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Re: [Xen-devel] [PATCH V6 net-next 0/5] xen-net{back, front}: Multiple transmit and receive queues

On Mon, 2014-03-03 at 11:47 +0000, Andrew J. Bennieston wrote:
> This patch series implements multiple transmit and receive queues (i.e.
> multiple shared rings) for the xen virtual network interfaces.
> The series is split up as follows:
>  - Patches 1 and 3 factor out the queue-specific data for netback and
>     netfront respectively, and modify the rest of the code to use these
>     as appropriate.
>  - Patches 2 and 4 introduce new XenStore keys to negotiate and use
>    multiple shared rings and event channels, and code to connect these
>    as appropriate.
>  - Patch 5 documents the XenStore keys required for the new feature
>    in include/xen/interface/io/netif.h
> All other transmit and receive processing remains unchanged, i.e. there
> is a kthread per queue and a NAPI context per queue.
> The performance of these patches has been analysed in detail, with
> results available at:
> http://wiki.xenproject.org/wiki/Xen-netback_and_xen-netfront_multi-queue_performance_testing
> To summarise:
>   * Using multiple queues allows a VM to transmit at line rate on a 10
>     Gbit/s NIC, compared with a maximum aggregate throughput of 6 Gbit/s
>     with a single queue.
>   * For intra-host VM--VM traffic, eight queues provide 171% of the
>     throughput of a single queue; almost 12 Gbit/s instead of 6 Gbit/s.

From the graphs it looks like 8 queues doesn't offer that much over 4
and the bulk of the improvement comes from going to just 2 queues.

Any idea what the bottleneck is? i.e. why does the graph flatten so

>   * There is a corresponding increase in total CPU usage, i.e. this is a
>     scaling out over available resources, not an efficiency improvement.

corresponding to the number of queues or the throughput improvement?
i.e. is it 8x or 1.71x with 8 queues?


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