Re: [Xen-users] HTPC + DUAL PC In one

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 07/17/2014 02:36 AM, Austin S Hemmelgarn wrote:
> On 07/16/2014 01:07 PM, Gordan Bobic wrote:
> > On 2014-07-16 16:08, Austin S Hemmelgarn wrote:
> > > Hyper-Threading shares a lot more than just the FPU between threads,
> > > IIRC the only thing in HT that isn't potentially shared is the
> > > registers, so unless the OS is built to schedule intelligently,
> > > performance on a HT cpu can be really terrible.
> > The throughput should always go up in the general case. Under
> > a saturating MySQL load it goes up about 10-15%, for example.
> > The reason HT helps is 2-fold:
> >
> > 1) It reduces the number of context switches because two
> > contexts are handled by the hardware at the same time
> >
> > 2) It improves benefits from CPU caches. Data in RAM is very
> > far away, and you don't want the CPU to sit idle while the MCH
> > is fetching it. While this is happening, if you have another
> > thread pre-loaded and ready to execute, you can check if the data
> > that threads requires is already in the cache, and if it is,
> > run it.
> I'm just trying point out that HT is a less efficient solution than for
> example SMT on a UltraSPARC T1.
Sure it is, but SPARC performance is do dire to begin with that in 
reality it doesn't really matter.
> Also, I don't entirely agree with either of your points:
> 1. Would be great, except that for 2-way HT you still have at-least 4
> different states things could be in context-wise (even ignoring
> interrupt and SMM contexts and different contexts for each individual
> process), and any time it transitions between states, both threads get
> stopped.
>
> 2. Would also be great, except that the L1-cache is shared between the
> multiple threads on a given core (on AMD's bulldozer processors, each
> 'core' gets it's own L1-cache, and the L2 and L3 are shared).
I was never making the comparison between Intel and AMD implementations, 
I was pointing out why having HT enabled improves performance over not 
having it, for the same number of cores.
> In general, most up-to-date OSes do a decent job of handling HT, but
> most of them (especially Windows and Solaris) still leave a lot to be
> desired.
It varies with different load profiles, too, even if the same 
application stack is used.


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