Intel 10Gb

[Andrew Gallatin]

Alexander Sack wrote:
 > On Fri, May 14, 2010 at 10:07 AM, Andrew Gallatin 
<gallatin at cs.duke.edu> wrote:
 >> Alexander Sack wrote:
 >> <...>
 >>>> Using this driver/firmware combo, we can receive minimal packets at
 >>>> line rate (14.8Mpps) to userspace.  You can even access this using a
 >>>> libpcap interface.  The trick is that the fast paths are OS-bypass,
 >>>> and don't suffer from OS overheads, like lock contention.  See
 >>>> http://www.myri.com/scs/SNF/doc/index.html for details.
 >>> But your timestamps will be atrocious at 10G speeds.  Myricom doesn't
 >>> timestamp packets AFAIK.  If you want reliable timestamps you need to
 >>> look at companies like Endace, Napatech, etc.
 >> I see your old help ticket in our system.  Yes, our timestamping
 >> is not as good as a dedicated capture card with a GPS reference,
 >> but it is good enough for most people.
 >
 > I was told btw that it doesn't timestamp at ALL.  I am assuming NOW
 > that is incorrect.

I think you might have misunderstood how we do timestamping.
I definately don't understand it, and I work there ;)
I do know that there is NIC component of it (eg, it is not 100%
done in the host).  I also realize that it is not is good as
something that is 1PPS GPS based.

 > Define *most* people.

I may have a skewed view of the market, but it seems like
some people care deeply about accurate timestamps, and
others (mostly doing deep packet inspection) care only
within a few milliseconds, or even seconds.

 > I am not knocking the Myricom card.  In fact I so wish you guys would
 > just add the ability to latch to a 1PPS for timestamping and it would
 > be perfect.
 >
 > We use I think an older version of the card internally for replay.
 > Its a great multi-purpose card.
 >
 > However with IPG at 10G in the nanoseconds, anyone trying to do OWDs
 > or RTT will find it difficult compared to an Endace or Napatech card.
 >
 > Btw, I was referring to bpf(4) specifically, so please don't take my
 > comments as a knock against it.
 >
 >>> PS I am not sure but Intel also supports writing packets directly in
 >>> cache (yet I thought the 82599 driver actually does a prefetch anyway
 >>> which had me confused on why that helps)
 >> You're talking about DCA.  We support DCA as well (and I suspect some
 >> other 10G NICs do to).  There are a few barriers to using DCA on
 >> FreeBSD, not least of which is that FreeBSD doesn't currently have the
 >> infrastructure to support it (no IOATDMA or DCA drivers).
 >
 > Right.
 >
 >> DCA is also problematic because support from system/motherboard
 >> vendors is very spotty.  The vendor must provide the correct tag table
 >> in BIOS such that the tags match the CPU/core numbering in the system.
 >> Many motherboard vendors don't bother with this, and you cannot enable
 >> DCA on a lot of systems, even though the underlying chipset supports
 >> DCA.  I've done hacks to force-enable it in the past, with mixed
 >> results. The problem is that DCA depends on having the correct tag
 >> table, so that packets can be prefetched into the correct CPU's cache.
 >> If the tag table is incorrect, DCA is a big pessimization, because it
 >> blows the cache in other CPUs.
 >
 > Right.
 >
 >> That said, I would *love* it if FreeBSD grew ioatdma/dca support.
 >> Jack, does Intel have any interest in porting DCA support to FreeBSD?
 >
 > Question for Jack or Drew, what DOES FreeBSD have to do to support
 > DCA?  I thought DCA was something you just enable on the NIC chipset
 > and if the system is IOATDMA aware, it just works.  Is that not right
 > (assuming cache tags are correct and accessible)?  i.e. I thought this
 > was hardware black magic than anything specific the OS has to do.

IOATDMA and DCA are sort of unfairly joined for two reasons: The DCA
control stuff is implemented as part of the IOATDMA PCIe device, and
IOATDMA is a great usage model for DCA, since you'd want the DMAs
that it does to be prefetched.

To use DCA you need:

- A DCA driver to talk to the IOATDMA/DCA pcie device, and obtain the tag
	table
- An interface that a client device (eg, NIC driver) can use to obtain
	either the tag table, or at least the correct tag for the CPU
	that the interrupt handler is bound to.  The basic support in
	a NIC driver boils down to something like:

nic_interrupt_handler()
{
   if (sc->dca.enabled && (curcpu != sc->dca.last_cpu)) {
      sc->dca.last_cpu = curcpu;
      tag = dca_get_tag(curcpu);
      WRITE_REG(sc, DCA_TAG, tag);
   }
}

Drew