Re: [RFC] Idea about increasing efficency of skb allocation in network devices

[Eric Dumazet <eric.dumazet@gmail.com>]

Brice Goglin a écrit :
> David Miller wrote:
>> From: Neil Horman <nhorman@tuxdriver.com>
>> Date: Sun, 26 Jul 2009 20:36:09 -0400
>>
>>   
>>> 	Since Network devices dma their memory into a provided DMA
>>> buffer (which can usually be at an arbitrary location, as they must
>>> cross potentially several pci busses to reach any memory location),
>>> I'm postulating that it would increase our receive path efficiency
>>> to provide a hint to the driver layer as to which node to allocate
>>> an skb data buffer on.  This hint would be determined by a feedback
>>> mechanism.  I was thinking that we could provide a callback function
>>> via the skb, that accepted the skb and the originating net_device.
>>> This callback can track statistics on which numa nodes consume
>>> (read: copy data from) skbs that were produced by specific net
>>> devices.  Then, when in the future that netdevice allocates a new
>>> skb (perhaps via netdev_alloc_skb), we can use that statistical
>>> profile to determine if the data buffer should be allocated on the
>>> local node, or on a remote node instead.
>>>     
>> No matter what, you will do an inter-node memory operation.
>>
>> Unless, the consumer NUMA node is the same as the one the
>> device is on.
>>
>> Because since the device is on a NUMA node, if you DMA remotely
>> you've eaten the NUMA cost already.
>>
>> If you always DMA to the device's NUMA node (what we try to do now) at
>> least the is the possibility of eliminating cross-NUMA traffic.
>>
>> Better to move the application or stack processing towards the NUMA
>> node the network device is on, I think.
>>   
> 
> Is there an easy way to get this NUMA node from the application socket
> descriptor?

Thats not easy, this information can change for every packet (think of
bonding setups, whith aggregation of devices on different NUMA nodes)

We could add a getsockopt() call to peek this information from the next
data to be read from socket (returns node id where skb data is sitting,
hoping that NIC driver hadnt copybreak it (ie : allocate a small skb and
copy the device provided data on it before feeding packet to network stack))


> Also, one question that was raised at the Linux Symposium is: how do you
> know which processors run the receive queue for a specific connection ?
> It would be nice to have a way to retrieve such information in the
> application to avoid inter-node and inter-core/cache traffic.

All this depends on the fact you have multiqueue devices or not, and
trafic spreads on all queues or not.

Assuming you have single queue device, only current way to handle
this is to do the reverse thinking.

Ie, bind NIC interrupts to the appropriate set of cpus, and
possibly bind user apps threads dealing with network trafic to same set.

Only background or cpu hungry threads should be allowed to run
on foreigns nodes.