Single-thread execution --> solution: parallelizing JavaScript

    • Main idea: automatically parallelizes JavaScript execution using concolic execution to take advantage of phones multi-core CPUs

• Parallelism Opportunities

  • Web workers are widely supported by browsers

  • Constrained APIs:

    • No access to DOM APIs

    • No access to the main global state

  • Legacy pages are highly amenable to safe parallelism

• Challenges

  • C1: ensuring correctness

    • The exact state accessed by parallelized JavaScript --> conservative signatures

      • Despite non-determinism and across all possible control paths

    • Conservative signatures

      • Dynamic analysis: track read / writes to page state

      • Concolic execution: explore all possible control paths

      • 
      • Server-side and offline

  • C2: web workers constrained APIs

    • How to parallelize execution with constrained APIs?

      • Client-side parallelization scheduler

        • Runs in the main browser thread

        • Only task: manage offloads in event-driven mode

  • C3: Offloading overheads

    • Pass-by-value I/O can take ~0-10ms

      • Root-function Granularity

      • Offloading Granularity

        • Trade-off: parallelization benefits vs. offloading overheads

        • Solution: offloading root-function invocations

        • 4x fewer offloads and 73% of parallelization benefits

• Evaluation

  • Impact on browser computation delays?

    • Total computation time (TCT)

  • Impact on end-to-end performance?

    • Page Load Time (PLT), Speed Index (SI)

  • Horcrux comparison to prior compute optimizations?

  • What do conservative signatures forgo?

  • How much are the server-side overheads?