netdev - RE: [patch v1, kernel version 3.2.1] net/ipv4/ip

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Date:	Mon, 16 Jan 2012 13:30:07 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>,
	Štefan Gula <steweg@...il.com>
Cc:	"Alexey Kuznetsov" <kuznet@....inr.ac.ru>,
	"David S. Miller" <davem@...emloft.net>,
	"James Morris" <jmorris@...ei.org>,
	"Hideaki YOSHIFUJI" <yoshfuji@...ux-ipv6.org>,
	"Patrick McHardy" <kaber@...sh.net>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [patch v1, kernel version 3.2.1] net/ipv4/ip_gre: Ethernet multipoint GRE over IP

> > +	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


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