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Date:      Wed, 20 Jul 2016 15:22:20 -0400
From:      Andrew Gallatin <gallatin@netflix.com>
To:        Ryan Stone <rysto32@gmail.com>
Cc:        freebsd-transport@freebsd.org
Subject:   Re: in_broadcast() called for almost every packet in ip_output()
Message-ID:  <559f1d0e-9d35-f458-bff5-b69a03297cf0@netflix.com>
In-Reply-To: <CAFMmRNyi1-K%2BKX6bY2bx7_hiq2PDPJa-QJatBf0QdtriGnJ5fw@mail.gmail.com>
References:  <CAFMmRNx%2Bx9GNDgDHO5oyoj-S%2BCK9bRvJhpNNFf3%2BPk0p2SQeSQ@mail.gmail.com> <CADLQ3sLyytm0HMsSChhWD108t6iPpquXH2E2R7Wc42FpaAP1Rw@mail.gmail.com> <CAFMmRNyi1-K%2BKX6bY2bx7_hiq2PDPJa-QJatBf0QdtriGnJ5fw@mail.gmail.com>

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On 07/20/2016 15:09, Ryan Stone wrote:
> On Wed, Jul 20, 2016 at 3:00 PM, Drew Gallatin <gallatin@netflix.com
> <mailto:gallatin@netflix.com>> wrote:
>
>     I'd certainly prefer not to add any overhead.  Properly managing
>     this (and other similar) lists is a job for some lightweight
>     lifecycle based mechanism like concurrency kit or rcu.
>
>     Unless you have a solid  reason to fix it, I'd suggest just adding
>     the locking commented out (the way rwatson did with IN_IFADDR_RLOCK
>     in ip_input(), so that the next person to trip over it will know
>     what's going on.
>
>     Drew
>
>
> I'm not clear on the details, but we have some internal tests that were
> provoking this exact crash after a couple of days of stress testing.
>
> I believe that the actual fix would involve caching the result in the
> pcb.  When the endpoint is already known, there's no reason to check for
> a broadcast packet on every single packet.  On a system that uses a
> large number of IP addresses on an interface (e.g. for jails) the
> overhead of just iterating over the list is going to add up no matter
> what synchronization mechanism you use.

Caching sounds reasonable, but.. won't the lock still add overhead
to paths like unconnected udp and packet forwarding?



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