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[lwip-users] TCP tuning to increase throughput

From: Mandeep Sandhu
Subject: [lwip-users] TCP tuning to increase throughput
Date: Thu, 1 Oct 2009 19:11:38 +0530

Hi All,

I have small HTTP proxy server implemented using the lwIP sequential API.

When a client connects to my webserver, the webserver proxies the
request to another server sitting somewhere on the internet. The
response of this server is then written back as-is to the client
connection. All this while the client is waiting on the open socket

Roughly this is what I do:

netconn_write( proxy_conn, <data to be proxied>, <length>, NETCONN_COPY);

   while ((proxy_resp = netconn_recv(proxy_conn)) != NULL ) {
       do {
           netbuf_data(proxy_resp, &data, &len);
           //Write response back to the client
           err = netconn_write(client_conn, data, len, NETCONN_COPY);

           if (err != ERR_OK) {
               goto error; // some error handler
       } while (netbuf_next(proxy_resp) >= 0);

(this code is based on the TCP echo example)

When I put debug's in the inner loop, I saw that the other end was
sending data in chunks of 128 bytes. Even after I increase the MSS to
256 bytes. I was using wget on the client side, and it was reporting a
speed of ~150 KB/s, where as a direct get of the same data from the
server directly would be ~ 4 MB/s.

Also, I saw that if the data was large (as in a large page ~200 KB), I
was getting mem alloc error:

memp_malloc: out of memory in pool TCP_PCB

I've increased the pool size, but that seems to only mitigate the
problem temporarily. Now the mem alloc error happens at slightly
larger values (~400KB).

I'm not very conversant with TCP, so any tips on how to increase
throughput to the max and avoid the mem issue? The PBUF buffer size is
set to 182 bytes.


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