anyone understand torvald's critique of freebsd?

Chuck Swiger cswiger at
Sun Apr 23 02:50:29 UTC 2006

Ted Mittelstaedt wrote:
> Then why does Linus think manipulating the VM page table mappings
> is bad?  That is, why does he -really- think it's bad, not the
> publically-given reason?  Is it because Linux is extrordinairly
> inefficient in page table mappings due to some structural decision
> that Linus made that cannot be reversed now, that it could never
> be any good at it?  Or is there some other reason?

I can't speak with certainty as to what someone else might think; no doubt Linus 
is entirely capable of explaining his own position should you wish to inquire, 
however....  :-)

I think Linus doesn't care much for Zero-copy sockets because for the common 
case of 1500/1504-byte MTU, you end up wasting at least 60% of a 4096-byte page 
for each packet, and maybe ?three? times that much if your hardware splits the 
packet into separate pages for the mbuf header, the packet headers, and the 
packet data.

I think FreeBSD doesn't enable Zero-copy sockets by default because those are 
legitimate criticisms, but there is some interest and potential for benefits 
from them, at least for some other circumstances like jumbo ethernet frames.


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