  The execution of similar units can be compared by their internal behaviors to
determine the causes of their potential performance issues. For instance, by
examining the internal behaviors of different fast or slow web requests more
closely and by clustering and comparing their internal executions, one can
determine what causes some requests to run slowly or behave in unexpected ways.
In this paper, we propose a method of extracting the internal behavior of web
requests as well as introduce a pipeline that detects performance issues in web
requests and provides insights into their root causes. First, low-level and
fine-grained information regarding each request is gathered by tracing both the
user space and the kernel space. Second, further information is extracted and
fed into an outlier detector. Finally, these outliers are then clustered by
their behavior, and each group is analyzed separately. Experiments revealed
that this pipeline is indeed able to detect slow web requests and provide
additional insights into their true root causes. Notably, we were able to
identify a real PHP cache contention using the proposed approach.
