Answer the number of scavenges that an object must on average
survive before being promoted to oldspace; this is however only
an estimate because objects that are reachable from oldspace have
a higher probability to be tenured soon, while objects that are
only reachable from thisContext have a lower probability to be
tenured. Anyway, if this number falls below 2-3 you should
definitely increment the size of eden and/or of survivor space,
because you are tenuring too often and relying too much on
global garbage collection to keep your heap clean; do the
measurement on a restarted image, so that the extra tenuring
incurred when creating long-lived objects such as classes or
methods is not considered.