Sent from my iPhone
> On Apr 1, 2014, at 7:27 AM, Ben Pfaff <b...@nicira.com> wrote:
>
>> On Tue, Apr 01, 2014 at 03:14:16PM +0300, Lori Jakab wrote:
>>> On 3/26/14, 2:16 AM, Ben Pfaff wrote:
>>>> On Mon, Mar 24, 2014 at 10:59:06AM -0700, Jarno Rajahalme wrote:
>>>> This patch shrinks the struct ofpbuf from 104 to 48 bytes on 64-bit
>>>> systems, or from 52 to 36 bytes on 32-bit systems (counting in the
>>>> 'l7' removal by the previous patch). This may help contribute to
>>>> cache efficiency, and will speed up initializing, copying and
>>>> manipulating ofpbufs. This is potentially important for the DPDK
>>>> datapath, but the rest of the code base may also see a little benefit.
>>>>
>>>> Changes are:
>>>>
>>>> - Remove 'l7' pointer (previous patch).
>>>> - Use offsets instead of layer pointers for l2_5, l3, and l4 using
>>>> 'l2' as basis. Usually 'data' is the same as 'l2', but this is not
>>>> always the case (e.g., when parsing or constructing a packet), so it
>>>> can not be easily used as the offset basis. Also, packet parsing is
>>>> faster if we do not need to maintain the offsets each time we pull
>>>> data from the ofpbuf.
>>>> - Use uint32_t for 'allocated' and 'size', as 2^32 is enough even for
>>>> largest possible messages/packets.
>>>> - Use packed enum for 'source'.
>>>> - Rearrange to avoid unnecessary padding.
>>>> - Remove 'private_p', which was used only in two cases, both of which
>>>> had the invariant ('l2' == 'data'), so we can temporarily use 'l2'
>>>> as a private pointer.
>>>>
>>>> Signed-off-by: Jarno Rajahalme <jrajaha...@nicira.com>
>>
>> Unfotunately I missed this patch until rebasing my layer 3 patch
>> series [1] today.
>>
>> Before this patch, I set the 'l2' pointer to NULL and 'l3' to the
>> start of the network header to represent a layer 3 packet in a
>> struct ofpbuf. The way struct ofpbuf looks after this patch makes
>> the assumption of always having layer 2 headers a lot stronger.
>>
>> I see two options for my patches going forward:
>> 1) Add back 'l3' and remove 'l3_ofs' from struct ofpbuf (keeping the
>> rest as is); or
>> 2) Encode a layer 3 packet by setting 'l2' to the network header
>> address and set l3_ofs to 0.
>
> Can we do the following?
> 3) Replace 'l2' by 'l2_ofs' also?
I'll look into this today. However, my earlier analysis was that this would
complicate things when the offset basis ('data') is changing a lot (e.g., when
pulling data from ofpbuf). Maybe the cleanest solution would be to have a
pointer to the start of packet and an additional l2 offset. However, as 0 is a
valid offset value, having a separate l2 offset seems redundant as l3_ofs 0
would be an unambiguous way of encoding the absence of the l2 header.
Jarno
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