Unix-compliant open source operating systems are often used as platforms of web service systems. Choosing an operating system can affect web system performance. In this paper, we present a source-code-level time measurement tool and measure a service time of each system call that is invoked by Squid proxy cache. We produce the log while running Squid 2.4.STABLE1 on Linux 2.4.2 and Solaris 8. As a result, we find that Linux 2.4.2 provides better services than Solaris 8. This can be used as a guide for selecting system software on building web service.