Re: [PATCH] Add prefetches in net/ipv4/route.c

[Rick Jones <rick.jones2@hp.com>]

> oprofile on AMD64 can gather lots of data, DATA_CACHE_MISSES for example...
> 
> But I think I know what happens...
> 
> nm -v /usr/src/linux/vmlinux | grep -5 rt_cache_stat
> 
> ffffffff804c6a80 b rover.5
> ffffffff804c6a88 b last_gc.2
> ffffffff804c6a90 b rover.3
> ffffffff804c6a94 b equilibrium.4
> ffffffff804c6a98 b ip_fallback_id.7
> ffffffff804c6aa0 B rt_cache_stat
> ffffffff804c6aa8 b ip_rt_max_size
> ffffffff804c6aac b ip_rt_debug
> ffffffff804c6ab0 b rt_deadline
> 
> So rt_cache_stat (which is a read only pointer) is in the middle of a 
> hot cache line (some parts of it are written over and over), that 
> probably ping pong between CPUS.
> 
> Time to provide a patch to carefully place all the static data from 
> net/ipv4/route.c into 2 parts : mostly readonly, and others... :)

Which of course begs the question - what cache line size should be ass-u-me-d 
when blocking these things?  I'll put-forth 128 bytes.

rick jones