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From: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Date: Sat, 14 Feb 2015 05:14:59 +0300
Message-ID: <CAHNKnsSm1GOUMONPB4oB1tJ-OeWOG1XOhw=HEY8LPXhDEHEjbg@mail.gmail.com> (sfid-20150214_032228_249262_CDB5E707)
Subject: Re: [RFC] mac80211: use rhashtable for station table
To: Johannes Berg <johannes@sipsolutions.net>
Cc: "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
	Thomas Graf <tgraf@suug.ch>,
	Johannes Berg <johannes.berg@intel.com>
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2015-02-14 0:47 GMT+03:00 Johannes Berg <johannes@sipsolutions.net>:
> We currently have a hand-rolled table with 256 entries and are
> using the last byte of the MAC address as the hash. This hash
> is obviously very fast, but collisions are easily created and
> we waste a lot of space in the common case of just connecting
> as a client to an AP where we just have a single station. The
> other common case of an AP is also suboptimal due to the size
> of the hash table and the ease of causing collisions.
>
> Convert all of this to use rhashtable with jhash, which gives
> us the advantage of a far better hash function (with random
> perturbation to avoid hash collision attacks) and of course
> that the hash table grows and shrinks dynamically with chain
> length, improving both cases above.
>
A nice change! Couple of years ago I did some tests with real sets of
MACs and jhash gives a better distribution than usage of a last octet.

BTW, why do you use full address and generic jhash? Hashing of two
least significant words could be faster. Isn't it?

-- 
Sergey