private double usableMemoryGb() {
        double usableMemoryGb = resources.memoryGiB() - Host.memoryOverheadGb;
        return usableMemoryGb * (1 - fractionOfMemoryReserved);
    }
Returns the memory we can expect will be available for the content node processes
config-model/src/main/java/com/yahoo/vespa/model/search/NodeResourcesTuning.java