RE: [patch v1, kernel version 3.2.1] net/ipv4/ip_gre: Ethernet multipoint GRE over IP

David Laight
2012-01-16T13:30:11Z

> > + for (i = 0; i < GRETAP_BR_HASH_SIZE; i++) {
> > + struct ipgre_tap_bridge_entry *entry;
> > + struct hlist_node *h, *n;
> > + hlist_for_each_entry_safe(entry, h, n,
> > + &tunnel->hash[i], hlist)
> > + {
> > + unsigned long this_timer;
> > + this_timer = entry->ageing_timer + delay;
> > + if (time_before_eq(this_timer, jiffies))
> > + ipgre_tap_bridge_delete(entry);
> > + else if (time_before(this_timer, next_timer))
> > + next_timer = this_timer;
> > + }
> > + }
> > + spin_unlock_bh(&tunnel->hash_lock);
> > + mod_timer(&tunnel->gc_timer, round_jiffies(next_timer + HZ/4));
>
> wow... why setup a 250 ms timer, if entries are valid 300 seconds ?

Isn't that code trying to wakeup 250ms after the first expiry of any
of its items?

Do you even need a timer at all?

It may be enough to just put a timestamp into each entry
and tidy up when scanning one of the hash chains in (say)
the 'add item' path - when you need write access anyway.

A hash table might not be the best structure either!
The hash lookup is still o(n) for n >> table_size.
It also may be likely that you'll do repeated lookups for a small
number of items - in which case using a hash to cache recent lookups
might be useful (maybe with some type of balanced tree structure)

David