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

[Luigi Rizzo]

On Wed, Aug 14, 2013 at 04:15:25PM +0400, Alexander V. Chernikov wrote:
> On 14.08.2013 16:05, Luigi Rizzo wrote:
> > On Wed, Aug 14, 2013 at 03:47:13PM +0400, Lev Serebryakov wrote:
> >> Hello, Luigi.
> >> You wrote 14 ?????????????? 2013 ??., 14:21:09:
> >>
> >> LR> Then the problem remains that we should keep a copy of route and
> >> LR> arp information in the socket instead of redoing the lookups on
> >> LR> every single transmission, as they consume some 25% of the time of
> >> LR> a sendto(), and probably even more when it comes to large tcp
> >> LR> segments, sendfile() and the like.
> >>    And we should invalidate this info on ARP/route changes, or connection
> >>   will be lost in such cases, am I right?.. So, on each such event code
> >>   should look into all sockets and check, if routing/ARP information is still
> >>   valid for them. Or we should store lists of sockets in routing and ARP
> >>   tables... I don't know, what is worse.
> > I think we should start by acknowledging that routing and ARP
> > information is inherently stale, and changes unfrequently.
> > So it is not a disaster if we have incorrect information for some
> > short amount of time (milliseconds) because in the end the remote
> > party that decides to change it and inform us may take much longer
> > than that to distribute the update.
> You can save rte&arp, however doing this
> gives you perfect chance to crash your kernel if egress interface is 
> destroyed (like vlan or ng or tun).

I hope I learned not to follow a stale ifp pointer :)
anyways ARP is really just the mac address so there is no
dandling pointer issue.

For the ifp associated to the route,
i do not see a huge problem in marking the route/ifp as
zombie and destroy it when the last reference goes away.

Not that the current way is any better -- you need to lock/unlock
the rte while you do the lookup, and hold a refcount to the ifp
until the packet is queued. So how does my suggestion make
things worse ?

cheers
luigi


> >
> >
> > Considering that each lookup takes between 100..300ns if you are
> > lucky (not many misses, relatively empty table etc.), one could
> > reasonably do the lookup at most once per millisecond or so (just
> > reading 'ticks', no need for a nanotime() if you have a slow clock),
> > or whenever we get an error related to the socket, either in the
> > forward path (e.g. ifp points to an interface that is down) or in
> > the reverse path (e.g. a dupack because we sent a packet to the
> > wrong place).
> This sounds like "Hey, the kernel lookup is slow (which is true), let's 
> make a hack and don't bother lookups".
> This approach gives us mtx-locked rte refcounts which are used (misused) 
> in many places making things worse and decreasing the ability to fix the 
> things up..
> >
> > cheers
> > luigi
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