On 9/12/2017 3:53 PM, Tom Herbert wrote:
On Tue, Sep 12, 2017 at 3:31 PM, Samudrala, Sridhar
<sridhar.samudr...@intel.com> wrote:
On 9/12/2017 8:47 AM, Eric Dumazet wrote:
On Mon, 2017-09-11 at 23:27 -0700, Samudrala, Sridhar wrote:
On 9/11/2017 8:53 PM, Eric Dumazet wrote:
On Mon, 2017-09-11 at 20:12 -0700, Tom Herbert wrote:
Two ints in sock_common for this purpose is quite expensive and the
use case for this is limited-- even if a RX->TX queue mapping were
introduced to eliminate the queue pair assumption this still won't
help if the receive and transmit interfaces are different for the
connection. I think we really need to see some very compelling results
to be able to justify this.
Will try to collect and post some perf data with symmetric queue
configuration.
Here is some performance data i collected with memcached workload over
ixgbe 10Gb NIC with mcblaster benchmark.
ixgbe is configured with 16 queues and rx-usecs is set to 1000 for a
very low
interrupt rate.
ethtool -L p1p1 combined 16
ethtool -C p1p1 rx-usecs 1000
and busy poll is set to 1000usecs
sysctl net.core.busy_poll = 1000
16 threads 800K requests/sec
=============================
rtt(min/avg/max)usecs intr/sec contextswitch/sec
-----------------------------------------------------------------------
Default 2/182/10641 23391 61163
Symmetric Queues 2/50/6311 20457 32843
32 threads 800K requests/sec
=============================
rtt(min/avg/max)usecs intr/sec contextswitch/sec
------------------------------------------------------------------------
Default 2/162/6390 32168 69450
Symmetric Queues 2/50/3853 35044 35847
Yes, this is unreasonable cost.
XPS should really cover the case already.
Eric,
Can you clarify how XPS covers the RX-> TX queue mapping case?
Is it possible to configure XPS to select TX queue based on the RX queue
of a flow?
IIUC, it is based on the CPU of the thread doing the transmit OR based
on skb->priority to TC mapping?
It may be possible to get this effect if the the threads are pinned to a
core, but if the app threads are
freely moving, i am not sure how XPS can be configured to select the TX
queue based on the RX queue of a flow.
If application is freely moving, how NIC can properly select the RX
queue so that packets are coming to the appropriate queue ?
The RX queue is selected via RSS and we don't want to move the flow based on
where the thread is running.
Unless flow director is enabled on the Intel device... This was, I
believe, one of the first attempts to introduce a queue pair notion to
general purpose NICs. The idea was that the device records the TX
queue for a flow and then uses that to determine receive queue in a
symmetric fashion. aRFS is similar, but was under SW control how the
mapping is done. As Eric mentioned there are scalability issues with
these mechanisms, but we also found that flow director can easily
reorder packets whenever the thread moves.
You must be referring to the ATR(application targeted routing) feature
on Intel
NICs wherea flow director entry is added for a flow based on TX queue
used for
that flow. Instead, we would like to select the TX queue based on the RX
queue
of a flow.
This is called aRFS, and it does not scale to millions of flows.
We tried in the past, and this went nowhere really, since the setup cost
is prohibitive and DDOS vulnerable.
XPS will follow the thread, since selection is done on current cpu.
The problem is RX side. If application is free to migrate, then special
support (aRFS) is needed from the hardware.
This may be true if most of the rx processing is happening in the interrupt
context.
But with busy polling, i think we don't need aRFS as a thread should be
able to poll
any queue irrespective of where it is running.
It's not just a problem with interrupt processing, in general we like
to have all receive processing an subsequent transmit of a reply to be
done on one CPU. Silo'ing is good for performance and parallelism.
This can sometimes be relaxed in situations where CPUs share a cache
so crossing CPUs is not not costly.
Yes. We would like to get this behavior even without binding the app
thread to a CPU.
At least for passive connections, we already have all the support in the
kernel so that you can have one thread per NIC queue, dealing with
sockets that have incoming packets all received on one NIC RX queue.
(And of course all TX packets will use the symmetric TX queue)
SO_REUSEPORT plus appropriate BPF filter can achieve that.
Say you have 32 queues, 32 cpus.
Simply use 32 listeners, 32 threads (or 32 pools of threads)
Yes. This will work if each thread is pinned to a core associated with the
RX interrupt.
It may not be possible to pin the threads to a core.
Instead we want to associate a thread to a queue and do all the RX and TX
completion
of a queue in the same thread context via busy polling.
When that happens it's possible for RX to be done on the completely
wrong CPU which we know is suboptimal. However, this shouldn't
negatively affect TX side since XPS will just use the queue
appropriate for running CPU. Like Eric said, this is really a receive
problem more than a transmit problem. Keeping them as independent
paths seems to be a good approach.
We are noticing that when majority of packets are received via busy
polling, it
should not be an issue if RX processing is handled by a thread running
on a core
that is different from the core that is associated with the RX
interrupt. Also, as
the TX completions on the associated TX queue are processed along with
the RX
processing via busy polling, we would like the Transmits also to happen
in the same
thread context.
Would appreciate any feedback or thoughts on optional configuration to
enable selection
of TX queue based on the RX queue of a flow.
Thanks
Sridhar
