route/arp lifetime (Re: it's the output, not ack coalescing (Re: TSO and FreeBSD vs Linux))

[Andre Oppermann]

On 19.08.2013 13:42, Alexander V. Chernikov wrote:
> On 14.08.2013 19:48, Luigi Rizzo wrote:
>> On Wed, Aug 14, 2013 at 05:40:28PM +0200, Marko Zec wrote:
>>> On Wednesday 14 August 2013 14:40:24 Luigi Rizzo wrote:
>>>> On Wed, Aug 14, 2013 at 04:15:25PM +0400, Alexander V. Chernikov wrote:
>> ...
>>> FWIW, apparently we already have that infrastrucure in place - if_rele()
>>> calls if_free_internal() only when the last reference to the ifnet is
>>> dropped, so with little care this should be usable for caching ifp pointers
>>> w/o fears for kernel crashes mentioned above.
>> maybe Alexander was referring to holding references to the rte entries
>> returned as a result of the lookup. The rte holds a reference to the ifp.
 >
> Yes. Since there is the only refcount which is protected (and is also a huge performance killer).
>
> Btw, there is a picture describing IPv4 packet flow from my still-not-written post related network
> stack performance,
> maybe it can be useful: http://static.ipfw.ru/images/freebsd_ipv4_flow.png

Wow, that's really cool.  Please note that a rmlock doesn't cost anything for
the read case (unless contended of course).  Whereas normal rlocks or rwlocks
write to the lock memory location and cause atomic bus lock cycles as well as
a lot of cache line invalidations across cores.  The same is true for refcounts.

-- 
Andre