What does cmd 'flush_all' use for?

[John.H]

Some body say this cmd will invalidate all item in the memcached server.
But why the curr_item STAT not set to zero?
And I read the source code:
in the do_item_flush_expired function,it give a condition:

            if (iter->time >= settings.oldest_live) {
                next = iter->next;
                if ((iter->it_flags & ITEM_SLABBED) == 0) {
                    do_item_unlink(iter);
                }
            } else {
                /* We've hit the first old item. Continue to the next queue.
*/
                break;
            }

settings.oldest_live = currenttime - 1
can iter->time >= settings.oldest_live be true??
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