netdev - Re: [PATCH 1/2] rps: core implementation

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Message-ID: <65634d660911200908l5acbf3d0v5fc9a2e1adb7c3a5@mail.gmail.com>
Date:	Fri, 20 Nov 2009 09:08:10 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] rps: core implementation

> The description reads: "This solution queues packets early on in the
> receive path on the backlog queues of other CPUs.", so I'm not sure
> it's intended.

That is precisely the intent.  Getting packets quickly distributed to
the target cpus maximizes parallelism and reduces latency.

Did you test it like this (and it was visibly worse)?:
>
>        if (cpu < 0 || cpu == smp_processor_id())
>
>> +             return __netif_receive_skb(skb);
>> +     else
>> +             return enqueue_to_backlog(skb, cpu);
>> +}
>> +
This increases overall latency due to head of line blocking which will
outweigh the benefits of optimizing for this one case.

Tom
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