Re: [dpdk-dev] DPDK Latency Issue

[Jun Han <junhanece@gmail.com>]

Thanks a lot Jeff for your detailed explanation. I still have open question
left. I would be grateful if someone would share their insight on it.

I have performed experiments to vary both the MAX_BURST_SIZE (originally
set as 32) and BURST_TX_DRAIN_US (originally set as 100 usec) in l3fwd
main.c.

While I vary the MAX_BURST_SIZE (1, 8, 16, 32, 64, and 128) and fix
BURST_TX_DRAIN_US=100 usec, I see a low average latency when sending a
burst of packets less than or equal to the MAX_BURST_SIZE.
For example, when MAX_BURST_SIZE is 32, if I send a burst of 32 packets or
less, then I get around 10 usec of latency. When it goes over it, it starts
to get higher average latency, which make total sense.

My main question are the following. When I start sending continuous packet
at a rate of 14.88 Mpps for 64B packets, it shows consistently receiving an
average latency of 150 usec, no matter what MAX_BURST_SIZE. My guess is
that the latency should be bounded by BURST_TX_DRAIN_US, which is fixed at
100 usec. Would you share your thought on this issue please?

Thanks,
Jun


On Thu, May 22, 2014 at 7:06 PM, Shaw, Jeffrey B
<jeffrey.b.shaw@intel.com>wrote:

> Hello,
>
> > I measured a roundtrip latency (using Spirent traffic generator) of
> sending 64B packets over a 10GbE to DPDK, and DPDK does nothing but simply
> forward back to the incoming port (l3fwd without any lookup code, i.e.,
> dstport = port_id).
> > However, to my surprise, the average latency was around 150 usec. (The
> packet drop rate was only 0.001%, i.e., 283 packets/sec dropped) Another
> test I did was to measure the latency due to sending only a single 64B
> packet, and the latency I measured is ranging anywhere from 40 usec to 100
> usec.
>
> 40-100usec seems very high.
> The l3fwd application does some internal buffering before transmitting the
> packets.  It buffers either 32 packets, or waits up to 100us (hash-defined
> as BURST_TX_DRAIN_US), whichever comes first.
> Try either removing this timeout, or sending a burst of 32 packets at
> time.  Or you could try with testpmd, which should have reasonably low
> latency out of the box.
>
> There is also a section in the Release Notes (8.6 How can I tune my
> network application to achieve lower latency?) which provides some pointers
> for getting lower latency if you are willing to give up top-rate throughput.
>
> Thanks,
> Jeff
>