Hi all, I realized I made a mistake on my previous post. Please note the changes below.
"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 greater than the MAX_BURST_SIZE. For example, when MAX_BURST_SIZE is 32, if I send a burst of 32 packets or larger, then I get around 10 usec of latency. When the burst size is less than 32, I see higher average latency, which make total sense." On Mon, May 26, 2014 at 9:39 PM, Jun Han <junhanece at gmail.com> wrote: > 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 at 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 >> > >
