Re: rx_copybreak value for non-i386 architectures

[David S. Miller]

Jun Sun writes:
 > I notice that many net drivers set rx_copybreak to 1518 (the max packet size)
 > for non-i386 architectures.  Once I thought I understood it and it seems
 > related to cache line alignment.  However, I am not sure exactly about the
 > reason now.  Can someone enlighten me a little bit?

Most non-x86 architectures take a large hit for unaligned accesses.
If the ethernet chip cannot land the beginning of the packet at an
arbitrary byte offset (a modulo 2 offset for ethernet is needed for an
aligned IP header) then the rx_copybreak is set to the ethernet MTU
so that all packets get copied into new buffers where they can have
their header aligned.

Later,
David S. Miller
[e-mail]