Re: [Xen-users] Xeon 5160 vs 5080

[Javier Guerra <javier@xxxxxxxxxxx>]

On Sunday 03 September 2006 8:55 pm, Nathan Allen Stratton wrote:
> Chip          Clock           HT      Cache   Bus Speed
> ---------------------------------------------------------
> 5080          3.7 GHz         YES     2MB     1066 MHz
> 5160          3.0 GHz         NO      4MB     1333 MHz
>
> Does the .7 GHz and HT worth more then 4MB cache and higher bus speed? The
> application is VoIP so there is not a lot of IO so I would not think Bus
> Speed would matter. I am finding mixed information on HT, some say it is
> great, others say it actually slows things down, could this be why the new
> chips done have HT?

HT helps to avoid context switches when used with an HT-aware scheduler.  it 
can be very significant for I/O in Xen because everything a DomU does has to 
pass through Dom0 for I/O. dedicating an HT thread exclusively for Dom0 is 
almost as good as giving it a full real core, but usually much cheaper.

AFAIK, all Xen schedulers, all modern Linux, BSD and Solaris kernels are fully 
HT-aware, and make good use of it.  I also think (but not sure) that some 
servicepacks for win2k and winXP gave them most, if not all, of the 
advantages of a modern scheduler.

the only remaining cases where HT impacts negatively are heavy single threaded 
tasks.  there, you'd like to dedicate the whole processor to a process during 
its timeslice, without other tasks taking up resources (cache, rename 
registers, FSB, etc)

with server-like workloads, even more with Xen, HT is usually a good thing.

i have no clue why newer chips doesn't have it, neither why AMD never bothered 
with something like that.  maybe there's some other consideration i don't 
know about.

-- 
Javier

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