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I hope I phrased this correctly. lshw gives:

         description: DIMM Synchronous 400 MHz (2.5 ns)
         product: M378B5773CH0-CH9
         vendor: Samsung
         physical id: 0
         slot: DIMM0
         size: 2GiB
         width: 64 bits
         clock: 400MHz (2.5ns)

And indeed the memory speed is set is set to 800MHz in the BIOS, which I think makes sense since it is a double rate.

On the other hand, Googling strongly suggests that to this product number corresponds the PC3-10600 type, which is 1333MHz, not 800MHz. And this seems to be confirmed in the BIOS, where if I select Auto for memory bus speed, 1333MHz is selected "based on SPD settings".

However in the latter case, the computer does not boot, i.e. the kernel panics, complaining that something attempted to kill the Idle process.

So, I am I am beginning to suspect that I have been given defective memory, the technician that installed saw this, and lowered the bus speed. Is this a possibility?

NEW DEVELOPMENT

The suggestions below have been very helpful, wish I could accept them both. Well, before running the memtest, I had the idea to reset the bios to its default settings, which I had not done on this computer. What do you know? Both the bus clock was set to Auto, i.e. 1333MHz and the computer booted normally, and seems to be able to pass the memtest (didn't let it finish). Before, I could not even get GRUB to load sometimes, and never the system.

So although I was paranoid about being scammed, it was some setting that the retailer en(dis)abled that caused the crashes. What it was, and why they had changed it will probably remain a mystery, since the days when I could understand HALF the options in my BIOS are long gone. :-)

I will post back if there is any more instability. Many thanks.

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Also this answers the original question: dmidecode and lshw list the EFFECTIVE frequencies. Since I booted with the new settings, they show the new value 1333/2=667 –  Panayiotis Karabassis Mar 30 '12 at 14:15

2 Answers 2

This could be one of the cases bellow :

  1. Your motherboard does not support that frequency 1333.
  2. defective memory (memtest - so you can prove it )
  3. the kernel that you use doesn't support motherboard / ram frequency combo .
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+1 for memtest86, or variant. Theoretical discussion is all very well, and I can understand your suspicions, but to really know if your memory works at a given speed, test it. –  MadHatter Mar 30 '12 at 9:54

To answer your "do I have bad RAM?" question first: this is a possibility, but only one among many.

The actual limit for the clock frequency on the memory bus will depend on a lot of factors, including your memory module configuration. Consult your system's (or mainboard's) documentation to rule architectural limits out as a possible cause.

It also might be worth removing a number of modules to get a meaningful minimal memory configuration (e.g. leaving just two DIMMs in place), raising the clock speed and retrying booting / running a memtest. If it fails, swap the two plugged modules for two different ones and try again.

Since those are cheapo non-ECC, unbuffered and unregistered modules, you might have a bad module which gets mostly unnoticed. But I also have seen memtest86 consistently reporting memory errors in a narrow address region where exchanging the modules did not help - circuit path or memory controller issues on your mainboard would look pretty much the same as a bad DIMM.

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