[dpdk-dev] KNI performance

[Jay Rolette]

On Fri, Jun 5, 2015 at 10:13 AM, Marc Sune <marc.sune at bisdn.de> wrote:

>
>
> On 05/06/15 17:06, Jay Rolette wrote:
>
>> The past few days I've been trying to chase down why operations over KNI
>> are so bloody slow. To give you an idea how bad it is, we did a simple
>> test
>> over an NFS mount:
>>
>> # Mount over a non-KNI interface (eth0 on vanilla Ubuntu 14.04 LTS)
>> $ time $(ls -last -R /mnt/sfs2008 > /dev/null)
>> real    11m58.224s
>> user    0m10.758s
>> sys     0m25.050s
>>
>> # Reboot to make sure NFS cache is cleared and mount over a KNI interface
>> $ time $(ls -last -R /mnt/sfs2008 > /dev/null)
>> real    87m36.295s
>> user    0m14.552s
>> sys     0m25.949s
>>
>> Packet captures showed a pretty consistent ~4ms delay. Get a READDIRPLUS
>> reply from NFS server and the TCP stack on the DPDK/KNI system took about
>> 4ms to ACK the reply. It isn't just on ACK packets either. If there was no
>> ACK required, there would be a 4ms delay before the next call was sent
>> (ACCESS, LOOKUP, another READDIR, etc.).
>>
>> This is running on top of a real DPDK app, so there are various queues and
>> ring-buffers in the path between KNI and the wire, so I started there.
>> Long
>> story short, worst case, those could only inject ~120us of latency into
>> the
>> path.
>>
>> Next stop was KNI itself. Ignoring a few minor optos I found, nothing in
>> the code looked like it could account for 4ms of latency. That wasn't
>> quite
>> right though...
>>
>> Here's the code for the processing loop in kni_thread_single():
>>
>>          while (!kthread_should_stop()) {
>>                  down_read(&kni_list_lock);
>>                  for (j = 0; j < KNI_RX_LOOP_NUM; j++) {
>>                          list_for_each_entry(dev, &kni_list_head, list) {
>> #ifdef RTE_KNI_VHOST
>>                                  kni_chk_vhost_rx(dev);
>> #else
>>                                  kni_net_rx(dev);
>> #endif
>>                                  kni_net_poll_resp(dev);
>>                          }
>>                  }
>>                  up_read(&kni_list_lock);
>>                  /* reschedule out for a while */
>>                  schedule_timeout_interruptible(usecs_to_jiffies( \
>>                                  KNI_KTHREAD_RESCHEDULE_INTERVAL));
>>
>> Turns out the 4ms delay is due to the schedule_timeout() call. The code
>> specifies a 5us sleep, but the call only guarantees a sleep of *at least*
>> the time specified.
>>
>> The resolution of the sleep is controlled by the timer interrupt rate. If
>> you are using a kernel from one of the usual Linux distros, HZ = 250 on
>> x86. That works out nicely to a 4ms period. The KNI kernel thread was
>> going
>> to sleep and frequently not getting woken up for nearly 4ms.
>>
>> We rebuilt the kernel with HZ = 1000 and things improved considerably:
>>
>> # Mount over a KNI interface, HZ=1000
>> $ time $(ls -last -R /mnt/sfs2008 > /dev/null)
>>
>> real    21m8.478s
>> user    0m13.824s
>> sys     0m18.113s
>>
>> Still not where I'd like to get it, but much, much better.
>>
>> Hopefully my pain is your gain and this helps other KNI users.
>>
>

> Jay,
>
> If you set CONFIG_RTE_KNI_PREEMPT_DEFAULT to 'n' you should see a reduced
> latency and delay since there is no preemption (though sacrifices 1 CPU for
> the kni kmod):
>
> http://patchwork.dpdk.org/dev/patchwork/patch/3304/
>
> However, KNI is still pretty slow. Even considering that there will always
> be at least 1 copy involved, I still think is too slow. I didn't had time
> to look closer yet.
>
> Marc
>

Hi Marc,

Thanks for the pointer to the patch. I did something similar as a test
before we started mucking with rebuilding the kernel. Skipping the call to
put the KNI kernel thread to sleep improved performance and reduced
latency, but oddly enough, it wasn't as fast for the end-app as the HZ=1000
change.

Here's what I got on that test:

# Mount over "no-sleep" KNI
$ time $(ls -last -R /mnt/sfs2008 > /dev/null)

real    37m49.004s
user    0m23.274s
sys     0m9.010s

Jay