netdev - Re: Problems with TPACKET_V3 delivery of wakeups (and empty buffer blocks)

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Message-Id: <C7B99DBD-8362-4A39-B9AD-D7AC14880DA5@alum.mit.edu>
Date:	Mon, 4 Aug 2014 21:52:27 -0700
From:	Guy Harris <guy@...m.mit.edu>
To:	chetan loke <loke.chetan@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Problems with TPACKET_V3 delivery of wakeups (and empty buffer blocks)


On Aug 4, 2014, at 8:23 PM, chetan loke <loke.chetan@...il.com> wrote:

> Empty buffer blocks have to be delivered. The patch explains it why.

If by "the patch" you mean

	https://lkml.org/lkml/2011/6/21/463

and the comment

> E2) Also implemented basic timeout mechanism to close 'a' current block.
>     That way, user-space won't be blocked forever on an idle link.
>     This is a much needed feature while monitoring multiple ports.
>     Look at 3) below.

then that's presumably referring to 3.4.2 in

> 3) Port aggregation analysis:
>    Multiple ports are viewed/analyzed as one logical pipe.
>    Example:
>    3.1) up-stream    path can be tapped in eth1
>    3.2) down-stream  path can be tapped in eth2
>    3.3) Network TAP splits Rx/Tx paths and then feeds to eth1,eth2.
> 
>    If both eth1,eth2 need to be viewed as one logical channel,
>    then that implies we need to timesort the packets as they come across
>    eth1,eth2.
> 
>    3.4) But following issues further complicates the problem:
>         3.4.1)What if one stream is bursty and other is flowing
>               at line rate?
>         3.4.2)How long do we wait before we can actually make a
>               decision in the app-space and bail-out from the spin-wait?

and presumably this is referring to some mechanism by which eth1 and eth2 are handled by *one* socket, because if it's handled by *two* sockets, you use select()/poll()/epoll() on the two sockets, and, when *either* of the devices either fills up a block or times out, a wakeup is delivered to its socket and userland wakes up.

And even if there's some reason why a wakeup has to be delivered when the timer expires even if *no* packets are available, what is the reason why you don't just deliver a wakeup but *no* buffer blocks, so that userland just says "well, I woke up, but there are no buffer blocks available for that particular socket, so there's nothing to process for that socket"?--
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