TCP issue with large packet under RTEMS

[Eric Norum]

On Feb 9, 2011, at 10:06 AM, João Rasta wrote:

> Hi all,
> 
> I implemented Yann's sugestion and it seemed to take a lot of time to return. So i profiled the recv() routine with the MSG_WAITALL flag and returned 5.70 seconds! With Eric's sugestion, the whole data is read in 0.60 seconds. This is odd, i would expect that, since apparently there are more system calls with Eric's method, recv() would be faster.
> 
> Any idea why this is happening?


> 
> Moreover, i'm transferring 436160*8 = 3489280 bits, so i would expect the total data read to happen in 3489280/10485760 = 0.33 seconds, given that i have a 10Mbit ethernet configuration. I don't think context switching for a couple of system calls is that heavy, what can be causing the 0.27 second overhead with Eric's solution?

1) Can the sender pump the data out at full speed?
2) TCP acknowledgements have to be formed and transmitted.
3) There's overhead in computing the IP and TCP checksums for each packet in and out.

-- 
Eric Norum
wenorum at lbl.gov