[THIN] Re: Hyperthreading

  • From: "Bernd Harzog" <Bernd.Harzog@xxxxxxxxxxx>
  • To: <thin@xxxxxxxxxxxxx>
  • Date: Fri, 1 Oct 2004 10:38:26 -0400

Tim,

Have the run the test that I alluded to? Fire up a rogue process that is 
isolated to CPU 3, and that only is allowed to run on CPU 3. My tests Windows 
2000 MetaFrame XP server that was a clone of a production server at a major 
healthcare organization, shows that that one process running on CPU 3 rendered 
the whole server unusable. Running that process on CPU's 0 or 1 had no 
discernable effect.

Cheers,

Bernd Harzog
CTO
RTO Software, Inc.
bernd.harzog@xxxxxxxxxxx
678-455-5506 x701
www.rtosoft.com

 -----Original Message-----
From:   Tim Mangan [mailto:tmangan@xxxxxxxxxxxx] 
Sent:   Friday, October 01, 2004 10:25 AM
To:     thin@xxxxxxxxxxxxx
Subject:        [THIN] Re: Hyperthreading

Although at times I have seen behavior that made me suspicious that one or
the other of the logical processors (as in the 0,2 pair or 1,3) is
"different", I have been unable to prove that out in tests when I try to
nail it down.  

I believe that both logical processors are essentially identical, both from
an Intel and a Microsoft point of view on Win2k.  The only thing Microsoft
will do differently on Win2k is that occasionally the threading algorithm
will select a lower numbered processor (sort of like the third tie-breaker
for making the playoffs).  This may explain why it at times may seem to be
unbalanced.

To the best of my knowledge all that happens is that if one (either) of the
logicals of a pair goes idle, some on-chip caching may be lost on it's other
half.  I suspect that this alters timing in that process as opposed to
corrupting anything.  I have never seen corruption at the chip level and
believe me I looked!  This makes sense in what we see in tests.

Tim Mangan

-----Original Message-----
From: thin-bounce@xxxxxxxxxxxxx [mailto:thin-bounce@xxxxxxxxxxxxx] On Behalf
Of Bernd Harzog
Sent: Thursday, September 30, 2004 12:18 PM
To: thin@xxxxxxxxxxxxx
Subject: [THIN] Re: Hyperthreading

Let me add some fuel to the fire. I do not know if this is pervasive, but I
have noticed on several customer systems. On a hyperthreaded dual, you have
processors 0,1,2 and 3. 0 and 1 are the "physical" processors, and 2 and 3
are the virtual processors. These four processors DO NOT behave identically.
Specifically, the virtual processors seem to have severe problems with them
that can negatively the performance of the whole server.

Here is what we have observed. TScale has a feature that allows you to set
the Affinity of a process. If you have an infrequently used process that
tends to go rogue, we recommend that you consider putting in a Priority rule
for that process to have its Priority be reduced when the process goes
rogue, and then also nailing all instances of that infrequently used process
to one of the N CPU's. That way, if two instances of that process are
launched and go rogue they can be isolated to one of the NT CPU's and set to
low priority so that they do not do damage to even that CPU.

The problem that we observed is that when you set Priority rules in TScale
they typically take the form of "reduce Priority when processname.exe uses
X% CPU for more than N seconds". So, it might say, reduce the Priority of
Acread32.exe when it takes more than 20% of CPU for 20 seconds. The theory
is that it might validly need lots of CPU for 20 seconds, but if it is doing
more than that, it is a hog and steps need to be taken to make sure that it
does not harm other processes and users.

Well what we discovered is that when a process is running on one of the
virtual CPU's (2 or 3) and goes rogue, it shuts down the whole server. So,
if something uses 25% of CPU (all of CPU 2 or 3) doing this on a virtual CPU
on Windows 2000 somehow clogs up the entire system. So, our recommendations
were; 1) don't use hyperthreading on Windows 2000, and if you insist upon
turning it on, don't use Affinity to assign any processes to the virtual
CPU's (only assign processes to the physical ones).

I would be interested in knowing if anyone else has seen anything like this.
If you have, and you could share with the list that would be most useful.

Best Regards,

Bernd Harzog
CTO
RTO Software, Inc.
bernd.harzog@xxxxxxxxxxx
678-455-5506 x701
www.rtosoft.com

 -----Original Message-----
From:   Brian Madden [mailto:brian@xxxxxxxxxxxxxxx] 
Sent:   Thursday, September 30, 2004 11:34 AM
To:     thin@xxxxxxxxxxxxx
Subject:        [THIN] Re: Hyperthreading

The main problems in W2K with Hyperthreading are:

(1) Stability. I haven't seen this personally, but it's something that
everyone says
(2) W2K doesn't realize that 4 hyperthreaded processors are really two
physical processors. The problem is that the system might try to "load
balance" some work evenly across processors 0 and 1, thinking it's doing
a good job but not realizing that they're really going to the same
place, all while procs 2 and 3 are sitting idle. Multiply this by
several billion ticks per second and it could in theory lead to worse
performance.

Again, it's only a few percentage points, and if someone is gung-ho for
THE on W2K, I won't stand in their way.

Brian

Brian Madden
brian@xxxxxxxxxxxxxxx
+1.202.302.3657
Sign up now for BriForum 2005: A 100% technical, 100% independent
server-based computing conference from April 11-12, 2005. Visit
www.briforum.com for details.

-----Original Message-----
From: thin-bounce@xxxxxxxxxxxxx [mailto:thin-bounce@xxxxxxxxxxxxx] On
Behalf Of Bob
Sent: Thursday, September 30, 2004 10:57 AM
To: thin@xxxxxxxxxxxxx
Subject: [THIN] Re: Hyperthreading

Thanks, Jim... great article on the benefits of HTE

-but-

Intel is criminally terse in their statement that it should be
disabled on Win2k.  I can't tell if their rationale is functional or
political.  I'm about to rock the boat here, and I'm going to be asked
tough questions.  I'm hoping somebody will weigh in with some personal
trials.

Thanks,
Bob



----- Original Message -----
From: Jim Kenzig http://thethin.net <jimkenz@xxxxxxxxxxxxxx>
Date: Thu, 30 Sep 2004 07:06:52 -0700 (PDT)
Subject: [THIN] Re: Hyperthreading
To: thin@xxxxxxxxxxxxx


See
http://www.2cpu.com/articles/42_1.html
http://thethin.net/faqs2.cfm?id=444&category=1&sortby=score
Leave it off for Win2K!
http://thethin.net/faqs2.cfm?id=443&category=1&sortby=score
 
Regards,
Jim Kenzig
http://thin.net


 

Bob <bobmails@xxxxxxxxx> wrote:


Has anybody experimented with Hyperthreading On -vs- Off on Win2k
servers? I've heard rumblings that it can be problematic, and perhaps
decrease performance. I noticed that most SpecInt. winners have it
disabled. I'm considering disabling it on one of my farm servers to
observe the results. If anybody else has experimented I'd love to
hear about it.

Thanks in advance,
Bob
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