Intel's Pentium Extreme Edition 955: 65nm, 4 threads and 376M transistors

Dual Core and Hyper Threading: Detriment or Not?

A question that we've always had is whether or not the inclusion of Hyper Threading support on Intel's dual-core Extreme Edition processors actually improves performance.  To answer that question, we have to look at two separate situations: multithreaded application performance and multitasking performance. 

For multithreaded application performance, we can now turn to a number of benchmarks.  We'll start off with 3dsmax 7 (higher numbers are better for the composite score, lower numbers are better for the rest of the numbers):

3dsmax 7	Composite Score	3dsmax 5 rays	CBALLS2	SinglePipe2	UnderWater
HT Enabled	3.0	12.922s	17.297s	83.515s	119.641s
HT Disabled	2.51	14.937s	21.141s	102.734s	141.641s

Here, the performance advantage is clear - enabling Hyper Threading provides Intel with another 14-19% over the base dual core Presler.  The same applies to almost all of the media encoding tests (if minutes or seconds are specified, lower numbers mean better performance):

Media Encoding	DVD Shrink	WME9	H.264	iTunes
HT Enabled	7.1m	46.5fps	9.96m	38s
HT Disabled	8.0m	38.6fps	8.53m	40s

Our Quicktime 7 H.264 encoding test is, generally speaking, an outlier from what we've seen of the impact of HT on multithreaded applications.  The rest of the applications show a clear benefit to being able to execute four threads simultaneously, even if the execution resources of the cores are shared with the remaining two threads. 

Armed with the latest SMP patches for Call of Duty 2 and Quake 4 (SMP was enabled in both games), we can also take a look at the impact of HT on Presler:

Gaming	Call of Duty 2	Quake 4
HT Enabled	68.4	142.3
HT Disabled	69.3	142.3

Call of Duty 2 is another example where HT actually reduces performance, but given that enabling SMP itself reduces performance, we'd venture a guess that you shouldn't really be drawing any conclusions based on its data.  Quake 4, on the other hand, shows no difference in performance with SMP on or off. 

From what we've seen, with most individual multithreaded applications, enabling HT will improve performance even if, you have a dual core processor.  The degree of performance improvement will vary from application to application, but generally speaking, it's going to be positive (if anything at all). 

The more interesting situation is what happens when you're multitasking - does Hyper Threading really help on top of the inherent benefits of a dual core processor?  To find out, we put together a couple of multitasking scenarios aided by a tool that Intel provided us to help all of the applications start at the exact same time.  We're not necessarily concerned with the actual performance of these applications, but rather with the impact that the number of simultaneous applications has on each other and how that varies with HT being enabled or not. 

We took five applications (Grisoft AVG Anti-Virus 7, Lame MP3 Encoder 3.97a, Windows Media Encoder 9, Info-ZIP extraction utility and Splinter Cell: Chaos Theory) and used various combinations of them to try to figure out if there are multitasking benefits to a dual core processor with Hyper Threading enabled.  Note that some of these applications are multithreaded themselves, so just because we chose five applications doesn't mean that there are only five threads of execution; in reality, there are many more. 

We tested four different scenarios:

1. A virus scan + MP3 encode
2. The first scenario + a Windows Media encode
3. The second scenario + unzipping files, and
4. The third scenario + our Splinter Cell: CT benchmark.

The graph below compares the total time in seconds for all of the timed tasks (everything but Splinter Cell) to complete during the tests:

AMD Athlon 64 X2 4800+	AVG	LAME	WME	ZIP	Total
AVG + LAME	22.9s	13.8s			36.7s
AVG + LAME + WME	35.5s	24.9s	29.5s		90.0s
AVG + LAME + WME + ZIP	41.6s	38.2s	40.9s	56.6s	177.3s
AVG + LAME + WME + ZIP + SCCT	42.8s	42.2s	46.6s	65.9s	197.5s

Intel Pentium EE 955 (no HT)	AVG	LAME	WME	ZIP	Total
AVG + LAME	24.8s	13.7s			38.5s
AVG + LAME + WME	39.2s	22.5s	32.0s		93.7s
AVG + LAME + WME + ZIP	47.1s	37.3s	45.0s	62.0s	191.4s
AVG + LAME + WME + ZIP + SCCT	40.3s	47.7s	58.6s	83.3s	229.9s

Intel Pentium EE 955 (HT Enabled)	AVG	LAME	WME	ZIP	Total
AVG + LAME	25.0s	13.3s			38.3s
AVG + LAME + WME	34.4s	21.6s	30.2s		86.2s
AVG + LAME + WME + ZIP	41.5s	28.1s	37.7s	54.2s	161.5s
AVG + LAME + WME + ZIP + SCCT	51.4s	33.0s	45.3s	71.1s	200.8s

As you can see, the Presler setup with HT enabled takes less time to complete the tasks as soon as you get beyond two simultaneous applications than the Presler system without HT enabled.  However, including the Athlon 64 X2 4800+ in the picture, we see that despite only being able to execute two threads at the same time, it does just as good of a job as the Presler HT system that can execute twice as many threads.  But to get the full picture, we have to measure one last data point: Splinter Cell performance. 

In the fourth scenario, we ran a total of five applications: AVG, Lame, WME, InfoZip and Splinter Cell.  The first four applications took a total of 197.5 seconds to complete on the Athlon 64 X2 4800+ system, ever so slightly quicker than the 200.8 seconds of the Presler HT system.  However, that does not take into account Splinter Cell performance - now let's see how our fifth application fared:

Splinter Cell: CT	Average	Min	Max
Intel Pentium EE 955 (no HT)	71.0 fps	27.8 fps	128.1 fps
Intel Pentium EE 955 (HT enabled)	77.2 fps	32.5 fps	139.6 fps
AMD Athlon 64 X2 4800+	66.9 fps	10.5 fps	185.0 fps

The Athlon 64 X2 4800+ actually is faster in the Splinter Cell: CT benchmark without anything else running, but here we see a very different story.  Although its 66 fps average frame rate is reasonably competitive with the Presler HT system, its minimum frame rate is barely over 10 fps - approximately 1/3 that of the Presler HT. 

While the regular Presler setup without HT managed to pull in higher frame rates than the AMD system, it did so while performing significantly worse in the remaining four applications.  The Presler HT vs. Athlon 64 X2 comparison is important because the two are virtually tied in the performance of the first four applications - but juggling all five of the applications is better done on the Presler HT system. 

We would say that if implemented properly, the benefits of a SMT system like Hyper Threading are definitely a good companion to a dual core desktop processor.  The usable limit, even for today's applications and usage models, is far from just two threads.