[openib-general] BandWidth doubt
krause at cup.hp.com
Fri Nov 10 09:26:54 PST 2006
At 02:02 AM 11/10/2006, john t wrote:
>I got following readings in one of my experiments:
>Single 64-bit xeon machine (2 dual-core 3.2 GHz Intel CPUs, linux FC4,
>OFED 1.0) with two Mellanox DDR (4x) HCAs (each having two ports and each
>connected to a PCI x8 interface) is connected to a switch (all the 4 DDR
>(4x) ports are connected to the switch).
>If I send data from mthca0-1 to mthca0-1 meaning from same port to the
>same port i.e. same port doing send/recv (also same cable doing send/recv)
>I get a BW of around 10 Gb/sec.
>Similarly, from mthca1-1 to mthca1-1 I get same i.e. around 10 Gb/sec.
>So, individual port-to-port gives 10 Gb/sec.
>But when I use them together i.e when I send the data from mthca0-1 to
>mthca0-1 AND from mthca1-1 to mthca1-1 at the same time (simultaneously) I
>get a BW of 6.7 Gb/sec on each port. This is less than 10 Gb/sec that is
>expected. Note that mthca0 and mthca1 are connected to two different
>PCI-x8 interfaces, so there is no question of bandwidth splitting. What
>could be causing such a behaviour ??
>Just to add if the same thing is done between two different hosts i.e. If
>I send data from mthca0-0 and mthca1-1 of one host to mthca0-0 and
>mthca1-1 of other host, I get expected BW i.e. 10 Gb/sec on each port/link.
You have two links pounding on a shared PCIe Root Complex / memory
controller. This sounds like a chipset issue not an IB / software issue
when it is placed under load.
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