hyper threading.

[em1897 at aol.com]

Test it yourself. I made a comment about making sure
you test before you assume that HT is helpful. I don't
feel compelled to convince you. Do what you want.

-----Original Message-----
From: John Pettitt <jpp at cloudview.com>
To: em1897 at aol.com
Cc: freebsd-questions at freebsd.org
Sent: Sat, 26 Mar 2005 17:23:40 -0800
Subject: Re: hyper threading.

Well you've proven than if you pick your benchmark you can get the
result you want.

So what that says it that the kernel network code doesn't get any
benefit from HT - given that HT is supposed to benefit diverse user
tasks  and no multiple copies of the same code this is not big news -
since you have a HT box how about running a less system code intensive
and more diverse  test?

John


em1897 at aol.com wrote:

> You can argue the technical theory all you want, but the
> measurements say otherwise.
>
>
> You guys have done it once again. Baited me into firing up a
> test that I already know the results of:
>
> Setup: Bridging em0 to em1
> Load: 500Kpps, 60 bytes
> 3.4Ghz P4 1MB Cache
>
> FreeBSD 4.9 -> Load: 38% (I put this in for fun :-)
>
> Freebsd 5.4-Pre UP (no HT) -> Load: high 55-60% range
>
> FreeBSD 5.4-Pre SMP/HT -> Load:  70-80% (much more jumping around)
>
> The bottom line is that if you don't test things to get real
> world results, you don't know crap.
>
>> If that were true, then it would be equally true of systems with 
actual
>> multiple physical processors.  In practice, multiple processors 
provide
>> an obvious performance gain, and hyperthreading does, too, although
>
> it's
>
>> much more modest than the gain obtained from physically independent
>> processors.
>
>
> this shows that you really are a bit foggy. Did you miss the part
> where with 2 processors you actually do have 2 processors?
>
> I can make an argument that networking with 1 processor on 5.4 is
> better than with 2. For example, with a test similar to the above, 
with
> 2 phyiscal processors FreeBSD 5.4 will start dropping packets way 
before
> it hits 500Kpps unless you increase the interrrupts/second, which of
> course increases the system load. And even with the dropped packets
> (which should reduce the load because it doesnt have to receive
> and transmit the packet), the load is still higher than for 4.x with
> a single processor.
>
> You and many others regulary say things like "SMP is obviously 
faster",
> or "Opterons are noticably faster", but those statements are only true
> for certain applications. I've tested an Opteron 2.0Ghz against a 
3.4Ghz
> P4, and the results are pretty interesting. For raw performance, ie
> interrupts/second handling, the P4 wins easily. The P4 wins out of the
> cache.  But once you grow out of the cache and get more memory
> intensive, the Opteron beats it handily.  So which is really faster? 
You
> could argue both depending on what benchmark you use. You
> have to test it in the environment where you plan to use it. Because
> the answer is almost never black and white.
>
>
>
> -----Original Message-----
> From: Anthony Atkielski <atkielski.anthony at wanadoo.fr>
> To: freebsd-questions at freebsd.org
> Sent: Sat, 26 Mar 2005 23:45:21 +0100
> Subject: Re: hyper threading.
>
> em1897 at aol.com writes:
>
>> Yes, the theory is very nice; you've done a nice
>> job reading Intel's marketing garb.
>
>
> I haven't read their marketing materials.  I'm simply going by the
> technical descriptions I've read of the architecture.
>
>> However if you don't have a specific hyperthreading-aware scheduler
>> and particularly well-written, threaded applications, you'll lose 
more
>> than you'll gain.
>
>
> If that were true, then it would be equally true of systems with 
actual
> multiple physical processors.  In practice, multiple processors 
provide
> an obvious performance gain, and hyperthreading does, too, although 
it's
> much more modest than the gain obtained from physically independent
> processors.
>
>> Since FreeBSDs network stack isn't particularly well threaded, nor is
>> the scheduler optimized for hyperthreading, you get a big mess at the
>> kernel level.
>
>
> Nothing needs to be specially optimized for hyperthreading.  All you
> need is at least two threads available for dispatch, with reasonably
> heterogenous instruction mixes that can use different parts of the
> processor hardware at the same time.  Real-world instruction mixes are
> often in this category in general-purpose operating systems.
>
>> So if you have a nice application that does a lot of threaded math
>> operations, you might think you've achieved something,
>
>
> Heavily math-oriented applications (or any group of applications that
> contains similar instruction mixes) are among the least likely to
> benefit from hyperthreading, because they will tend to use the same
> processor logic at the same time, effectively rendering hyperthreading
> moot.
>
>> But what you've missed is that the overhead to manage
>> the "better utilization" of the dual-pipelines created
>> by HT costs more than it gains.
>
>
> Unless FreeBSD is very poorly written indeed, the gain from
> hyperthreading should still exceed the slight increase in overhead
> incurred by multiprocessing logic.
>
>> Hence, the loss of performance.
>
>
> Where can I see this loss of performance documented?
>
>> The poblem is not at the application level, but at the kernel level.
>> The SMP overhead is so substantial, and the OS is working thinking it
>> has 2 processors, that process switching and interrupt handling slow
>> down considerably.
>
>
> How much is "so substantial"?  Where can I see this documented?
>
>> A machine with a 50% load UP will run 65-70% load with
>> HT/SMP running. Like I said, its easily measurable.
>
>
> Then you can show me the measurements.  Where are they?
>
> A 40% increase in system load just because of multiprocessing is
> enormous.  Where did you get this figure?
>
>> Thats at the kernel level (say routing or bridging performance).
>
>
> But the kernel is only a small fraction of overall processor
> utilization.
>
>> Now if the machine isn't a server, it may be just fine.
>> Thats why I suggested testing. But for a network server
>> HT is bad. Very Bad.
>
>
> It doesn't matter whether the machine is a server or a desktop.  What
> matters is the specific mix and nature of applications.
>
>> Not only that, but FreeBSD 5.x actually has a higher
>> capacity network-wise with 1 processor than 2 ...
>
>
> Here again, I need to see this documented.
>
>> ... and I'm sure you can theorize why 2 processors should be
>> faster than one. The theory only matters if you have
>> well written code to handle it properly. FreeBSD is
>> a long way off from that.
>
>
> Where can I see the measurements?
>
> --
> Anthony
>
>
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