From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Drozdov Subject: Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3 Date: Fri, 06 Feb 2015 07:49:36 +0300 Message-ID: <54D447E0.6040702@gmail.com> References: <1423115891-3578-1-git-send-email-al.drozdov@gmail.com> <474D8071-4AEC-48F4-B0DC-02D9091CF3DB@alum.mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , Daniel Borkmann , Eric Dumazet , Al Viro , "Michael S. Tsirkin" , Network Development , linux-kernel , Dan Collins To: Guy Harris , Willem de Bruijn Return-path: In-Reply-To: <474D8071-4AEC-48F4-B0DC-02D9091CF3DB@alum.mit.edu> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 06.02.2015 00:16:30 +0300 Guy Harris wrote: > On Feb 5, 2015, at 12:01 PM, Willem de Bruijn wrote: > >> On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov 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)