[lwip-devel] [task #7040] Work on tcp_enqueue

[Iordan Neshev]

Follow-up Comment #13, task #7040 (project lwip):

I can confirm that by sending 
MSS-sized blocks of data I gained huge
performance boost. My application
generates small chunks of data (20-30)
bytes, which I cached in a MSS-sized
application buffer (not a pbuf).
I'm sending files over GPRS connection
and I am saturating the link, so I
do not need zero-copy.

At first I accomplished transfer rate of
at most 200-300 bytes/sec. Now I'm sending
3-3.5 KBytes/sec :)
At application level I use something like this:
int my_enque(*pcb, u8_t *src, u16_t len, u8_t push_flag);

Most of the times it is called with len = 20-30, push_flag= FALSE.
These bytes are accumulated in the buffer and
when it reaches MSS, I call tcp_write() and tcp_output()
to send it (note: I do not call directly tcp_enque()).

When I want to send the data eventhough the temp buffer
is not filled up to MSS, I pass push_flag = TRUE;
This is used when I'm enqueing the last chunk just
before I close the file and also when I send commands
to the FTP server over the control connection.

Maybe it wold be best if I add the small amounts of bytes
directly to the last pbuf. Then I should not
need the temp buffer any more but I did not want to
touch the core.

The price I have to pay is the extra MSS-sized buffer
and very small amount of code which is not problem
in my case. This approach does not support zero-copy,
but even if it did I would not be able to benefit from
it since the GPRS net is extremely slow and the RTT is
huge compared to Ethernet (an ACK comes afte [500ms - 30-40 sec]!!!

To me integrating somewhere in
the stack something like this:
 1.   if( len > pcb->mss )
       len -= len % pcb->mss;
 2.   Add len bytes to current pbuf
 3.   ...

looks pretty good. Everyone should benefit
from this change. 

I'm a bit conservative about changing the API. 
Also, I don't think that tcp_sndbuf(pcb) should 
return a multiple of MSS. 

We could add new function which does what my_enque()
does. It should be 
implemented in different ways with #ifdefs according to user
needs. So everyone would be free to chose which
approach is best for his application and if he 
wants to use my_enque at all because this works for me,
but for most of you it will not.

Greetings,
Iordan


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