[petsc-users] Understanding the memory bandwidth

[Åsmund Ervik]

>> So this is a pretty low fraction (55%) of 59.7*2 = 119.4.  I suspect
>> your memory or motherboard is at most 1600 MHz, so your peak would be
>> 102.4 GB/s.
>
>> You can check this as root using "dmidecode --type 17", which should
>> give one entry per channel, looking something like this:
>>
>> Handle 0x002B, DMI type 17, 34 bytes
>> Memory Device
>>         Array Handle: 0x002A
>>         Error Information Handle: 0x002F
>>         Total Width: Unknown
>>         Data Width: Unknown
>>         Size: 4096 MB
>>         Form Factor: DIMM
>>         Set: None
>>         Locator: DIMM0
>>         Bank Locator: BANK 0
>>         Type: <OUT OF SPEC>
>>         Type Detail: None
>>         Speed: Unknown
>>         Manufacturer: Not Specified
>>         Serial Number: Not Specified
>>         Asset Tag: Unknown
>>         Part Number: Not Specified
>>         Rank: Unknown
>>         Configured Clock Speed: 1600 MHz
>
>I have no root access. Is there another way to confirm the clock speed?

Also note: even in the case where your motherboard, RAM and CPU all say
1866 on the label, if there are more memory DIMMs (chips) per node than
channels, say 16 DIMMs on your 8 channels, you will see a performance
reduction on the order of 20-30%. This is more likely if you are using
nodes in a "high-memory queue" or similar where there's >= 128 GB memory
per node. (This will change in the future when/if people start using
DDR4 LRDIMMs.) There's a series of in-depth discussions here:
http://frankdenneman.nl/2015/02/20/memory-deep-dive/ and there's also
lots of interesting memory-stuff on John McCalpin's blog:
https://sites.utexas.edu/jdm4372/

Regards,
Åsmund

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