I'm trying to figure out if this PR: https://github.com/elastic/gosigar/pull/71/files
Has been incorporated into the 6.4.3 release of Metricbeat, which is what we are using. This might help explain the discrepancy between metricbeat memory usage reports and Pingdom Server Monitor reports (which uses this code: https://github.com/pingdomserver/server_metrics/blob/master/lib/server_metrics/collectors/memory.rb#L40)