Christiaan Simons wrote:
Let's quote (and learn from) McKusick and Karels (from the BSD kernel
malloc paper):
"A generalized memory allocator is needed to reduce the complexity of
writing code inside the kernel.
Rather than providing many semi-specialized ways of allocating memory, the
kernel should provide a single
general purpose allocator. With only a single interface, programmers do not
need to figure out the most
appropriate way to allocate memory. If a good general purpose allocator is
available, it helps avoid the syndrome
of creating yet another special purpose allocator."
Search'n'replace "kernel" with "lwip" and you get my point.
Too many hackers here, with too many good ideas for their own lwIP setup,
I'm afraid.
Generalised memory allocators only work sensibly with enough spare space
left over that fragmentation is an infrequent occurence. Unlike the BSD
kernel, lwIP users are unlikely to have the freedom of lots of space for
the heap - quite the opposite. Maybe that's not too bad for PBUF_RAM
allocations - apps can possibly handle not being able to send data for a
little while (in the hope that more space will free itself later), but for,
failures in, say, renewing a DHCP lease, you could lose your interface. And
that could happen just because the SNMP agent was feeling a bit hungry with
its own stuff. If there are more users, the problem compounds - a low
priority subsystem can deprive a high priority subsystem of memory, either
directly due to using it, or indirectly due to fragmentation.
Also in the above quote, the emphasis is on "good". The mem_malloc
allocator is not "good". It is simple. This is only right in the context of
lwIP. But it does not resist fragmentation in the way most other memory
allocators endeavour to do (however it does avoid the larger code and space
penalty).
I would favor solving this problem by 'fixing' savannah bug #3031
submitted by Leon Woestenberg, which basically proposes getting rid
of PBUF_RAM and using pools instead.
Different issue. #3031 is nice to have, and may solve some other odd
corners.
Doesn't solve fragmentation.
Since pbufs can be chained, I'm not sure what you mean?
I'm open for fragmentation test results and test cases for our
mem_malloc() though...
It entirely depends on the size of MEM_SIZE, compared with the allocation
pressure.
Jifl
--
eCosCentric http://www.eCosCentric.com/ The eCos and RedBoot experts
------["The best things in life aren't things."]------ Opinions==mine
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