[Xen-devel] [PATCH RFC 0/4]: xen-net{back, front}: Multiple transmit and receive queues

["Andrew J. Bennieston" <andrew.bennieston@xxxxxxxxxx>]

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.

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.

One open issue is how to deal with the tx_credit data for rate limiting.
This used to exist on a per-VIF basis, and these patches move it to
per-queue to avoid contention on concurrent access to the tx_credit
data from multiple threads. This has the side effect of breaking the
tx_credit accounting across the VIF as a whole. I cannot see a situation
in which people would want to use both rate limiting and a
high-performance multi-queue mode, but if this is problematic then it
can be brought back to the VIF level, with appropriate protection.
Obviously, it continues to work identically in the case where there is
only one queue.

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.

Queue-specific XenStore entries for ring references and event channels
are stored hierarchically, i.e. under .../queue-N/... where N varies
from 0 to one less than the requested number of queues (inclusive). If
only one queue is requested, it falls back to the flat structure where
the ring references and event channels are written at the same level as
other vif information.

--
Andrew J. Bennieston

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