[GRLUG] FYI - old mysql and new gear

[Mike Williams]

On 9/22/2011 12:22 PM, scott.tanner at comcast.net wrote:
> I'm going to try using affinity to limit the CPU access and run a few 
> benchmarks - should be an interesting test.
>
> I've heard there's a fair amount of overhead when running MySQL in 
> virtual machines, specifically due to the abstraction layer between 
> the raid controller and guest.  It would be interesting to test, but 
> difficult to setup now that the server is running in production - It 
> doesn't look like you can limit the number of processors for Dom0.
>
> We purposely don't run anything else on these servers to reduce any 
> contention, so limiting the CPU's isn't a very big deal, however 
> wasting 4 / 16 cores  (real / +HT) is pretty silly.
>
Have you tried just turning off hyperthreading? In quite a few cases, 
the scheduler doesn't do a very good job of handling the virtual CPU's 
of hyperthreading and actually makes things worse. At best, HT is only 
going to help when you several different types of tasks going on at the 
same time, like a floating point calculation and an integer one. If 
mysql is mostly integer, which I suspect it is, then at best HT is going 
to give you nothing. If the scheduler gets it wrong and is trying to 
simultaneously run 8 operations on only 4 real cores all you're going to 
do is double (or worse) the amount of task switches.

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