[Discuss] Next AMD FX-8120 update

[markw at mohawksoft.com (markw at mohawksoft.com)]

> On 03/18/2012 12:02 PM, Richard Pieri wrote:
>> What you will typically find is that if your threads are CPU bound then
>> you will see better performance over the long term with HT disabled.
>> The reason is that the phantom CPUs that HT provides need to share cache
>> and memory bandwidth and there is some extra switching overhead.  The
>> upshot is that if you have 1 CPU with 2 HT threads and 4 CPU-bound jobs
>> to run, the total time to run all 4 jobs will be less with HT disabled.
>> As an aside for anyone running a Condor pool, disabling HT is
>> recommended for this reason.
>>
>> On the other hand, if you are not CPU-bound across all of your threads,
>> or in environments where concurrency is more important than throughput,
>> then HT may be a win.
>>
>> AMD's Bulldozer architecture has less resource contention than Intel's
>> HT implementations (less overhead) but two threads on 1 core still have
>> to share some resources and you will usually see results similar to what
>> I described.
> In Toronto they always turn off HT. I ran a quick test and found that
> the RiskWatch application runs better with no HT. There is certainly
> some benefit to HT under some circumstances.

The problem isn't with hyperthreads per se' it is a problem with system
schedulers not knowing the difference or how to use them. Hyperthreads are
sort of a micro-NUMA environment. Sometimes, it is best to put a HT
semi-core to sleep instead of using it because there is no appropriate job
for it to run and running another job would affect its peer.

One of the things I was concerned about the FX-8120 was the shared
resources of the cores. So far it doesn't seem too bad. Even though core
pairs share a numeric processor and some caching, they seem to schedule
fairly well independently.

So, like I said, they aren't truly full cores, but they don't seem
similarly limited.

> --
> Jerry Feldman <gaf at blu.org>
> Boston Linux and Unix
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