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From: Alexander Drozdov <al.drozdov@gmail.com>
To: Guy Harris <guy@alum.mit.edu>, Willem de Bruijn <willemb@google.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <dborkman@redhat.com>,
	Eric Dumazet <edumazet@google.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Network Development <netdev@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Dan Collins <dan@dcollins.co.nz>
Subject: Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3
Date: Fri, 06 Feb 2015 07:49:36 +0300	[thread overview]
Message-ID: <54D447E0.6040702@gmail.com> (raw)
In-Reply-To: <474D8071-4AEC-48F4-B0DC-02D9091CF3DB@alum.mit.edu>

On 06.02.2015 00:16:30 +0300 Guy Harris <guy@alum.mit.edu> wrote:
> On Feb 5, 2015, at 12:01 PM, Willem de Bruijn <willemb@google.com> wrote:
>
>> On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov <al.drozdov@gmail.com> 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)

  reply	other threads:[~2015-02-06  4:49 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-05  5:58 Alexander Drozdov
2015-02-05 20:01 ` Willem de Bruijn
2015-02-05 21:16   ` Guy Harris
2015-02-06  4:49     ` Alexander Drozdov [this message]
2015-02-06  6:54   ` Alexander Drozdov
2015-02-07  1:45     ` Willem de Bruijn
2015-02-24  5:18 ` [RESEND PATCH] " Alexander Drozdov
2015-02-24 21:09   ` David Miller

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