[perf] Fix bug in perf modeling -- use memory round trip time.
[perf] Fix bug in perf modeling -- use memory round trip time.

fhijaz/Graphite,victorisildur/Graphite,mit-carbon/Graphite,8l/Graphite,8l/Graphite,nkawahara/Graphite,fhijaz/Graphite,mit-carbon/Graphite-Cycle-Level,fhijaz/Graphite,fhijaz/Graphite,mit-carbon/Graphite-Cycle-Level,victorisildur/Graphite,victorisildur/Graphite,8l/Graphite,8l/Graphite,mit-carbon/Graphite-Cycle-Level,victorisildur/Graphite,mit-carbon/Graphite,mit-carbon/Graphite-Cycle-Level,mit-carbon/Graphite,nkawahara/Graphite,mit-carbon/Graphite,nkawahara/Graphite,nkawahara/Graphite