netdev - Re: [PATCH net-next] net: bridge: use rhashtable for fdbs

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Date:   Tue, 12 Dec 2017 10:07:13 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:     netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
        roopa@...ulusnetworks.com, srn@...mr.com, davem@...emloft.net
Subject: Re: [PATCH net-next] net: bridge: use rhashtable for fdbs

On Tue, 12 Dec 2017 16:02:50 +0200
Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:

> Before this patch the bridge used a fixed 256 element hash table which
> was fine for small use cases (in my tests it starts to degrade
> above 1000 entries), but it wasn't enough for medium or large
> scale deployments. Modern setups have thousands of participants in a
> single bridge, even only enabling vlans and adding a few thousand vlan
> entries will cause a few thousand fdbs to be automatically inserted per
> participating port. So we need to scale the fdb table considerably to
> cope with modern workloads, and this patch converts it to use a
> rhashtable for its operations thus improving the bridge scalability.
> Tests show the following results (10 runs each), at up to 1000 entries
> rhashtable is ~3% slower, at 2000 rhashtable is 30% faster, at 3000 it
> is 2 times faster and at 30000 it is 50 times faster.
> Obviously this happens because of the properties of the two constructs
> and is expected, rhashtable keeps pretty much a constant time even with
> 10000000 entries (tested), while the fixed hash table struggles
> considerably even above 10000.
> As a side effect this also reduces the net_bridge struct size from 3248
> bytes to 1344 bytes. Also note that the key struct is 8 bytes.
> 
> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
> ---

Thanks for doing this, it was on my list of things that never get done.

Some downsides:
 * size of the FDB entry gets larger.
 * you lost the ability to salt the hash (and rekey) which is important
   for DDoS attacks
 * being slower for small (<10 entries) also matters and is is a common
   use case for containers.