DP performance

[Danial Thom]

--- Marko Zec <zec at xxxxxxxx> wrote:

> On Wednesday 30 November 2005 16:18, Danial
> Thom wrote:
> > --- Hiten Pandya <hmp at xxxxxxxxxxxxx> wrote:
> > > Marko Zec wrote:
> > > > Should we be really that pessimistic
> about
> > > > potential MP performance,
> > > > even with two NICs only?  Typically
> packet
> > > > flows are bi-directional,
> > > > and if we could have one CPU/core taking
> care
> > > > of one direction, then
> > > > there should be at least some room for
> > > > parallelism, especially once the
> > > > parallelized routing tables see the
> light.
> > > > Of course provided that
> > > > each NIC is handled by a separate core,
> and
> > > > that IPC doesn't become the
> > > > actual bottleneck.
> > >
> > > On a similar note, it is important that we
> add
> > > the *hardware* support
> > > for binding a set of CPUs to particular
> > > interrupt lines.  I believe that
> > > the API support for CPU-affinitized
> interrupt
> > > threads is already there
> > > so only the hard work is left of converting
> the
> > > APIC code from physical
> > > to logical access mode.
> > >
> > > I am not sure how the AMD64 platform
> handles
> > > CPU affinity, by that I
> > > mean if the same infrastructure put in
> place
> > > for i386 would work or not
> > > with a few modifications here and there. 
> The
> > > recent untangling of the
> > > interrupt code should make it simpler for
> > > others to dig into adding
> > > interrupt affinity support.
> >
> > This, by itself, it not enough, albeit
> useful.
> > What you need to do is separate transmit and
> > receive (which use the same interrupts, of
> > course). The only way to increase capacity
> for a
> > single stream with MP is to separate tx and
> rx.
> 
> Unless doing fancy oubound queuing, which
> typically doesn't make much 
> sense at 1Gbit/s speeds and above, I'd bet that
> significantly more CPU 
> cycles are spent in the "RX" part than in the
> "TX", which basically 
> only has to enqueue a packet into the devices'
> DMA ring, and recycle 
> already transmitted mbufs.  The other issue
> with having separate CPUs 
> handling RX and TX parts of the same interface
> would be the locking 
> mess -> you would end up with the
> per-data-structure locking model of 
> FreeBSD 5.0 and later, which DragonFly diverted
> from.
> 
> Cheers,
> 
> Marko

The issue is that RX is absolute, as you cannot
"decide" to delay or selectively drop since you
don't know whats coming. Better to have some
latency than dropped packets. But if you don't
dedicate to RX, then you have an unknown amount
of cpu resources doing "other stuff". The
capacity issues always first manifest themselves
as rx overruns, and they always happen a lot
sooner on MP machines than UP machines. The LINUX
camp made the mistake of not making RX important
enough, and now their 2.6 kernels drop packets
all over the ranch. But audio is nice and
smooth... 

How to do it or why its difficult is a designer's
issue. I've not yet been convinced that MP is
something thats suitable for a network intensive
environment as I've never seen an MP OS that has
come close to Freebsd 4.x UP performance.

DT


		
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