Red Hat Bugzilla – Bug 604239
Memtest86+ clock runs fast.
Last modified: 2011-06-27 14:18:39 EDT
I've noticed that on my computer the memtest walltime clock runs fast.
The problem is *probably* that it's using the TSC to measure time.
I have a 3 GHz Intel CPU which is BIOS configured to start at 2 GHz.
This is an EIST configuration (ie. multiplier = 6 instead of the default/max 9).
However because of 'constant_tsc cpu flag' the TSC continues to cycle at 3 GHz (even though the cpu only executes at 2 GHz).
So Memtest probably sees a 2 GHz CPU and uses TSC for the walltime, but the TSC is cycling at 3GHz instead of 2 (ie. 150%). This is inline with the observed behaviour of the wall clock taking 40 seconds to measure off a minute (ie. speedup of 150%).
My guess is this may also be a problem on newer cpu's (i7's) which have turboboost and other dynamic accelerations (IDA), since [I believe] their TSC cycle frequency is actually the maximum turbo boost frequency (and not the 'normal' frequency of the cpu). Although the difference should be much smaller than the 150% I'm seeing.
You can get some information about EIST out of MSRs 0x198 and 0x199. So you might be able to detect the scaling factor...
At a very rough first approximation, the scaling factor is bits 45-40 of MSR 0x198 divided by bits 13-8 of MSR 0x199 (with bits 46 and 14 increasing the relevant number by 0.5). Unfortunately this is definitely nowhere near a full solution (it fails to handle non-zero bits 47 and 15 for example, not sure if it even deals with i7 turboboost or IDA - it should handle your basic garden variety Intel EIST without super-low-power laptop mode).
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