Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3

[Alexander Drozdov]

On 06.02.2015 00:16:30 +0300 Guy Harris <guy@xxxxxxxxxxxx> wrote:
> On Feb 5, 2015, at 12:01 PM, Willem de Bruijn <willemb@xxxxxxxxxx> wrote:
>
> > On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov <al.drozdov@xxxxxxxxx> wrote:
> > > Don't close an empty block on timeout. Its meaningless to
> > > pass it to the user. Moreover, passing empty blocks wastes
> > > CPU & buffer space increasing probability of packets
> > > dropping on small timeouts.
> > >
> > > Side effect of this patch is indefinite user-space wait
> > > in poll on idle links. But, I believe its better to set
> > > timeout for poll(2) when needed than to get empty blocks
> > > every millisecond when not needed.
> > This change would break existing applications that have come
> > to depend on the periodic signal.
> >
> > I don't disagree with the argument that the data ready signal
> > should be sent only when a block is full or a timer expires and
> > at least some data is waiting, but that is moot at this point.
> For what it's worth, the BPF packet capture mechanism (which really needs a new name, to distinguish itself from the BPF packet filter language and its implementation(s), but I digress) has the same issue - when the timer expires, a wakeup is delivered even if there are no packets to read.
>
> *However*, if there are no packets available, the buffers aren't rotated, so the empty buffer is left around to be filled up with packets, rather than being made the hold buffer.
>
> Given that before the previous TPACKET_V3 change, wakeups were delivered when packets arrived rather than when a block was closed, presumably code using TPACKET_V3 was capable of dealing with wakeups being delivered when no new blocks had been made available to userland; could TPACKET_V3 work a bit more like BPF and deliver a wakeup when the timer expires *without* closing the empty block?
Thank you all for your comments! I'll try to create two patches:
1. Wakeup by timeout without closing the empty block
2. Allow to not wakeup by timeout (the feature should be explicitly requested by a user)
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