[netperf-talk] Confusing result of the throughput in wireless network with different UDP packet length.

[Peng Wang]

Hi,

I did the benchmark on wired LAN, and it shows the same characteristic as in
the wireless LAN. The performance is shown in the attachment.

I did not add CPU utilization options.

The send throughput and the receive throughput is roughly the same. If I
plot them in one figure, it will be hard to distinguish them.

Peng

On Sat, Feb 21, 2009 at 2:19 AM, Rick Jones <rick.jones2 at hp.com> wrote:

> Andrew Gallatin wrote:
>
>> Peng Wang wrote:
>>
>>
>>  The benchmark platform I use contains two buffalo wireless g router. One
>>> router is the sender, and one is the receiver. The operating system in
>>> them is Linux with 2.4.7 version kernel.
>>>
>>> The result of the benchmark confuses me. From the following figure we
>>> can see that the throughput is highest when the UDP packet size is about
>>> 20k bytes which is much bigger than the MTU. I guess it is because of
>>> the buffer in the system, but I am not quite sure. Do you see it before?
>>> Could you explain it?
>>>
>>
>>
>> I'm not at all familiar with performance characteristics of
>> wireless networks, but on fast (10GbE) wired networks I
>> sometimes see similar behavior when the host has expensive
>> system calls.   The larger the message size, then the more data
>> is written on the socket using a single system call.
>>
>
> It doesn't necessarily have to be an "expensive" system call either.  When
> sending 1000 bytes at a time that is 20 system calls and 20 calls through
> the UDP code into IP and then 20 trips through the driver and onto the
> "wire."  With 20K bytes at a time, that is one system call, one call through
> UDP into IP and then 20000/MTU trips through the driver.  If MTU is larger
> than 1000 bytes then that will be rather less than 20 trips through the
> driver - perhaps only about 15 or 16.
>
>  So, a few questions:
>>
>> 1) Does a test via a 100Mb or 1Gb connection between the routers show
>>   the same characteristic?   How about a local (loopback) test
>>   from a machine to itself?
>>
>> 2) Have you measured the system call overhead using something like
>>   lmbench?
>>
>
> I would ask:
>
> 3) Where are the netperf-measured service demands? (ie add CPU utilization
> options to your test)
>
> 4) Does the graph show reported send throughput, or actual receive
> throughput?
>
> 5) Was send throughput == receive throughput?
>
> rick jones
>
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