Author: Matt Taylor
Date: 00:10:48 03/08/03
Go up one level in this thread
On March 05, 2003 at 18:11:25, Robert Hyatt wrote: >On March 05, 2003 at 15:13:41, Aaron Gordon wrote: > >>On March 05, 2003 at 11:36:30, Robert Hyatt wrote: >> >>>On March 05, 2003 at 02:05:30, Aaron Gordon wrote: >>> >>>>On March 04, 2003 at 21:57:58, Nolan Denson wrote: >>>> >>>>>There are no draw backs when looking to get a computer that using >>>>>Hyper-Threading. If you do not like the feature you can just simply turn it off. >>>>>There are many system on the market that claims to be Hyper Threading enabled. >>>>>Intel has a utility program that checks for Hyper Threading ... >>>>> >>>>>1. You must have the proper CPU's. >>>>>2. You must have the proper motherboard. >>>>>3. You bios must support it. >>>>>4. Your Operating system must support it. >>>>> >>>>>Once these things are met your system will indicate that Hyper Threading is >>>>>enable before booting into the Operating System. >>>>> >>>>>It is not an indication that you will see when the computer is turned on and the >>>>>Bios is posting. >>>>> >>>>>Soon all of Intel processcor's will have Hyper Threading. So if what you are >>>>>using your processor slows down with Hyper Threading, just simply turn it off >>>>>via the Bios. >>>> >>>>How convenient. Have to reboot 20 times per day to turn it on/off just so your >>>>applications run optimally. I'll be sticking with the cooler running, faster, >>>>and cheaper AMD chips. :) >>> >>>You don't have to do this. Just use an SMT aware operating system and just run >>>two >>>tasks, although I think there is _zero_ chance that a parallel algorithm that >>>works with >>>two threads on a dual processor will behave _worse_ using two threads on a >>>single SMT >>>cpu than it does using just one thread. SMT is not going to _worsen_ >>>performance, >>>particularly if spinlocks and spinwaits are used correctly. Since most programs >>>don't even >>>use 'em, it isn't an issue at all. >> >>If you read around you'll see when SMT is enabled it can hurt performance up to >>20% just as it helps in some cases up to 20%. Be it SMT or bad programming, it >>does it. I dislike P4's anyway (hot, slow, etc) and if there's something that >>can potentially hurt it's performance even more.. well, it's time to glue the >>cpu to the wall and let the cat use it as a cat scratcher. Thats about all it'd >>be worth to me. > > >SMT can _not_ hurt performance if you only run one thread. Which I believe I >clearly said. If you run on an SMT-aware O/S, and have a dual xeon with SMT >on or off, you will get the _same_ results for any pair of applications you care >to >run if you only run two compute threads total, one will run on each physical CPU >and there is no interaction. > >I don't see how SMT can hurt a program, unless it is using a lot of >spinlocks/spinwaits and >it doesn't follow the "fix" posted by Intel. A sloppy spinlock/spinwait can >certainly hurt, >but then the same program running two threads on a single CPU would do _badly_ >as >well, so it is a problem either way. You mean using all available processors. If it's one thread, other programs can compete for limited resources such as execution bandwidth and trace cache space. -Matt
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