Implementation of a new scheduling policy in web servers

Abstract

Recently, the Shortest-Remaining-Processing-Time (SRPT) has been proposed for scheduling static HTTP requests in web servers to reduce the mean response time. The SRPT assumes that the response time of the requested file is strongly proportional to its size. This assumption is unwarranted in Internet environment. Thus, we proposed the Shortest-Remaining-Response-Time (SRRT) that better estimates the response time for static HTTP. The SRRT prioritizes requests based on a combination of the current round-trip-time (RTT), TCP congestion window size (cwnd) and the size of what remains of the requested file. We compare SRRT to SRPT and Processor-Sharing (PS) policies. The SRRT shows the best improvement in the mean response time. SRRT gives an average improvement of about 7.5% over SRPT. This improvement comes at a negligible expense in response time for long requests. We found that under 100Mbps link, only 1.5% of long requests have longer response times than under PS. The longest request under SRRT has an increase in response time by a factor 1.7 over PS. For 10Mbps link, only 2.4% of requests are penalized, and SRRT increases the longest request time by a factor 2.2 over PS.