  Multiserver-job (MSJ) systems, where jobs need to run concurrently across
many servers, are increasingly common in practice. The default service ordering
in many settings is First-Come First-Served (FCFS) service. Virtually all
theoretical work on MSJ FCFS models focuses on characterizing the stability
region, with almost nothing known about mean response time.
  We derive the first explicit characterization of mean response time in the
MSJ FCFS system. Our formula characterizes mean response time up to an additive
constant, which becomes negligible as arrival rate approaches throughput, and
allows for general phase-type job durations.
  We derive our result by utilizing two key techniques: REduction to Saturated
for Expected Time (RESET) and MArkovian Relative Completions (MARC).
  Using our novel RESET technique, we reduce the problem of characterizing mean
response time in the MSJ FCFS system to an M/M/1 with Markovian service rate
(MMSR). The Markov chain controlling the service rate is based on the saturated
system, a simpler closed system which is far more analytically tractable.
  Unfortunately, the MMSR has no explicit characterization of mean response
time. We therefore use our novel MARC technique to give the first explicit
characterization of mean response time in the MMSR, again up to constant
additive error. We specifically introduce the concept of "relative
completions," which is the cornerstone of our MARC technique.
