[ovs-dev] Userspace Netlink MMAP status

Zoltan Kiss zoltan.kiss at citrix.com
Thu May 1 16:09:53 UTC 2014

On 29/04/14 17:36, Thomas Graf wrote:
> On Tue, Apr 29, 2014 at 05:17:07PM +0100, Zoltan Kiss wrote:
>> On 23/04/14 22:56, Thomas Graf wrote:
>>> On 04/23/2014 10:12 PM, Ethan Jackson wrote:
>>>> The problem has actually gotten worse since we've gotten rid of the
>>>> dispatcher thread.  Now each thread has it's own channel per port.
>>>> I wonder if the right approach is to simply ditch the per-port
>>>> fairness in the case where mmap netlink is enabled.  I.E. we simply
>>>> have one channel per thread and call it a day.
>>>> Anyways, I don't have a lot of context on this thread, so take
>>>> everything above with a grain of salt.
>>> I agree with Ethan's statement. Even with a reduced frame size the cost
>>> of an individual ring buffer per port is likely still too large and
>>> we lose the benefit of zerocopy for large packets which are typically
>>> the expensive packets.
>> My expectation is that such large packets shouldn't go to the
>> userspace very often, as ideally the TCP handshake packets already
>> established the flow. Do you have a use case where this is not true?
> The common use case is a flow expiring during the lifetime of a TCP
> connection. It will result in multiple data packets being sent upwards.
> It's much less likely in the megaflows era though.
>>> As we extend the GSO path into the upcall and make use of the new DPDK
>>> style ofpbuf to avoid the memcpy() for the mmap case
>> Can you elaborate a bit more on this?
> The current upcall code does segmentation which is not required and is
> expensive for the above mentioned case. A single 64K GSO packet will
> automatically result in up to 50 upcalls.
> Also, right now, the first thing we do in the mmap case is copy the
> buffer into an ofpbuf. This is not required at all and the copy is
> expensive,  instead, we should make use of the shared memory just like
> in the DPDK case and only release the buffer after the packet has been
> fully processed.

So you suggest userspace should directly access the linear buffer and 
the frags, instead of copying them into the shared buffer?

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