Announcing Silk: a silky smooth fiber runtime for ClickHouse

Summary

Silk is a C++ stackful-fiber library and scheduler created for ClickHouse’s highly concurrent, I/O-bound components, including object-storage I/O, distributed cache lookups, replica coordination, and HTTP fan-out. Its design combines one pinned OS scheduler thread per CPU, NUMA-aware work stealing, io_uring for asynchronous I/O and timers, and zero heap allocation in the steady-state hot path; fiber stacks are mmap’d from per-fiber pools with guard pages, while runtime containers are intrusive. The post reports benchmarks including roughly 3.6-nanosecond fiber yields with cross-CPU stealing, 5.9 million file IOPS in a working configuration, and about fifteen times boost::asio’s throughput at one connection. At 10,000 concurrent S3-style requests, internal measurements showed a roughly 65% improvement in 99.9th-percentile latency versus an equivalent thread-pool executor, with identical median throughput; distributed cache is the first intended ClickHouse integration.

Takeaways

1. 1

   Silk’s topology-aware stealing orders candidates by estimated cost: hyperthread siblings, same-socket cores, then cross-socket cores, with randomization within each tier.

2. 2

   The reported benchmarks include roughly 3.6 nanoseconds per fiber yield with cross-CPU work stealing, 7.6 microseconds for an io_uring ping-pong, and 5.9 million file IOPS in a working configuration.

3. 3

   At 10,000 concurrent S3-style requests, internal benchmarks reported roughly 65% better 99.9th-percentile latency than an equivalent thread-pool executor, while median throughput was identical.