Re: [lwip-users] memory pools question

[Simon Goldschmidt]

jblackarty <address@hidden> wrote:
> It's totally unclear are mem and memp independent mechanisms, or mem
> utilizes memp, or memp uses mem, or they are cross-references each
> other ? Custom pools are part of pools (memp), or pools are part of
> custom pools ?
> 
> There are complicated set of options:

Yes, that's the nature of lwIP being adjustable to the needs of your (possibly 
small or otherwise restricted) target hardware.

I'll start explaining the 'standard' options. Basically, lwIP has 2 strategies 
for memory allocation: pools and a heap. The heap is mainly only used for 
outgoing PBUFs allocated with the type PBUF_RAM. Then there is one memory pool 
per structure type:

> MEMP_NUM_*,

These control the amount of structures of that type (e.g. MEMP_NUM_UDP_PCB sets 
the amount of available udp connections).

> MEM_SIZE,

This is the size of the heap (used for PBUF_RAM allocation).

> PBUF_POOL_SIZE.

As it says in opt.h (did you read that file???), the number of pbufs in the 
pbuf pool (where PBUF_POOL_BUFSIZE controls the size of these pbufs). The pbufs 
from the pool are of type PBUF_POOL.


Nnow to the more advanced options:

> MEM_LIBC_MALLOC,

Some people don't care about the danger of heap fragmentation and want to use 
the C library's malloc/free functions instead of the lwIP-internal memory 
allocation (if only to save code because the C library's functions are included 
in their binary, anyway). Set this to 1 to do so.

> MEMP_MEM_MALLOC,

This is mainly only usefule with the previous option: when set to 1, memp pool 
allocation uses the heap allocation functions (this was introduced to let memp 
allocations be using the C library's malloc.

> MEM_USE_POOLS,

This is the other way round than the previous: when you're scared of heap 
fragmentation, let mem_malloc() allocate from specific pools. The size of these 
pools (and the number of entries in them) needs to be configured *and* you have 
to enable the next option.

> MEMP_USE_CUSTOM_POOLS,

This enables custom memp pools. They are needed for the previous option but you 
can also define more pools, for example to put your semaphores or mboxes into.

> How they related to each other ?
> For MEMP_MEM_MALLOC a comment says "Use mem_malloc/mem_free instead of
> the lwip pool allocator.". But mem_malloc is part of pool mechanisms.

No, memp_malloc (not the 'p') allocates from pools, mem_malloc allocates from 
the heap!

> How it can be disabled ? Also "Especially useful with MEM_LIBC_MALLOC
> ..." confuses. Isn't libc allocator completely disables pool mechanism
> ? These options must exclude each other.

No, why should they???

> Is it possible disable allocating standard structs in pool at all ?

Yes, see above

> Does it makes sense configure MEMP_NUM_* options if MEM_LIBC_MALLOC
> enabled ?

No.

> MEM_SIZE includes only pool of standart structures or also pbufs pool ?

None of them. It controls the heap size and has nothing todo with pools.

> Questions may be continued indefinitely.

Is that so?

> lwip source code doesn't
> describe architecture because it's heavily stuffed with complex macro
> conditions
> nested each in other.

lwIP relies on people's contribution, so if there's anything missing for a 
beginner, please don't hesitate to contribute. Writing something on the wiki is 
the easiest part to help future beginners getting started faster.

> "Custom_memory_pools" article on wiki doesn't
> help too because it doesn't describe whole memory mechanism completely.

Right, it only describes how to enable custom memory pools. I guess it's rather 
targeted at people already using lwIP as a guideline of how to get custom pools 
running.

> I will appreciate if someone throw light upon this subject.

Hope I did. :-)

Simon
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