lkml.org 
[lkml]   [2001]   [Feb]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: rx_copybreak value for non-i386 architectures
Date
From
> 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?

A lot of pci net cards can only start packets on a 4 byte boundary. A lot
of CPU's need 4 byte aligned read/writes for performance. The ethernet header
is howerver 14 bytes long.

For CPU's with poor unaligned performance it turns out better to copy or
copy/checksum the data so the IP/TCP headers are aligned. If the card can
hit 16bit boundaries then you will see card drivers doing


alloc_skb(blah)

skb_reserve(skb, 2);

to align the header of he buffer so that the IP data is aligned


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:22    [W:1.872 / U:0.016 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site