On 5/2/13 1:24 PM, Benny Amorsen wrote:
joel jaeggli <joe...@bogus.com> writes:
There's literally no options in between. so a 1/10Gb/s TOR like the
force10 s60 might have 2GB of shared packet buffer, while an like an
arista 7050s-64 would have 9MB for all the ports, assuming you run it
as all 10Gb/s rather than 100/1000/10000/40000 mixes of ports it can
cut-through-forward to every port which goes a long way toward
ameliorating your exposure to shallow buffers.
Why does cut-through help so much? In theory it should save precisely
one packets worth of memory, i.e. around 9kB per port. 500kB extra
buffer for the whole 50-port switch does not seem like a lot.
Until there's contention for the output side, you should only have one
packet in the output queue at a time for each port on a cut through
switch. which is like 96K of buffer for 1500 byte frames on a 64 port switch
Store and forward means you hold onto the packet a lot longer
mechanically even if nominally you are able to forward at line rate so
long as there's always a packet in the ouput queue to put on the wire.
consider that the fastest cut-through 10Gb/s switches now are around
.4usec and your 1500 byte packet takes ~1.2usec to arrive.
when adding rate conversion, consider that when having a flow come from
a 10Gb/s to 1Gb/s port that another 1500byte packet can arrive every
~1.2usec but you can only clock them back out every 12usec. jumbos just
push the opportunities to queue for rate conversion out that much furthure
Lots of people say that cut-through helps prevents packet loss due to
lack of buffer, so something more complicated must be happening.
/Benny
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