Re: [lwip-users] Sizes

[Jeff Barber]

On Wed, May 27, 2009 at 9:47 AM, John Kennedy
<address@hidden> wrote:
> I’m using Lwip with sockets on MicroBlaze.  I have an MTU of 1500.  I want
> to maximize the Ethernet TX/RX data rates (bits/sec) in my embedded system.
> Memory is not an issue (512Mbytes avail).
>
> How does one determine the values for:
>
> ·         MEM_SIZE
>
> ·         PBUF_POOL_SIZE
>
> ·         PBUF_POOL_BUFSIZE
>
> ·         TCP_MSS
>
> ·         TCP_SND_BUF
>
> ·         TCP_SND_QUEUELEN
>
>  to maximize the Ethernet TX/RX data rates?

I wish there were a better source of information on this as well.  I
spent quite a while trolling through the source trying to understand
what all the different settings are for.  Some I figured out, others
I'm still not sure of.

Like you, my environment is not memory-constrained.  Furthermore, I am
using my own malloc implementation rather than the lwip built-in
mem_malloc and memp_malloc (this is partially because I trust my own
allocation functions and partially because I had a hard time
understanding how to configure the lwIP implementation :-).  As best I
can tell, when using one's own allocation routines, several of the
memory-related defines are "don't care".  So MEM_SIZE and all the
MEMP_* and PBUF_POOL_SIZE are (I think) not actually used -- beyond
the sanity tests in lwip/src/core/init.c.

Caveat: my port is very young -- my driver came up for the first time
a couple of days ago, and it hasn't yet done much more than answer
ping packets and ARP (no TCP yet).  FWIW, though, here's my lwipopts.h
(for everything else, I accept the default values from <lwip/opt.h>):

#define NO_SYS                      1
#define MEM_LIBC_MALLOC             1
#define MEMP_MEM_MALLOC             1
#define MEM_ALIGNMENT               4
#define MEM_SIZE                    (4 * 1024 * 1024)
#define MEMP_NUM_PBUF               1024
#define MEMP_NUM_UDP_PCB            20
#define MEMP_NUM_TCP_PCB            20
#define MEMP_NUM_TCP_PCB_LISTEN     16
#define MEMP_NUM_TCP_SEG            128
#define MEMP_NUM_REASSDATA          32
#define MEMP_NUM_ARP_QUEUE          10
#define PBUF_POOL_SIZE              512
#define LWIP_ARP                    1
#define IP_REASS_MAX_PBUFS          64
#define IP_FRAG_USES_STATIC_BUF     0
#define IP_DEFAULT_TTL              255
#define IP_SOF_BROADCAST            1
#define IP_SOF_BROADCAST_RECV       1
#define LWIP_ICMP                   1
#define LWIP_BROADCAST_PING         1
#define LWIP_MULTICAST_PING         1
#define LWIP_RAW                    0
#define TCP_WND                     (4 * TCP_MSS)
#define TCP_MSS                     1460
#define TCP_SND_BUF                 (8 * TCP_MSS)
#define TCP_LISTEN_BACKLOG          1
#define LWIP_NETIF_STATUS_CALLBACK  1
#define LWIP_NETIF_LINK_CALLBACK    1
#define LWIP_NETIF_HWADDRHINT       1
#define LWIP_NETCONN                0
#define LWIP_SOCKET                 0
#define LWIP_STATS_DISPLAY          1
#define ETHARP_TRUST_IP_MAC         0

#define mem_init()
#define mem_free                    my_free
#define mem_malloc                  my_malloc
#define mem_calloc(c, n)            my_zalloc((c) * (n))
#define mem_realloc                 my_realloc


For the MSS value, specifically, I worked backwards from the standard
max ethernet frame size: 1514.  Subtract 14 bytes for layer 2 header;
subtract 20 bytes for IP header and another 20 for TCP header; that
leaves 1460.  There are a couple of ports in contrib/ that define it
to 1476 but I couldn't figure out how they came up with that.  I will
have a direct ethernet connection to my peer so don't need to worry
about any additional fragmentation.

BTW, any comments on my settings would be greatly appreciated.

Hope this helps.
Jeff