AGP FastWrite לא עובד...

[GuyGesh]

טוב לגבי שאלתי המקורית - חשיבות ה- FastWrite התשובה המלאה ניתנה ב- RAGE3D ע"י darkmage :

"

I posted this like 4-5 times since I registered here ( Dec/2001 ) since the fastwrites issue keeps popping up - so here goes again:

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Hi everyone and welcome to Darkmage´s Ultra-Boring "Fast Writes : A REALLY USELESS feature" Tutorial!

ACT 1 : NVidia´s Amazing Creation

Let me start saying that fast writes ARE NVidia´s creation, even though they´re part of AGP 2.0 specs; Back with my Geforce256 ( first card ever to support FW ) I used to dream how much better my games would run if I ever had a mobo with 4x AGP and Fast-Writes - Silly me

Now let´s picture the following , a system WITHOUT fast writes ;

Data will move this way :

CPU -> Chipset -> System Memory -> Chipset ---------> Graphics Card

***The chipset -> Graphics card move is done with AGP BUS***

So, everything MUST run through system memory, right ? Every data the CPU wants to transfer, must first go through memory, then reach graphics card by AGP.

So, if you have a system w/ 900mb/s bandwidth , and your agp4x can handle 1.06 gb/s , the agp bandwidth is actually wasted - no more than 900mb/s will ever be used. Fast writes will do the following :

CPU -------------------------------> Graphics Card

**This data moving process is done with AGP BUS**

Just that. All the bandwidth fully put to use, no bottlenecks.

( End of Act 1 )

Interlude :

At this point I ask you who are still reading this to take a look at Nvidia´s original paper, where I took all the info above from :

http://www.nvidia.com/view.asp?IO=T...al_Brief_AGP_4X

there´s a doc and a pdf there.

( End of Interlude )

Now, Act 2 : The Truth! - this time by myself

After reading all this, seems like fastwrites are perfect, fantastic and really needed for high performance. Hmmmm.

If you take a look at NVidia´s Paper, or even at my amazing ASCII art above, you´ll find out something weird, and that´s indeed the one reason that makes fast writes so useless in the real world, as everyone nowadays ( = years after NVIDIA introduced it ) notices:

EVERY SINGLE BYTE on the original NVIDIA example originates from the CPU. That´s not really how real-world applications works.

Unless we have CPU´s with a ... 5GB cache, the data itself STILL LIES on the Main Memory. So, here´s how data really moves without FW:

Main Memory --> CPU --> Chipset ---------------> Graphics Card

(Chipset-> Graphics Card uses AGP BUS)

OR, with fast writes :

Main Memory --> CPU ---------------------------> Graphics Card

(CPU -> Graphics Card with AGP BUS)

Not hard to realize the bottleneck between

Main Memory <-> CPU

WILL EXIST IN BOTH CASES.

Now, NVIDIA offers an OpenGL extension - wglAllocateMemoryNV() that allows you to store graphics data into AGP local memory. That helps, but then again you´re using valuable Video Card memory to store data that can stay on main memory...and in the end you may end fetching other data from main memory as well .

As for the small performance differences noticed from FW on/off , it´s two things :

1. CPU cache : If the data is there, fine, Fast Writes sends it straight to Video card. But most of the time it is not. And it´s not a lot of data, actually - how big is a CPU cache compared to a 64/128mb video card anyway ?

2. Static data/display lists, or Vertex Blended data - no need to fetch anything from memory save for an identifier, so that´s more effective.

So, in a few words:

1. NVIDIA seems to think CPU´s have 5gb caches , or they simply generate data from ether. I really can´t believe they did´t see that when designing Fast Writes back in ´99 ( correct year? )

2. If your video card has 512mb AND your API supports the local memory extension AND your favorite game uses it, you may see fast writes working.

3. The SLOWER the system memory, the bigger the improvement - if your system memory bandwidth matches your AGP bandwidth forget FW forever or build a slower system ... heheh.

4. Enough, I REALLY doubt there´s anyone still reading this !!

Dark "´Nuff typing for two years" Mage

אך השאלה האחרונה עדיין רלוונטית ואשמח אם מישהו יענה עליה :

מה ההבדלים בין ה- Catalist לבין הדריברים

שמצואים באתר של Hercules

[SpiritKnight]

הדריברים של ATI מתוכננים לצ'יפים, הדריברים של הרקולס מתוכננים לכרטיס.

עדיף שתוריד את ATI.

[R0ck]

פשוט תוריד מהלינק שהבאתי יא גיא... זה מיועד לכרטיסי PBA בלבד!