public long firstFill() {
        long first = Long.MAX_VALUE;
        if (tracks.isEmpty()) {
            return 0L;
        }
        for (TimeTracker t : tracks) {
            long candidate = t.firstFill();
            if (candidate == 0L) {
                continue;
            }
            first = Math.min(first, t.firstFill());
        }
        return first;
    }
The time of the start of the first document fill requested.
container-search/src/main/java/com/yahoo/search/statistics/ElapsedTime.java