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More info?)
John Lewis wrote:
> See:-
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>
http://www.anandtech.com/video/showdoc.aspx?i=2461
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> and the extensive introductory article
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http://www.anandtech.com/video/showdoc.aspx?i=2453
>
"Clever marketing however, will always try to fool the consumer."
In our last article we had a fairly open-ended discussion about many of
the challenges facing both of the recently announced next-generation
game consoles. We discussed misconceptions about the Cell processor
and its ability to accelerate physics calculations, as well as touched
on the GPUs of both platforms. In the end, both the Xbox 360 and the
PlayStation 3 are much closer competitors than you would think based on
first impressions.
The Xbox 360's Xenon CPU features more general purpose cores than the
PlayStation 3 (3 vs. 1), however game developers will most likely only
be using one of those cores for the majority of their calculations,
leveling the playing field considerably.
The Cell processor derives much of its power from its array of 7 SPEs
(Synergistic Processing Elements), however as we discovered in our last
article, their purpose is far more specialized than we had thought.
Speaking with Epic Games' head developer, Tim Sweeney, he provided a
much more balanced view of what sorts of tasks could take advantage of
the Cell's SPE array.
The GPUs of the next-generation platforms also proved to be quite
interesting. In Part I we speculated as to the true nature of
NVIDIA's RSX in the PS3, concluding that it's quite likely little
more than a higher clocked G70 GPU. We will expand on that discussion
a bit more in this article. We also looked at Xenos, the Xbox 360's
GPU and characterized it as equivalent to a very flexible 24-pipe R420.
Despite the inclusion of the 10MB of embedded DRAM, Xenos and RSX
ended up being quite similar in our expectations for performance; and
that pretty much summarized all of our findings - the two consoles,
although implementing very different architectures, ended up being so
very similar.
So we've concluded that the two platforms will probably end up
performing very similarly, but there was one very important element
excluded from the first article: a comparison to present-day PC
architectures. The reason a comparison to PC architectures is
important is because it provides an evaluation point to gauge the
expected performance of these next-generation consoles. We've heard
countless times that these new consoles would offer better gaming
performance than anything we've had on the PC, or anything we would
have for a matter of years. Now it's time to actually put those
claims to the test, and that's exactly what we did.
Speaking under conditions of anonymity with real world game developers
who have had first hand experience writing code for both the Xbox 360
and PlayStation 3 hardware (and dev kits where applicable), we asked
them for nothing more than their brutal honesty. What did they think
of these new consoles? Are they really outfitted with the PC-eclipsing
performance we've been lead to believe they have? The answer is
actually quite frequently found in history; as with anything, you get
what you pay for.
Page 2
Learning from Generation X
The original Xbox console marked a very important step in the evolution
of gaming consoles - it was the first console that was little more than
a Windows PC.
The original Xbox was basically a PC
It featured a 733MHz Pentium III processor with a 128KB L2 cache,
paired up with a modified version of NVIDIA's nForce chipset (modified
to support Intel's Pentium III bus instead of the Athlon XP it was
designed for). The nForce chipset featured an integrated GPU,
codenamed the NV2A, offering performance very similar to that of a
GeForce3. The system had a 5X PC DVD drive and an 8GB IDE hard drive,
and all of the controllers interfaced to the console using USB cables
with a proprietary connector.
For the most part, game developers were quite pleased with the original
Xbox. It offered them a much more powerful CPU, GPU and overall
platform than anything had before. But as time went on, there were
definitely limitations that developers ran into with the first Xbox.
One of the biggest limitations ended up being the meager 64MB of memory
that the system shipped with. Developers had asked for 128MB and the
motherboard even had positions silk screened for an additional 64MB,
but in an attempt to control costs the final console only shipped with
64MB of memory.
Developers wanted more memory, but the first Xbox only shipped with
64MB
The next problem is that the NV2A GPU ended up not having the fill rate
and memory bandwidth necessary to drive high resolutions, which kept
the Xbox from being used as a HD console.
Although Intel outfitted the original Xbox with a Pentium III/Celeron
hybrid in order to improve performance yet maintain its low cost, at
733MHz that quickly became a performance bottleneck for more complex
games after the console's introduction.
The combination of GPU and CPU limitations made 30 fps a frame rate
target for many games, while simpler titles were able to run at 60 fps.
Split screen play on Halo would even stutter below 30 fps depending on
what was happening on screen, and that was just a first-generation
title. More experience with the Xbox brought creative solutions to the
limitations of the console, but clearly most game developers had a wish
list of things they would have liked to have seen in the Xbox
successor. Similar complaints were levied against the PlayStation 2,
but in some cases they were more extreme (e.g. its 4MB frame buffer).
Given that consoles are generally evolutionary, taking lessons learned
in previous generations and delivering what the game developers want in
order to create the next-generation of titles, it isn't a surprise to
see that a number of these problems are fixed in the Xbox 360 and
PlayStation 3.
One of the most important changes with the new consoles is that system
memory has been bumped from 64MB on the original Xbox to a whopping
512MB on both the Xbox 360 and the PlayStation 3. For the Xbox, that's
a factor of 8 increase, and over 12x the total memory present on the
PlayStation 2.
The other important improvement with the next-generation of consoles is
that the GPUs have been improved tremendously. With 6 - 12 month
product cycles, it's no surprise that in the past 4 years GPUs have
become much more powerful. By far the biggest upgrade these new
consoles will offer, from a graphics standpoint, is the ability to
support HD resolutions.
There are obviously other, less-performance oriented improvements such
as wireless controllers and more ubiquitous multi-channel sound
support. And with Sony's PlayStation 3, disc capacity goes up thanks
to their embracing the Blu-ray standard.
The Xbox 360: two parts evolution, one part mistake?
But then we come to the issue of the CPUs in these next-generation
consoles, and the level of improvement they offer. Both the Xbox 360
and the PlayStation 3 offer multi-core CPUs to supposedly usher in a
new era of improved game physics and reality. Unfortunately, as we
have found out, the desire to bring multi-core CPUs to these consoles
was made a reality at the expense of performance in a very big way.
Page 3
Problems with the Architecture
At the heart of both the Xenon and Cell processors is IBM's custom
PowerPC based core. We've discussed this core in our previous
articles, but it is best characterized as being quite simple. The core
itself is a very narrow 2-issue in-order execution core, featuring a
64KB L1 cache (32K instruction/32K data) and either a 1MB or 512KB L2
cache (for Xenon or Cell, respectively). Supporting SMT, the core can
execute two threads simultaneously similar to a Hyper Threading enabled
Pentium 4. The Xenon CPU is made up of three of these cores, while
Cell features just one.
Each individual core is extremely small, making the 3-core Xenon CPU in
the Xbox 360 smaller than a single core 90nm Pentium 4. While we
don't have exact die sizes, we've heard that the number is around
1/2 the size of the 90nm Prescott die.
Cell's PPE is identical to a single core in Xenon. The die area of the
Cell processor is 221 mm^2, note how little space is occupied by the
PPE - it is a very simple core.
IBM's pitch to Microsoft was based on the peak theoretical floating
point performance-per-dollar that the Xenon CPU would offer, and given
Microsoft's focus on cost savings with the Xbox 360, they took the
bait.
While Microsoft and Sony have been childishly playing this flops-war,
comparing the 1 TFLOPs processing power of the Xenon CPU to the 2
TFLOPs processing power of the Cell, the real-world performance war has
already been lost.
Right now, from what we've heard, the real-world performance of the
Xenon CPU is about twice that of the 733MHz processor in the first
Xbox. Considering that this CPU is supposed to power the Xbox 360 for
the next 4 - 5 years, it's nothing short of disappointing. To put it
in perspective, floating point multiplies are apparently 1/3 as fast on
Xenon as on a Pentium 4.
The reason for the poor performance? The very narrow 2-issue in-order
core also happens to be very deeply pipelined, apparently with a branch
predictor that's not the best in the business. In the end, you get
what you pay for, and with such a small core, it's no surprise that
performance isn't anywhere near the Athlon 64 or Pentium 4 class.
The Cell processor doesn't get off the hook just because it only uses
a single one of these horribly slow cores; the SPE array ends up being
fairly useless in the majority of situations, making it little more
than a waste of die space.
We mentioned before that collision detection is able to be accelerated
on the SPEs of Cell, despite being fairly branch heavy. The lack of a
branch predictor in the SPEs apparently isn't that big of a deal,
since most collision detection branches are basically random and
can't be predicted even with the best branch predictor. So not
having a branch predictor doesn't hurt, what does hurt however is the
very small amount of local memory available to each SPE. In order to
access main memory, the SPE places a DMA request on the bus (or the PPE
can initiate the DMA request) and waits for it to be fulfilled. From
those that have had experience with the PS3 development kits, this
access takes far too long to be used in many real world scenarios. It
is the small amount of local memory that each SPE has access to that
limits the SPEs from being able to work on more than a handful of
tasks. While physics acceleration is an important one, there are many
more tasks that can't be accelerated by the SPEs because of the
memory limitation.
The other point that has been made is that even if you can offload some
of the physics calculations to the SPE array, the Cell's PPE ends up
being a pretty big bottleneck thanks to its overall lackluster
performance. It's akin to having an extremely fast GPU but without a
fast CPU to pair it up with.
Page 4
What About Multithreading?
We of course asked the obvious question: would game developers rather
have 3 slow general purpose cores, or one of those cores paired with an
array of specialized SPEs? The response was unanimous, everyone we
have spoken to would rather take the general purpose core approach.
Citing everything from ease of programming to the limitations of the
SPEs we mentioned previously, the Xbox 360 appears to be the more
developer-friendly of the two platforms according to the cross-platform
developers we've spoken to. Despite being more developer-friendly, the
Xenon CPU is still not what developers wanted.
The most ironic bit of it all is that according to developers, if
either manufacturer had decided to use an Athlon 64 or a Pentium D in
their next-gen console, they would be significantly ahead of the
competition in terms of CPU performance.
While the developers we've spoken to agree that heavily multithreaded
game engines are the future, that future won't really take form for
another 3 - 5 years. Even Microsoft admitted to us that all developers
are focusing on having, at most, one or two threads of execution for
the game engine itself - not the four or six threads that the Xbox 360
was designed for.
Even when games become more aggressive with their multithreading,
targeting 2 - 4 threads, most of the work will still be done in a
single thread. It won't be until the next step in multithreaded
architectures where that single thread gets broken down even further,
and by that time we'll be talking about Xbox 720 and PlayStation 4. In
the end, the more multithreaded nature of these new console CPUs
doesn't help paint much of a brighter performance picture -
multithreaded or not, game developers are not pleased with the
performance of these CPUs.
What about all those Flops?
The one statement that we heard over and over again was that Microsoft
was sold on the peak theoretical performance of the Xenon CPU. Ever
since the announcement of the Xbox 360 and PS3 hardware, people have
been set on comparing Microsoft's figure of 1 trillion floating point
operations per second to Sony's figure of 2 trillion floating point
operations per second (TFLOPs). Any AnandTech reader should know for a
fact that these numbers are meaningless, but just in case you need some
reasoning for why, let's look at the facts.
First and foremost, a floating point operation can be anything; it can
be adding two floating point numbers together, or it can be performing
a dot product on two floating point numbers, it can even be just
calculating the complement of a fp number. Anything that is executed
on a FPU is fair game to be called a floating point operation.
Secondly, both floating point power numbers refer to the whole system,
CPU and GPU. Obviously a GPU's floating point processing power doesn't
mean anything if you're trying to run general purpose code on it and
vice versa. As we've seen from the graphics market, characterizing GPU
performance in terms of generic floating point operations per second is
far from the full performance story.
Third, when a manufacturer is talking about peak floating point
performance there are a few things that they aren't taking into
account. Being able to process billions of operations per second
depends on actually being able to have that many floating point
operations to work on. That means that you have to have enough
bandwidth to keep the FPUs fed, no mispredicted branches, no cache
misses and the right structure of code to make sure that all of the
FPUs can be fed at all times so they can execute at their peak rates.
We already know that's not the case as game developers have already
told us that the Xenon CPU isn't even in the same realm of performance
as the Pentium 4 or Athlon 64. Not to mention that the requirements
for hitting peak theoretical performance are always ridiculous; caches
are only so big and thus there will come a time where a request to main
memory is needed, and you can expect that request to be fulfilled in a
few hundred clock cycles, where no floating point operations will be
happening at all.
So while there may be some extreme cases where the Xenon CPU can hit
its peak performance, it sure isn't happening in any real world code.
The Cell processor is no different; given that its PPE is identical to
one of the PowerPC cores in Xenon, it must derive its floating point
performance superiority from its array of SPEs. So what's the issue
with 218 GFLOPs number (2 TFLOPs for the whole system)? Well, from
what we've heard, game developers are finding that they can't use the
SPEs for a lot of tasks. So in the end, it doesn't matter what peak
theoretical performance of Cell's SPE array is, if those SPEs aren't
being used all the time.
Don't stare directly at the flops, you may start believing that they
matter.
Another way to look at this comparison of flops is to look at integer
add latencies on the Pentium 4 vs. the Athlon 64. The Pentium 4 has
two double pumped ALUs, each capable of performing two add operations
per clock, that's a total of 4 add operations per clock; so we could
say that a 3.8GHz Pentium 4 can perform 15.2 billion operations per
second. The Athlon 64 has three ALUs each capable of executing an add
every clock; so a 2.8GHz Athlon 64 can perform 8.4 billion operations
per second. By this silly console marketing logic, the Pentium 4 would
be almost twice as fast as the Athlon 64, and a multi-core Pentium 4
would be faster than a multi-core Athlon 64. Any AnandTech reader
should know that's hardly the case. No code is composed entirely of
add instructions, and even if it were, eventually the Pentium 4 and
Athlon 64 will have to go out to main memory for data, and when they
do, the Athlon 64 has a much lower latency access to memory than the
P4. In the end, despite what these horribly concocted numbers may lead
you to believe, they say absolutely nothing about performance. The
exact same situation exists with the CPUs of the next-generation
consoles; don't fall for it.
Page 5
Why did Sony/MS do it?
For Sony, it doesn't take much to see that the Cell processor is eerily
similar to the Emotion Engine in the PlayStation 2, at least
conceptually. Sony clearly has an idea of what direction they would
like to go in, and it doesn't happen to be one that's aligned with much
of the rest of the industry. Sony's past successes have really come,
not because of the hardware, but because of the developers and their
PSX/PS2 exclusive titles. A single hot title can ship millions of
consoles, and by our count, Sony has had many more of those than
Microsoft had with the first Xbox.
Sony shipped around 4 times as many PlayStation 2 consoles as Microsoft
did Xboxes, regardless of the hardware platform, a game developer won't
turn down working with the PS2 - the install base is just that
attractive. So for Sony, the Cell processor may be strange and even
undesirable for game developers, but the developers will come
regardless.
The real surprise was Microsoft; with the first Xbox, Microsoft
listened very closely to the wants and desires of game developers.
This time around, despite what has been said publicly, the Xbox 360's
CPU architecture wasn't what game developers had asked for.
They wanted a multi-core CPU, but not such a significant step back in
single threaded performance. When AMD and Intel moved to multi-core
designs, they did so at the expense of a few hundred MHz in clock
speed, not by taking a step back in architecture.
We suspect that a big part of Microsoft's decision to go with the Xenon
core was because of its extremely small size. A smaller die means
lower system costs, and if Microsoft indeed launches the Xbox 360 at
$299 the Xenon CPU will be a big reason why that was made possible.
Another contributing factor may be the fact that Microsoft wanted to
own the IP of the silicon that went into the Xbox 360. We seriously
doubt that either AMD or Intel would be willing to grant them the right
to make Pentium 4 or Athlon 64 CPUs, so it may have been that IBM was
the only partner willing to work with Microsoft's terms and only with
this one specific core.
Regardless of the reasoning, not a single developer we've spoken to
thinks that it was the right decision.
Page 6
The Saving Grace: The GPUs
Although both manufacturers royally screwed up their CPUs, all
developers have agreed that they are quite pleased with the GPU power
of the next-generation consoles.
First, let's talk about NVIDIA's RSX in the PlayStation 3. We
discussed the possibility of RSX offloading vertex processing onto the
Cell processor, but more and more it seems that isn't the case. It
looks like the RSX will basically be a 90nm G70 with Turbo Cache
running at 550MHz, and the performance will be quite good.
One option we didn't discuss in the last article, was that the G70 GPU
may feature a number of disabled shader pipes already to improve yield.
The move to 90nm may allow for those pipes to be enabled and thus
allowing for another scenario where the RSX offers higher performance
at the same transistor count as the present-day G70. Sony may be
hesitant to reveal the actual number of pixel and vertex pipes in the
RSX because honestly they won't know until a few months before mass
production what their final yields will be.
Despite strong performance and support for 1080p, a large number of
developers are targeting 720p for their PS3 titles and won't support
1080p. Those that are simply porting current-generation games over
will have no problems running at 1080p, but anyone working on a truly
next-generation title won't have the fill rate necessary to render at
1080p.
Another interesting point is that despite its lack of "free 4X AA" like
the Xbox 360, in some cases it won't matter. Titles that use longer
pixel shader programs end up being bound by pixel shader performance
rather than memory bandwidth, so the performance difference between no
AA and 2X/4X AA may end up being quite small. Not all titles will push
the RSX to the limits however, and those titles will definitely see a
performance drop with AA enabled. In the end, whether the RSX's lack
of embedded DRAM matters will be entirely dependent on the game engine
being developed for the platform. Games that make more extensive use
of long pixel shaders will see less of an impact with AA enabled than
those that are more texture bound. Game developers are all over the
map on this one, so it wouldn't be fair to characterize all of the
games as falling into one category or another.
ATI's Xenos GPU is also looking pretty good and most are expecting
performance to be very similar to the RSX, but real world support for
this won't be ready for another couple of months. Developers have just
recently received more final Xbox 360 hardware, and gauging performance
of the actual Xenos GPU compared to the R420 based solutions in the G5
development kits will take some time. Since the original dev kits
offered significantly lower performance, developers will need a bit of
time to figure out what realistic limits the Xenos GPU will have.
Page 7
Final Words
Just because these CPUs and GPUs are in a console doesn't mean that we
should throw away years of knowledge from the PC industry - performance
doesn't come out of thin air, and peak performance is almost never
achieved. Clever marketing however, will always try to fool the
consumer.
And that's what we have here today, with the Xbox 360 and PlayStation
3. Both consoles are marketed to be much more powerful than they
actually are, and from talking to numerous game developers it seems
that the real world performance of these platforms isn't anywhere near
what it was supposed to be.
It looks like significant advancements in game physics won't happen on
consoles for another 4 or 5 years, although it may happen with PC games
much before that.
It's not all bad news however; the good news is that both GPUs are
quite possibly the most promising part of the new consoles. With the
performance that we have seen from NVIDIA's G70, we have very high
expectations for the 360 and PS3. The ability to finally run at HD
resolutions in all games will bring a much needed element to console
gaming.
And let's not forget all of the other improvements to these
next-generation game consoles. The CPUs, despite being relatively
lackluster, will still be faster than their predecessors and increased
system memory will give developers more breathing room. Then there
are other improvements such as wireless controllers, better online play
and updated game engines that will contribute to an overall better
gaming experience.
In the end, performance could be better, the consoles aren't what they
could have been had the powers at be made some different decisions.
While they will bring better quality games to market and will be better
than their predecessors, it doesn't look like they will be the end of
PC gaming any more than the Xbox and PS2 were when they were launched.
The two markets will continue to coexist, with consoles being much
easier to deal with, and PCs offering some performance-derived
advantages.
With much more powerful CPUs and, in the near future, more powerful
GPUs, the PC paired with the right developers should be able to bring
about that revolution in game physics and graphics we've been hoping
for. Consoles will help accelerate the transition to multithreaded
gaming, but it looks like it will take PC developers to bring about
real change in things like game physics, AI and other non-visual
elements of gaming.
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