mbuf doubts

[Giovanni P. Tirloni]

Hi,

 I have been reading the D&I chapter about IPC, specially the mbuf
 section and *many* doubts were raised by my mind. I sending them here
 in the hope that someone can clarify some bits for me so I can proceed.

 Reading sys/param.h and sys/mbuf.h I came to the conclusion that there
 are four types of mbufs regarding it's allocation of memory for data.
 Is the listing below correct? 

 struct mbuf *m;

  1. Normal mbuf using m->M_databuf
  2. Normal mbuf with external storage (cluster?) in m->m_hdr->mh_data
  3. Header mbuf using m->m_pktdat;
  4. Header mbuf with ext. storage (cluster?) in m->m_ext->ext_buf

 Other questions:

  1. When using ext. storage is the space allocated by M_databuf wasted?
  
  2. How the system decides 256 bytes for each mbuf isn't enough and it
     needs a mbuf cluster? Isn't chaining useful there?

  3. How does changing MSIZE affects the whole thing?
  
  4. What about MCLBYTES?
  
 Sorry to make so many questions at once but I find it very interesting
 and I'm really willing to learn how the building blocks of the network
 stack work. Perhaps my questions are out of reality.. it's risk.

 Thanks,

 --
 Giovanni P. Tirloni <gpt at tirloni.org>
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