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Re: [Xen-devel] [PATCH V2 net-next 0/5] xen-net{back, front}: Multiple transmit and receive queues



On 14/02/14 14:06, Wei Liu wrote:
On Fri, Feb 14, 2014 at 11:50:19AM +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.
   * There is a corresponding increase in total CPU usage, i.e. this is a
     scaling out over available resources, not an efficiency improvement.
   * Results depend on the availability of sufficient CPUs, as well as the
     distribution of interrupts and the distribution of TCP streams across
     the queues.

Queue selection is currently achieved via an L4 hash on the packet (i.e.
TCP src/dst port, IP src/dst address) and is not negotiated between the
frontend and backend, since only one option exists. Future patches to
support other frontends (particularly Windows) will need to add some
capability to negotiate not only the hash algorithm selection, but also
allow the frontend to specify some parameters to this.


This has an impact on the protocol. If the key to select hash algorithm
is missing then we're assuming L4 is in use.

This either needs to be documented (which is missing in your patch to
netif.h) or you need to write that key explicitly in XenStore.

I also have a question what would happen if one end advertises one hash
algorithm then use a different one. This can happen when the
driver is rogue or buggy. Will it cause the "good guy" to stall? We
certainly don't want to stall backend, at the very least.

I'm not sure I understand. There is no negotiable selection of hash algorithm here. This paragraph refers to a possible future in which we may have to support multiple such. These issues will absolutely have to be addressed then, but it is completely irrelevant for now.

Andrew.

I don't see relevant code in this series to handle "rogue other end". I
presume for a simple hash algorithm like L4 is not very important (say,
even a packet ends up in the wrong queue we can still safely process
it), or core driver can deal with this all by itself (dropping)?

Wei.



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