IBM T.J. Watson Research Center
Yorktown Heights, NY
- Vivek Pai
Department of Computer Science
We investigate the origin and components of network server latency under various loads and find that filesystem-related kernel queues exhibit head-of-line blocking, which leads to bursty behavior in event delivery and process scheduling. In turn, these problems degrade the existing fairness and scheduling policies in the operating system, causing requests that could have been served in memory, with low latency, to unnecessarily wait on disk-bound requests. While this batching behavior only mildly affects throughput, it severely degrades latency. This problem manifests itself in fairness and service quality degradation, a phenomenon we call service inversion.
We show a portable solution that avoids these problems without kernel or filesystem modifications, We modify two different Web servers to use this approach, and demonstrate a qualitatively different change in their latency profiles, generating more than an order of magnitude reduction in latency. The resulting systems are able to serve most requests without being tied to disk performance, and they scale better with improvements in processor speed. These results are not dependent on server software architecture, and can be profitably applied to experimental and production servers.