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Re: G5 bandwidth

From what I heard from an Apple Xserve person yesterday the performance of the dual G5 tower will be within 1% of the duan G5 Xserve. This suggests that they did not change the motherboard/controller to address the bandwidth issue.

Its amazing that the potentially great performance is being hampered significantly.

Sean


On Jan 23, 2004, at 9:41 AM, Avi Purkayastha wrote:

Sean,


I've seen this trend of poor scaling on the motherboard using three
different codes. So its not a code-specific problem Also, as Roger
Kylin mentioned, the Xeons take a bigger hit than the G5.

Do the Opteron boards have dual memory controllers?

Yes the Opteron design is different. Each memory controller is attached
separately with the CPU, so it scales, because there is no contention at any
point, say like the single FSBs in the Xeons or the single memory controller
in the dual G5 towers. But because the Opteron's have a NUMA like
architecture, the shared memory latency is higher than say the Xeon's or the
G5s. But I did get excellent memory B/W for single proc on the G5 about 3.2
GB/s for loads or about 80%. So if you plan to work on single CPU nodes you
should eventually get very good performance from the G5 towers.

You ask the important question about potential improvements in the
XServe. Unfortunately Apple is not selling a single CPU Xserve Cluster
Node (at a decent price).

Apple has to fix the point of contention or the single memory controller
design aspect for the dual nodes and hopefully they have already done so for
Xserve's.


I'll be speaking to the Opteron vendor today and get as much relevant
information as possible.

The main limiting factor there is the memory speed because their
hypertransport b/w is faster. Also the shared memory latency will only get
worse if they go 4-way or higher nodes. Its still ok for two-way nodes.

    -- Avi


On Jan 22, 2004, at 9:09 PM, Avi Purkayastha wrote:

But its also true, assuming Sean's single processor performance is
optimal
or close to it, he would not see much scaling anyway, given that the
G5 2p
system does not scale well, as a single memory controller is shared
between
the two DIMMs? At least when I did the STREAM benchmark's that was
clearly
the case. If his code is optimal he should see good b/w for a single
proc
(3-4 GB/s range) and in the same nbhd for the 2p case.

The important question is, has the design been tweaked for improved
scaling
in a dual Xserve node.

    -- Avi

On 1/22/04 7:19 PM, "Yusuf Abdulghani" <email@hidden> wrote:

Sean,

Did you profile your code and see where are you spending your time in
the code? What is your memory access pattern? What compiler/compiler
options are you using to compile your code?

Profile using Shark (profiling tool that comes with CHUD performance
tools) and see if you can determine why you are not seeing as much
scaling as you were expecting to see on the dual G5 system.

Yusuf

On Jan 22, 2004, at 3:45 PM, Sean C. Garrick wrote:

I've been doing some more benchmarking using two homegrown CFD codes,
this time with Opterons and PPC-970s (G5).

Here is a typical single processor result :

Small memory code (110 MB): Time Opteron / Time G5 = 2.3
Opteron: 85 seconds
G5: 36 seconds

Large memory code (450MB)  Time Opteron / Time G5 = 1.9
Opteron: 454 seconds
G5: 237 seconds

While the G5 is faster, the Opteron seems to have a better memory
access/bandwidth.

Also, I  ran a job one job on the G5 using 1 processor and another
using 2 processors. The speed-up was only 50%. I did the same thing
on
the Opteron and the speed-up was 83%!!!

I'm not sure what type of board the Opteron was built on but this is
very surprising as Apple was touting its motherboard as having the
most bandwidth. I think the performance is quite poor and I'm
wondering its worth getting 2 CPUs per board or just go with one?

Any ideas?

Thanks,
Sean
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References: 
 >Re: G5 bandwidth (From: Avi Purkayastha <email@hidden>)



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