Re: [lwip-devel] How to estimate heap memory usage (TCP case)

[narke]

On 13 March 2012 00:48, Simon Goldschmidt <address@hidden> wrote:
>
> narke <address@hidden> wrote:
>> > My TCP_MSS is 1388 and TCP_SND_BUF is 4 * TCP_MSS.
>> Does anyone have an idea?  Thank in advance.
>
> Keep in mind that to the MSS, various protocol headers plus struct pbuf is 
> also allocated from the heap. That alone would require at least ~5.9KByte 
> heap.

Thanks for your reply Simon.  Did you get the number 5.9KB from my
TCP_MSS value?  How did you calculate this magic number?


> Without using TCP_OVERSIZE, every write gets its own pbuf, so if you call 
> tcp_write with a size != MSS, you risk losing another 72 byte for every call 
> of tcp_write that doesn't start a new segment. Added to that, the heap needs 
> its own organization structures, so you cannot allocate the full byte count 
> configured with MEM_SIZE.

I am not quite understand the TCP_OVERSIZE.  My current setting is
TCP_OVERSIZE = TCP_MSS. What does it mean?  And, with a higher or
lower TCP_OVERSIZE value, will lwIP use more or less momory?  What's
the idea TCP_OVERSIZE setting to archive minimum memory use without
worrying about my segment get fragmented?


>
> Other than that, have a look at the sources to see who else calls 
> mem_malloc() (e.g. DHCP, AutoIP, slipif, ppp, your own code?). The stats also 
> help you there. Ideally, before calling tcp_write(), the heap should not be 
> used (when DHCP and AutoIP are correctly initialized and not using slip/ppp).

Yes, this I understand. I will separably count my other mem_alloc() uses.


>
> Simon
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narke
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