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RAM latency too high when system is idle

Hello,

I have noticed that two of the systems I have access are exhibiting RAM latencies that are too high (500ns), but only when the system is idle. The result of this is the unprecedented behavior that an application's performance is enhanced if there is another application running concurrently in the system.

The latency for a given thread is better even when all the other cores in the system are performing memory accesses at maximum throughput than when only one said thread is running.

This can be observed while running the Intel® Memory Latency Checker loaded latency test.

For one of the systems, the output is this:

Inject  Latency Bandwidth
Delay   (ns)    MB/sec
==========================
 00000  380.24    68920.1
 00002  380.22    68886.4
 00008  380.38    68881.2
 00015  380.11    68846.8
 00050  376.15    68501.7
 00100  372.86    68304.9
 00200  282.19    69399.9
 00300  115.94    51590.0
 00400   96.13    39300.5
 00500   92.62    31755.3
 00700   87.91    23047.5
 01000   85.88    16441.6
 01300   84.75    12858.1
 01700   83.99    10033.5
 02500   83.41     7084.0
 03500   85.19     5267.4
 05000   92.23     3857.7
 09000  122.13     2283.1
 20000  219.69     1083.2

 

As expected, the latency descreases as the load from the rest of the system decreases, but when this load decreases below a certain point, the latency increases back. When the load is close to nonexistent, the latency is at its highest.

This happens in two of the dual-socket Xeon systems which I have access (4114 and 4214). Both these systems are from the same supplier, and I can't observe this in the other systems which were bought through other vendors.

I've attached the full outputs for the latency checker (mlc) and dmidecode.

Does someone have a clue of what's going on here? I guess it's a configuration issue but I don't have access to the BIOS of these systems at the moment.

 

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Moderator
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Hello DragonX1979,


Thank you for contacting Intel Xeon Processor Communities.


I was reviewing your thread and sometimes a high System Idle Process can be related to a software problem.

If you see a high idle percent and do not think it should be idling that high, check to see if any open programs are locked up or not responding in general. Sometimes a program can get stuck in a memory loop or stop responding, causing the processor to become idle due to not having any commands to process from an open program.


Also, I checked the dmidecode log file and I noticed that you System Manufacturer is Dell and the system model is a PowerEdge R740.


My best recomendation would be to verify with Dell directly the system configuration and provide them the Tech Support Report from the iDRAC in order to verify if there are some ECC reports correctable memory errors related with the system.


Have a wonderful day.



Best regard,


Emeth O.

Intel Server Specialist.


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