Re: Sub-millisecond NTP synchronization for local network

Kevin Oberman <oberman@xxxxxx> wrote:
Before this gets too far off track, standards compliant Ethernet
uses frames of 1500 bytes, regardless of speed. This is true for 10M
or 10G Ethernet.

Most modern cards have support for jumbo frames. These are, by
definition, non-standard, and are available only when specifically
configured. Unless you turn them on, frames are 1500 data bytes plus
framing of up to 22 bytes. When jumbos are enabled, the size
currently recommended by the US federal Joint Engineering Team is
9000 bytes to allow the transmission of 8K of actual user data when
allowing for framing, VLAN and MPLS tags and such with a comfortable
margin over that. (Most modern systems page memory in 8K chunks, so
this optimizes performance.)

I like JumboFrames. I think they are quite useful. Still...

Back in the 1990's we were pleased that the MTU of FDDI was 4500ish
bytes because it was larger than the typical page size. Then at least
up in the server space the pages becamse variably sized and rather
larger. Bye bye page flipping.

Today x86 systems might use an 8K page size, but they also have
increasingly used support for huge pages. It might be possible to
page flip today with a 9000 byte MTU, but don't expect it to last much
longer than it did with FDDI.

"All" the 10G NICs and I suspect a decent number of the 1G NICs
support TSO or TCP/Transport Segmentation Offload. For the sender at
least that can be considered a "poor man's jumbo frame." Many (most?)
of the curent 10G NICs also support LRO or Large Receive Offload.
Together they do a decent if not complete job of getting the bulk
transfer performance gains of Jumbo Frames. There is of course some
additional benefit of using them on top of Jumbo Frames although it is
in the realm of diminishing returns.

rick jones
--
The computing industry isn't as much a game of "Follow The Leader" as
it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
- Rick Jones
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
.

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