RE: FW: ixg(4) performances

["Terry Moore" <tmm%mcci.com@localhost>]

> -----Original Message-----
> From: Hisashi T Fujinaka [mailto:htodd%twofifty.com@localhost]
> Sent: Sunday, August 31, 2014 12:39
> To: Terry Moore
> Cc: tech-kern%netbsd.org@localhost
> Subject: RE: FW: ixg(4) performances
> 
> I may be wrong in the transactions/transfers. However, I think you're
> reading the page incorrectly. The signalling rate is the physical speed of
> the link. On top of that is the 8/10 encoding (the Ethernet controller
> we're talking about is only Gen 2), the framing, etc, and the spec
> discusses the data rate in GT/s. Gb/s means nothing.
 
Hi,

I see what the dispute is. The PCIe 3.0 spec *nowhere* uses "transfers" in
the sense of UNIT INTERVALs (it uses UNIT INTERVAL).  The word "transfer" is
not used in that way in the spec.  Transfer is used, mostly in the sense of
a larger message. It's not in the glossary, etc.  

However, you are absolutely right, many places in the industry (Intel,
Wikipedia, etc.) refer to 2.5GT/s to mean 2.5G Unit Intervals/Second; and
it's common enough that it's the de-facto standard terminology. I only deal
with the spec, and don't pay that much attention to the ancillary material.

We still disagree about something: GT/s does mean *something* in terms of
the raw throughput of the link. It tells the absolute upper limit of the
channel (ignoring protocol overhead). If the upper limit is too low, you can
stop worrying about fine points.  If you're only trying to push 10% (for
example) and you're not getting it, you look for protocol problems. At 50%,
you say "hmm, we could be hitting the channel capacity".  

I think we were violently agreeing, because the technical content of what I
was writing was (modulo possible typos) identical to 2.5GT/s. 2.5GT/s is
(after 8/10 encoding) a max raw data rate of 250e6 bytes/sec, and when you
go through things and account for overhead, it's very possible that 8 lanes
(max 2e9 bytes/sec) of gen1 won't be fast enough for 10G Eth.  

Best regards,
--Terry