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

[Eric Dumazet <dada1@xxxxxxxxxxxxx>]

David S. Miller a écrit :
> From: Eric Dumazet <dada1@xxxxxxxxxxxxx>
> Date: Thu, 28 Jul 2005 22:56:32 +0200
>
>
> > But in case of DDOS, prefetches are a win.
>
>
> Numbers please, I'm simply curious.

I have no profiling info for this exact patch, I'm sorry David.

On a dual opteron machine, this thing from ip_route_input() is very expensive :

 RT_CACHE_STAT_INC(in_hlist_search);

ip_route_input() use a total of 3.4563 % of one cpu, but this 'increment' takes 
1.20 % !!!

0.0047   mov    2123529(%rip),%rax        # ffffffff804b4a60 <rt_cache_stat>
1.1898   not    %rax
         mov    %gs:0x34,%edx
0.0042   movslq %edx,%rdx
         mov    (%rax,%rdx,8),%rax
         incl   0x38(%rax)

Sometime I wonder if oprofile can be trusted :(

Maybe we should increment a counter on the stack and do a final
   if (counter != 0)
       RT_CACHE_STAT_ADD(in_hlist_search, counter);

Eric