AGP and PCI Express Performance

In our first article we compared the performance of DDR versus DDR2 on the ASRock 775Dual-VSTA motherboard and found there was very little difference between DDR-400 and DDR2-533 on this platform. In our second article we compared the performance of DDR against DDR2 on several different platforms that included the VIA PT880 Pro, Intel 865, Intel 945P, Intel P965, and Intel 975X chipsets. Our results showed that while there were differences in memory performance between each chipset and speed setting, it mattered little in the overall performance of our system. This was mainly due to our selection of mid-range components that likely would be used when upgrading to our motherboard and processor choice.

Our article today will look at the performance differences between AGP and PCI Express graphics cards on the ASRock 775Dual-VSTA motherboard with Intel's E6300 Core 2 Duo. We will state up front that our article today is not a video card review. Instead we are verifying if AGP performance on our test motherboards is acceptable when comparing it to PCI Express performance on the VIA PT880 Pro chipset used on our ASRock test platform. Our tests today will be utilizing the EVGA 7600GS and 6800 Ultra series of video cards in both PCI Express and AGP configurations. Both series of cards offer decent performance that is well suited for 17" or 19" LCD monitors running at resolutions up to 1280x1024. While both cards will struggle with current games such as Oblivion -- a game that will bring most systems to their knees -- they still offer a fair amount of performance for games released the past couple of years and can certainly handle any normal desktop application work with ease.

The typical user who will purchase a motherboard of this type or the AGP only ASRock 775i65G are primarily concerned about extending their current component investments while upgrading to the latest Core 2 Duo processor series from Intel. These component investments usually include AGP graphics, DDR memory, along their current power supply, storage, and optical drives. Based upon this profile our tests will utilize DDR memory only and video cards that represent typical performance in the mid to lower range of graphics capability at this time. Additional graphics performance results when utilizing DDR2 memory combined with other chipsets can be found in our last article.

The ASRock 775Dual-VSTA motherboard offers AGP 8X/4X capability along with PCI Express X4 graphics performance. However, our PCI Express video cards will theoretically be at a disadvantage due to the bandwidth differences between X4 mode and our AGP 8X capability. We would like to see if the difference in bandwidth affects performance, though realistically we're only measuring the performance of one specific design as opposed to AGP versus PCI Express; we cannot say for certain that the PCI Express implementation of the PT880 chipset is fully competitive with other PCI Express implementations. Still, it is possible that the X4 slot will saturate the PCI-E bus, which can certainly occur in some cases based upon graphics settings and applications.

AGP 8X has up to 2.1GB/s of shared bandwidth. The typical GPU can make use of nearly all the bandwidth, but the upstream bandwidth isn't as important. Various requirements for upstream transfers end up limiting the maximum upstream throughput to around 266 MB/s, and switching back and fortch between reads and writes can incur a further performance penalty. Typical desktop applications tend to utilize the downstream bandwidth (read from system) the greatest amount of time while upstream bandwidth (write to system) is seldom used and is rarely an issue. The design of AGP took this into account, which is why the realizeable upstream bandwidth is so much lower than the downstream bandwidth.

PCI Express X16 operation has 8GB/s of theoretical bandwidth that is segregated for upstream and downstream paths due to its serial bus design. Technically, the PCI-E bus transmits at 2.5 Gbps on each link, so 2.5 Gbps x 16 = 40 Gbps. Converting to bytes that gives 5 GB/s, but like most serial buses there is a 20% transaction overhead that reduces the useable bandwidth to about 4 GB/s. Thus we arrive at the result of 4 GB/s of read and write speed for each direction at maximum bus capacity. Once again, the majority of application usage is spent on the read side with the write side capacity being wasted for the most part in current PCI Express graphics systems, but there is the potential for it to become more important in the future, and SLI/CrossFire implementations using the PCI-E bus to transmit data certainly benefit.

Since our test board is operating in X4 mode we have one fourth of the bandwidth available which equates to about 1000MB/s upstream and 1000MB/s downstream. This is slower than AGP 8X since the available read bandwidth is limited and can be saturated by certain applications, making AGP 8X potentially more effective. PCI Express X4 offers about half the available read bandwidth of AGP 8X, and the write performance advantage of PCI-E goes largely unused in most applications.

Let's see if this theory holds true in our test results -- again, recognizing that we are only comparing performance on one specific chipset.

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  • Shoal07 - Tuesday, August 15, 2006 - link

    “Pairing the motherboard with a top end GPU results in performance that is up to 10% slower than competing motherboards, but it is unlikely anyone would be looking to purchase a $300+ GPU to use with a $50 motherboard.)”

    I disagree with this statement. Many of us have good/great AGP cards, gigs of memory laying around (or in the old systems to upgrade) and basically want a new, and faster, processor. In order to do this (AGP, DDR memory and a C2D) we need a mobo that can support all 3. Of which there is... 1? I think we’ll see more people buying high end processors to use with this budget mobo then we will with any other budget mobo. The motivations are different. When was the last time we were really offered the opportunity to upgrade overtime?

    Existing AGP and DDR --> (Phase 1) New budget mobo and high end processor --> (Phase 2) Upgrade to high end memory or PCI-X video card --> (Phase 3) Ditto --> (Phase 4) replace mobo with High end mobo. Walla! High end system in steps. Saves the wallet and the wife aggro.
  • Orbitr8 - Saturday, August 26, 2006 - link

    I just dropped $400 into an e6400, Asrock VSTA, and another SATA drive.

    Took quite some time to get the SATA to work for some reason, and I could never get it to be the boot drive, so I'm using it as the Program drive...

    Aside from that, I really did NOT want to shell out another $400 for DDR2 ram and a PCI-e vid card, since I just bought a couple gigs of XMS for my aging X3400, and a 6800XT AGP card not too long ago.

    Once up and running, I have to admit, it flies. Literally, I had to tie the case down. No, really. ;)
    anyway. A simple flick of the keyboard in the CPU setting in the BIOS to 300, and no sneezes at all.
    Using DDR ram absolutely does not present any speed issues over DDR2. NONE !! In fact, my ram is faster than DDR2. Go figure.

    As for my 'aging' 6800XT, I gained roughly 75 to 100+ FPS in UT2004 with no vid card unlocking !!
    BF2 now plays smooth as butter ~ I can only imagine how nice a 7800 would be, but I'm not into spending that kind of money for games.

    All in all, this article and the tests performed were exactly what I wanted to see, because the scenario fit me to a 'T', and I'm sure there are many more of us out there.

    Just because some of you insist on being bleeding edge, even if the changes don't offer any performance gains, doesn't mean the rest of us are like that. I'm into actually getting my $$ worth out of my gear before I upgrade. The key is to seriously consider the upgrade path and all the options before spending.

    This upgrade was probaby one of the best experiences I've had so far in the last 10 years, the first being the change from a 266 PII to a 1GHz Athlon.

    So, Thumbs UP for both AnandTech and Asrock for being responsible for me actually writing a post.
    a WTG from me.
  • joex444 - Wednesday, August 16, 2006 - link

    Walla! Erm, ahm, VOILA!
  • yyrkoon - Tuesday, August 15, 2006 - link

    Owning a Asrock AM2NF4G-SATA2 motherboard, and using it in conjunction with a AMD 64 AM2 3800+, I also have to disagree. While my preffered motherboard IS a highend motherboard (ABIT AN9 32x for AM2, and ABIT AB9 Pro for Conroe), I find these Asrock boards for the most part very good motherboards, especially for the price. The only real drawback for my current AM2 board, is that it seems it wont enter into windows XP pro SP2 setup with a SATA drive attached (which means I have to install to a IDE drive). Anyhow, I find myself considering upgrading my older socket A 3200+ XP system using this board, and migrating the rest of the components, with of course a C2D CPU.

    Anyhow, least we forget, Asrock IS a division of Asus . . .
  • Myrandex - Tuesday, August 15, 2006 - link

    just so you know, PCI-X and PCIe is not the same thing. There really aren't PCI-X video cards..
  • Uwe - Tuesday, August 15, 2006 - link

    So would something like a vanilla 6600 AGP be worth transferring over to this ASUS board with an E6300? I like articles like these because it doesn't deal with parts I can't afford. I've been hanging onto XP 2500+ Barton for a while now. The ASUS board has perked my interest in an affordable upgrade. Thanks!
  • kalrith - Thursday, August 17, 2006 - link

    It would be worth it if it allowed you to do your upgrades in steps. For about $270 you could get this mobo and a E6300. Then in a few months or whenever, you could upgrade your video card. Then you could later pick up 2GB of DDR2.

    Just keep in mind that you will get little or no improvement in gaming performance with this upgrade (unless you are running a lot of stuff in the background while gaming).
  • VooDooAddict - Wednesday, August 16, 2006 - link

    I think it all depends on if you are happy with the video performance of the vanilla 6600 AGP.
  • Gary Key - Tuesday, August 15, 2006 - link

    I basically agree with your viewpoint. There was a difference of opinion on the staff about my original comments/thoughts that is now back in place. ;-) However, to a certain degree I believe if you are going to spend $500 on a GPU solution intended for gaming that you would probably want a better performing motherboard (one with a true PCIe x16 slot) to maximize that investment. It is a toss up decision that could go either way depending on the individual circumstances. Thank you for the comments
  • yyrkoon - Tuesday, August 15, 2006 - link

    I for one will probably NEVER spend more on a GPU, than I spend on a motherboard / CPU combination. Since most of the time, my upgrade monies are rather limited, I tend to upgrade as little as possible, and opt for mid-range GPUs.

    My current system is: Asrock AM2NF4G-SATA2 | AMD 64 AM2 3800+ | 2GB Corsair DDR2 6400 | eVGA 7600GT KO | Seagate Barracuda 80 GB IDE | Seagate Barracuda 250GB SATA | Antec 450SL 450W PSU. Now I think its fairly obvious what I had to upgrade, and I spent around $600-$650 for CPU, motherboard, memory, and video card (plus a Lian Li PC-G50 case), including shipping. In any computer system, there are three things I will never skimp on for my own personal PC (and will protest having to do so for a customer), these three things are: Motherboard, memory, and PSU. Despite the fact that Asrock boards are in-expencive, and a bit quirky, they make very solid motherboards, and know quite a few people who use them in server setups all the time (some even use them exclusively).

    Anyhow, back to my point, WHY on gods earth would I spend $500 on a GPU, when I only spend $600 (ish) on my whole upgrade . . . and trust me, I'm not alone.

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