Subject: Re: Bad count of CPUs
To: None <firstname.lastname@example.org>
From: Michael <email@example.com>
Date: 10/11/2006 17:39:10
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On Oct 11, 2006, at 16:01, Joseph A. Dacuma wrote:
> Is it wise to enable HT? I came across various sites where they say
> Threading makes a system unstable.
Not that I played much with HT Xeons, especially not recent ones, but
it highly depends on what you're doing. For instance, we had Oracle 9i
and some monstrous Java app on a 2.something GHz Xeon a couple years
ago and benchmarking showed that with HT enabled the whole thing ran
about 10% slower than without HT. This specific app mind you. Others
have reported a slight speedup with HT so your mileage will vary.
For what I can tell - if you have two threads running bad code with
plenty of pipeline stalls on a HT CPU it's likely to help since most of
the core would be idle anyway and HT masks that to some degree - when
one pipeline stalls the other one will happily continue working. If you
have two threads that run highly optimized code which uses pretty much
everything the CPU has and hardly ever stalls then they're likely to
get into each other's way. HT would also help masking latency issues -
while one pipeline waits for the cache to fetch something from RAM
there's a chance that the other one will not.
Don't expect huge performance differences though. If you have the time
benchmark your app with and without HT and see what happens.
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