Re: A question regarding MTU: how it can effect TCP performance + other queries
- From: "Arsalan Ahmad" <arsal__@xxxxxxxxxxx>
- Date: Thu, 30 Nov 2006 10:58:41 +0100
Hi,
Just to add, I am sending one IP packet that I receive over virtual
interface to the physical one using socket. Now depending on its size, this
packet (packet recieved on virtual interface) could break into two packets
in case of physical NIC depending on the MTU. For UDP test, Iperf is using
datagram size of 1470 bytes and UDP buffer size of 8KB.
So any idea why UDP throughput is reported by Iperf to be greater in case of
virtual NIC as compared to physical NIC?
Thanks,
Arsalan
"Pankaj Garg" <pankajg@xxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:Pine.WNT.4.64.0611292332321.2364@xxxxxxxxxxxxxxxxx
*Removing multiple newsgroups as cross posting usually generates tons of
extra traffic.*
What packet size IO are you doing in case of TCP? Can you check if your
physical NIC has TCP large send offload enabled?
The reason for the large difference in TCP performance may be due to the
offload happening in case of physical NIC.
I can't think of anything for the UDP case however, that just seems
strange to me. You probably need to dig deeper and tell us what exactly is
happening here. How many packets are being sent, what was the size of
these packets? Are you grouping multiple UDP packets in one TCP packet?
--
Pankaj Garg
2006-11-29 at 11:32pm
http://www.intellectualheaven.com
On Wed, 29 Nov 2006, Steve Dispensa wrote:
I'm sorry, I misread the UDP part of your question. You're right,
something seems off there. Are you buffering somewhere somehow? Are some
packets hitting the bit bucket after being delivered to the virtual NIC,
perhaps?
As for TCP case, you said:
What I found that for TCP, throughput that I am getting on physical
interface is around 900 Mbps and on virtual interface i am getting
around 90 Mbps (only 10% of the physcial one). All the tests were
performed with Iperf with 15 parallel streams.
and
So, for TCP packets over virtual interface, I am sending a TCP packet
encapsulated within another TCP packet when passed to physical
interface, while for UDP I am sending UDP packet encapsulated within
TCP packet when passed to physical interface.
Nagle would matter for UDP packets encapsulated within TCP packets,
depending (a great deal) on the characteristics of the traffic you are
testing with. Nagle's has an adverse affect on transmission speed,
because it introduces an artificial queuing delay, regardless of what
it's carrying. I suggested turning on NODELAY for the "outside" TCP
connection, not (necessarily) the "inside" connection.
At any rate, your results do seem strange to me; not a whole lot else
comes to mind though. You might want to instrument your miniport and
track where the packets spend their time between receipt at your send
handler and final send() to the other system.
Good luck.
-sd
On 2006-11-29 07:37:48 -0600, "Arsalan Ahmad" <arsal__@xxxxxxxxxxx> said:
My question was that Iperf is showing less UDP throughput over physical
interface (around 550Mbps for 15 parallel streams) while for virtual NIC
it is showing greater throughput (around 1Gbps for 15 parallel streams).
So why is this? because what I thought that UDP thoughput should be high
in case of physical interface which is not I am getting. And how
Nagle's. Turn on NODELAY for TCP can effect UDP throughput performance?
Thanks,
Arsalan
"Steve Dispensa" <dispensa@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
message
news:2006112900064250073-dispensa@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On 2006-11-28 04:03:23 -0600, "Arsalan Ahmad" <arsal__@xxxxxxxxxxx>
said:
Hello all,
I have been doing some throughput tests with Iperf for my VNIC and
physical NIC. I connect two P-4 3.4 GHz computers with 1Gbps NIC cards
together via cross-over cable and then performed my experiments.
What I found that for TCP, throughput that I am getting on physical
interface is around 900 Mbps and on virtual interface i am getting
around 90 Mbps (only 10% of the physcial one). All the tests were
performed with Iperf with 15 parallel streams.
However, for UDP its totally different. With Iperf with 15 parallel
streams, I am getting only around 500 Mbps throughput on physical
interface while approximately 1000Mbps (1Gbps) on virtual interface. I
am unable to figure out why am I getting this strange behaviour. Can
you guess?
Yes, I can guess; are you tunneling the packets? Do you have Nagle's
turned on? Did you do the math on the bandwidth-delay product and make
your TCP window was set appropriately?
My virtual NIC pass all the packets to a user-level application which
then send all the IP packets of virtual interface to user-level
application running on the other computer. The communication between
the two user-level application is via TCP.
Windows is sensitive to the exact structure of your sockets code. To
get the best possible performance from the OS, you may want to look
into using I/O Completion for your tunnel code on both ends, and make
sure you keep enough buffers pending such that the protocol is never
waiting on you.
So, for TCP packets over virtual interface, I am sending a TCP packet
encapsulated within another TCP packet when passed to physical
interface, while for UDP I am sending UDP packet encapsulated within
TCP packet when passed to physical interface.
Given this, I'm voting for Nagle's. Turn on NODELAY and re-test.
-sd
.
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