[dpdk-dev] Mbuf memory alignment constraints for (micro)architectures

[Jerin Jacob Kollanukkaran]

CC:  Arch and platform maintainers

While reviewing the mempool objection allocation requirements in the code, 

A) it's found that in the default case, mempool objects have padding 
in the object trailer to have start addresses of objects among the different channels,
to enable equally load on the DRAM channel to have better performance

# More documentation is here
https://doc.dpdk.org/guides/prog_guide/mempool_lib.html
in section 8.3. Memory Alignment Constraints

B) The optimize_object_size() does the channel distribution requirement
by the following formula

        new_obj_size = (obj_size + RTE_MEMPOOL_ALIGN_MASK) / RTE_MEMPOOL_ALIGN;
        while (get_gcd(new_obj_size, nrank * nchan) != 1)
               new_obj_size++;


C) The formula mentioned in the (B) is NOT generic. At least of the octeontx2 SoC
The memory/DDR controller works in different way. Where by:
# It does XOR operation of some  of physical address lines(not the user space VA address)
to compute the hash and that the function defines the actual channel.

The XOR(kind of CRC) scheme is useful because there is natural  channel distribution
based on the address i.e No need to have padding to waste memory

So, in short the padding scheme does not need for some SoC. I trying to send the patch
to fix it. So the questions is,

# Is PPC and other ARM SoC has formula (B)  to compute DRAM channel distribution ? or
Is it specific to x86? That would define where the hooks needs to added to have proper fix.