Monday, May 12, 2014

The Truth on OpenGL Driver Quality

The driver landscape is something that any practicing GL dev must face unless you like having only a fraction of potential customers able to enjoy your product. (These are the drivers you'll have to work with in order to actually ship a product today or within the next year or so. If you're just a dev playing at home with one driver you'll probably not have to deal with any of this gritty real-world stuff.) 

If all you've ever done is use D3D then you better strap yourself in because the available GL drivers for Windows/Linux are all over the map. Here's my current opinion on driver quality:

Vendor A

What most devs use because this vendor has the most capable GL devs in the industry and the best testing process. It's the "standard" driver, it's pretty fast, and when given the choice this vendor's driver devs choose sanity (to make things work) vs. absolute GL spec purity. Devs playing at home use this driver because it has the sexiest, most fun to play with extensions and GL support. Most of what you hear about the amazing things GL will be able to do in order to compete against D3D12/Mantle are by devs playing with this driver. Unfortunately, we can't just target this driver or we miss out on large amounts of market share. 

Even so, until Source1 was ported to Linux and Valve devs totally held the hands of this driver's devs they couldn't even update a buffer (via a Map or BufferSubData) the D3D9/11-style way without it constantly stalling the pipeline. We're talking "driver perf 101" stuff here, so it's not without its historical faults. Also, when you hit a bug in this driver it tends to just fall flat on its face and either crash the GPU or (on Windows) TDR your system. Still, it's a very reliable/solid driver.

Vendor A supports a zillion extensions (some of them quite state of the art) that more or less work, but as soon as you start to use some of the most important ones you're off the driver's safe path and in a no man's land of crashing systems or TDR'ing at the slightest hickup.

This vendor's tools historically completely suck, or only work for some period of time and then stop working, or only work if you beg the tools team for direct assistance. They have enormous, perhaps Dilbert-esque tools teams that do who knows what. Of course, these tools only work (when they do work) on their driver.

This vendor is extremely savvy and strategic about embedding its devs directly into key game teams to make things happen. This is a double edged sword, because these devs will refuse to debug issues on other vendor's drivers, and they view GL only through the lens of how it's implemented by their driver. These embedded devs will purposely do things that they know are performant on their driver, with no idea how these things impact other drivers.

Historically, this vendor will do things like internally replace entire shaders for key titles to make them perform better (sometimes much better). Most drivers probably do stuff like this occasionally, but this vendor will stop at nothing for performance. What does this mean to the PC game industry or graphics devs? It means you, as "Joe Graphics Developer", have little chance of achieving the same technical feats in your title (even if you use the exact same algorithms!) because you don't have an embedded vendor driver engineer working specifically on your title making sure the driver does exactly the right thing (using low-level optimized shaders) when your specific game or engine is running. It also means that, historically, some of the PC graphics legends you know about aren't quite as smart or capable as history paints them to be, because they had a lot of help.

Vendor A is also jokingly known as the "Graphics Mafia". Be very careful if a dev from Vendor A gets embedded into your team. These guys are serious business.

Vendor B

A complete hodgepodge, inconsistent performance, very buggy, inconsistent regression testing, dysfunctional driver threading that is completely outside of the dev's official control. Unfortunately this vendor's GPU is pretty much standard and is quite capable hardware wise, so you can't ignore these guys even though as an organization they are idiots with software. Basic stuff like glTexStorage() crashes (on a shipped title) for months on end with this driver. B's driver devs try to follow the spec more closely than Vendor A, but in the end this tends to do them no good because most devs just use Vendor A's driver for development and when things don't work on Vendor B they blame the vendor, not the state of GL itself.

Vendor B driver's key extensions just don't work. They are play or paper extensions, put in there to pad resumes and show progress to managers. Major GL developers never use these extensions because they don't work. But they sound good on paper and show progress. Vendor B's extensions are a perfect demonstration of why GL extensions suck in practice.

This vendor can't get key stuff like queries or syncs to work reliably. So any extension that relies on syncs for CPU/GPU synchronization aren't workable. The driver devs remaining at this vendor pine to work at Vendor A. 

Vendor B can't update its driver without breaking something. They will send you updates or hotfixes that fix one thing but break two other things. If you single step into one of this driver's entrypoints you'll notice layers upon layers of cruft tacked on over the years by devs who are no longer at the company. Nobody remaining at vendor B understands these barnacle-like software layers enough to safely change them.

I've occasionally seen bizarre things happen on Vendor B's driver when replaying GL call streams of shipped titles into this driver using voglreplay. The game itself will work fine, but when the GL callstream is replayed we'll see massive framebuffer corruption (that goes away if we flush the GL pipeline after every draw). My guess: this driver is probably using app profiles to just turn off entire features that are just too buggy.

Interestingly, Vendor B has a tiny tools team that actually makes some pretty useful debugging tools that actually work much of the time - as long as you are using vendor B's GPU. Without Vendor B's tools togl and Source1 Linux would have taken much longer to ship.

This could be a temporary development, but Vendor B's driver seems to be on a downward trend on the reliability axis. (Yes, it can get worse!)

On the bright side, and believe it or not, Vendor B knows the OpenGL spec inside and out - to the syllable. If you can get them to assist you, their advice is more or less reasonable about plain GL matters (not extensions).

Vendor C - Driver #1

It's hard to ever genuinely get angry at Vendor C. They don't really want to do graphics, it's really just a distraction from their historically core business, but the trend is to integrate everything onto one die and they have plenty of die space to spare. They are masters at hardware, but at software they aren't all that interested really. They are the leaders in the open source graphics driver space, and their hardware specs are almost completely public. These folks actually have so much money and their org charts are so deep and wide they can afford two entirely different driver teams! (That's right - for this vendor, on one platform you get GL driver #1, and another you get GL driver #2, and they are completely different codebases and teams.)

Anyhow, this vendor's HR team is smart: it directly hires open source wiz kids to keep driver #1 plodding forward. This driver is the least advanced of the major drivers, but it more or less works as long as you don't understand or care what "FPS" means. If it doesn't work and you're really motivated you can git your hands dirty and try to fix it and submit a patch. If you're really good at fixing this driver and submitting patches then you may get a job offer from this vendor.

Anyhow, driver #1 is unfortunately pretty far behind on the GL standard, but maybe in 1-2 years they'll catch up and implement the spec as of last year. But you can't ignore this driver because they have a significant and strategically growing market share. So as a developer who wants to reach this market, you can't afford to use those fancy extensions or the latest trendy "modern" GL supported by vendors A and B. You must do a min() operation across all the drivers and in many cases this driver gates what you can do.

Vendor C has no GL tools at all for either platform. Sorry - want to debug that graphics problem you're having? Welcome to 1999.

Vendor C - Driver #2

A complete disaster. This team's driver is barely used by any titles because GL on this platform is totally a second class citizen, so many codepaths in there just don't work. They can't update a buffer without massive, random corruption. This team will do stuff like give you a different, unique, buggy driver drop for every title in your back catalog for perf analysis or testing. This team will honestly ask you if "perf" or "correctness" is more important.

I've seen one well-known engine team spend over a year attempting to get their latest GL 4.x+trendy extensions backend working at all on this team's driver. Hey guys - this driver just doesn't work, just move on already and implement a plain GL 3.x backend with workarounds (just like togl and other shipping titles do today).

On the bright side, Vendor C feeds this driver team more internal information about their hardware than the other team. So it tends to be a few percent faster than driver #1 on the same title/hardware - when it works at all.

Other drivers:

In addition to the above major drivers, there are several open source drivers, mostly developed by the community, for hardware from vendors A and B. They tend to be behind the times from a GL perspective, but I hear they mostly work. I don't have any real experience or hard data with these drivers, because I've been fearful that working with these open source/reverse engineered drivers would have pissed off each vendor's closed source teams so much that they wouldn't help.

Vendor A hates these drivers because they are deeply entrenched in the current way things are done. These devs have things like mortgages and college funds (or whatever) to keep funding, so there's a massive amount of inertia from this camp. There's no way they are going to release their Top Secret GPU Specs to the public, or (gasp!) open source their driver. Vendor A will have to jump on the open source driver bandwagon soon in order to better compete against Vendor C's open model, whether they like it or not.

Vendor B halfheartedly helps their open source driver by funding a tiny team to keep the thing working. At some point, the open source driver for Vendor B's GPU may be a more viable path forward then their half-functional closed source driver.

Conclusion

To ship a major GL title you'll need to test your code on each driver and work around all the problems. May the "GL Gods" help you if you experience random GPU corruption, heap corruption, lockups, or TDR's. Be very nice to the driver teams and their managers/execs, because without them your chances aren't nearly as good.


Sunday, May 11, 2014

Things that drive me nuts about OpenGL

Here's a brain dump of the things that sometimes drive me crazy about OpenGL. (Note these are strictly my own opinions, not those of Valve or my coworkers. I'm also in a ranty-type mood today after grappling with OpenGL for several years now..) My major motivation to posting this: the GL API needs a reboot because IMO Mantle/D3D12 are going to most likely eat it for lunch soon, so we should start talking and thinking about this stuff now.

Some are minor issues, and some are specific to tracing the API, but all these issues add up to API "friction" that sometimes make it difficult to encourage other devs to get into the GL API or ecosystem:

1. 20 years of legacy, needs a reboot and major simplification pass
Circle the wagons around a core-style API only with no compatibility mode cruft.
Simplify, KISS principle, "if in doubt throw it out"!
Mantle and D3D12 are going to thoroughly leave GL behind (again!) on the performance and developer "mindshare" axes very soon.
Global context state and the binding pattern sucks. The DSA (direct state access)-style API should be standard/required.

Some bitter medicine/tough love: Most devs will take the easy path and port their PS4/Xbone rendering code to D3D12/Mantle. They will not bother to re-write their entire rendering pipeline to use super-aggressive batching, etc. like the GL community has been recently recommending to get perf up. GL will be treated like a second-class citizen and porting target until the API is modernized and greatly simplified.

2. GL context creation hell:
Creating modern GL contexts can be hair raisingly and mind numbingly tricky and incredibly error prone ("trampoline" contexts anyone?). The process is so error prone, and platform (and occasionally even driver) specific that I would almost always recommend to never go directly to the glX, wgl, etc. API's, and instead always use a library such as SDL or GLFW (and something like GLEW to retrieve the function/extension pointers).

The de-facto requirement to always pick from a small set of large 3rd party libraries just to get a real context rolling sucks. The API should be simplified and standardized so using a 3rd party lib shouldn't be a requirement just to get a real context going.

3. The thread's current context may be an implied "this" pointer:
Function pointers returned by GetProcAddress() cannot (or should not - depending on the platform!) be used globally because they may be strongly tied to the context ("context-dependent" vs. "context-independent" in GL-speak). In other words, calling GetProcAddress() on one context and using the returned func pointer on another is either bad form or just crashes.
So is GL a C API or not?
Can we just simplify and standardize all this cruft?

4. glGet() API deficiencies:
This is probably too tracing specific, but it impacts regular devs indirectly because if the tools suck or are non-existent because the API is hard to trace your life as a developer will be harder.
The glGet() series of API's (glGetIntegerv, glGetTexImage, etc.) don't have a "max_size" parameter, so it's possible for the driver to overwrite the passed in buffer depending on the passed in parameters or even the global context state. These functions should accept a "max_size" parameter and the functions should fail if the supplied max_size is too small, not overwrite memory.
Computing the exact size of texture buffers the driver will read or write depends on various global context state - bad bad bad.
There are hundreds of possible glGet() pname enum's, some accepted by only some drivers. If you're writing a tracer or some sort of debug helper, there is no official way to determine how many values will be written by the driver given a specific pname enum. There are no official tables to determine if the indexed variants of glGet() can be used with a specified enum, or determine the optimal (lossless) type to use given a specific enum.
Also, the behavior of indexed vs. non-indexed gets & sets is not always clear to new users of the API.
Alternately, perhaps just add some glGet() metadeta API's vs. publishing tables.

5. glGetError()
There is no glSetLastError() API like Win32, making tracing needlessly complex.
Some apps never call it, some call it once per frame, some only call it while creating resources. Some call it thousands of times at init, and never again. I've seen major shipped GL apps with per-frame GL errors. (Is this normal? Does the developer even know?)

6. Can't query key things such as texture targets
(I know some of this is being worked on - thanks Cass!) This makes tracing/snapshotting more complex due to shadowing.
Shadowing deeply interacts with glGetError()'s (we can't update our shadow until we know the call succeeded, which involves a call to glGetError(), which absorbs the context's current GL error requiring even more fancy footwork to not diverge the traced app's view of GL errors).

About the recent talk of getting rid of all glGet()'s: IMO either all state should be queryable (which is almost the case today), or the API should be written with maximum perf and scalability in mind like D3D12/Mantle. The value added by the API is clearly understood in either of these extremes.
Getting rid of glGet()'s will make writing tracers & context snapshotters even trickier.

7. DSA (Direct State Access) API variants are still not standard and used/supported everywhere
DSA can make a huge difference to call overhead in some apps (such as Source1's GL backend). Just get rid of the reliance on global state, please, and make DSA standard for once and for all.

8. Official spec is still not complete in 2014:
The XML spec still lacks strongly typed param information everywhere. For example:

 <command>
    <proto>void <name>glBindVertexArray</name></proto>
    <param><ptype>GLuint</ptype> <name>array</name></param>
    <glx type="render" opcode="350"/>
  </command>

apitrace's glapi.py is still the only known public, reliable source of this information that I know of:

  GlFunction(Void, "glBindVertexArray", [(GLarray, "array")]),

Notice how glapi.py defines the type as "GLarray", while the official spec just has the nondescript "GLuint" type.

Add glGet info() to official spec: Mentioned above. How many values does the pname enum return? What are the optimal types to use to losslessly retrieve the driver's shadow of this state? Is the pname ok to use with the indexed variants?

9. GLSL version of the week hell:
For early versions, the GLSL version may not sync up with the GL version it was first defined in, making things even more needlessly confusing. And this is before you add in things like GLSL extensions (*not* GL extensions). Can be overwhelming to beginners.

10. No equivalent of standard/official D3DX lib+tools for GL:
Texture/pixel format conversion helpers that don't rely on using the driver or a GL context
KTX format interchange hell: The few tools that read/write the KTX format (GL's equivalent of DDS) can't always read/write eachother's files.
Devs just need the equivalent of Direct3D's DXTEX tool, with source.
The KTX examples just show how to load a KTX file into a GL texture. We need tools to convert KTX files to/from other standard formats, such as DDS, PNG, etc.
A GLSL compiler should be part of this lib (just like you can compile HLSL shaders with D3DX).

11. GL extensions are written as diffs vs the official spec
So if you're not a OpenGL Specification Expert it can be extremely difficult to understand some/many extensions.

Related: The official spec is written for too many audiences. Most consumers of the spec will not be experts in parsing it. The spec should be divided up into a developer-friendly spec vs a deeper spec for the driver writers. Extensions should not be pure delta's vs. the spec - who can really understand that?

12. Documentation hell
We've got 20 years of GL API cruft out there that adds noise to Google searching for GL API information, and beginners can get easily tripped up by bad/legacy documentation/examples.

13. MakeCurrent() hell
Can be extremely expensive, hidden extra variable cost with some extensions (I'm looking at you NV bindless texturing!), can crash drivers (or even the GPU!) if called within a glBegin/glEnd bracket, etc.
The behavior and performance of this call needs to be better specified and communicated to devs.

14. Drivers should not crash the GPU or CPU, or lock up when called in undefined ways via the API
Should be obvious by now. Please hire real testers and bang on your drivers!
Better yet: Structure the API to minimize the # of undefined or unsafe patterns that are even possible to express via the API.

15. Object deletion with multiple contexts, cross-context refcounting rules, "zombie" objects:
Good luck if the object being deleted is currently bound on another context.
Trying to call glGet()'s on a deleted object (that is still partially "live" because it's bound or attached somewhere) - behavior can differ between drivers.
All of this is needless overhead/complexity IMO.
Makes 100% reliable snapshotting and restoring GL context state very, very difficult.
I see world-class developers screw this up without knowing it, which is a clear sign that the API and/or tool ecosystem is broken.

16. Shader compiling/program linking hell
Major performance implications to shader compiling/linking.
Tokenized shader programs work. Direct3D is the existence proof that this approach works. The overall amount of pain GLSL has caused developers porting from D3D and end users (due to slow load times) is incredible, yet GL still only accepts textual GLSL shaders.
Performance drastically varies between drivers. Shader compiling can be effectively a no-op on some drivers, but extremely expensive on others.
Program linking can take *huge* amounts of time.
Some drivers cache linked programs, some don't.
Program linking time can be unpredictable: fast if the program is cached, but there's no way to query if the program is already cached or not. Also no way to query if the driver even supports caching.
Some drivers support threaded compilation, some don't. No way to query if the driver supports threaded compilation.
Some drivers just deadlock or have race conditions when you try to exploit threaded compilation.
Just a bad API, making it hard to trace and snapshot: Shaders can be detached after linking. Lots of linked program state is just not queryable at all, requiring link time shadowing by tracers.
Just copy & paste what D3D is doing (again, it works and devs understand it).

17. Difficult API to trace, replay, and snapshot/restore
Hurts tool ecosystem, ultimately impacts all users of API.
API should either be written to be easily traced/replayed/snapshotted, or incredibly performant/scalable like Mantle/D3D12. Right now GL has none of these properties, putting it in a bad spot from a value proposition perspective.
API authors should focus more on VALUE ADDED and less on how things should work, or how we are different from D3D because we're smarter.

18. Endless maze of GL functions (thousands of them!)
Hey - do we really need dozens of glVertexAttrib variants? Who really even uses this API?
API needs a reboot/simplification pass. Boost the "signal to noise" ratio, please.

19. Legacy complexities around v3.x API transition:
"Forward compatible", "compatibility" vs. "core" profiles etc. etc. etc.
Devs should not have to master this stuff to just use the API to render shaded triangles.
"Core" should not even be in the lexicon.

20. Reliably locking a buffer with DISCARD-semantics on all drivers without stalling the pipeline:
Do you use a map flag? BufferData() with NULL? Both, either, etc.?
What lock flag or flags do you use? Or does the driver just completely ignore the flag?
Trivial in D3D, difficult to do reliably in GL without being an expert or having direct access to driver developers.
This stuff is incredibly important!
Special note to driver developers: What separates the REAL driver devs from wannabees is how well you implement and test stuff like this. Pipeline stalling is not an option in 2014!

21. BufferSubData() stalls when called with "too much" data on threaded drivers
No way to query what "too much" data is. Is it 4k? 8k? 256k?

22. Pipeline stalling
No official (or any) way to get a callback or debug message when the driver decides to throw up its hands and insert a giant pipeline stall into your rendering thread
This can be the #1 source of rendering bottlenecks, yet we still have almost zero tools (or API's to help us build these tools) to track them down

23. Threaded drivers hell
Some manufacturers decide to forceably auto-enable their buggy multithreaded drivers months after titles have been shipped & thoroughly tested by the developer. (And to make matters worse, they do this without informing the developer of the "app profile" change or additions.)
Some multithreaded drivers have buggy glGet()'s when threading is enabled - makes snapshotting a nightmare.
No official way to query or control whether or not the driver will use multithreading.
No way to specify to the driver that a tracer is active which may issue a lot of glGet()'s (that the app would not normally do)
Bone headed threaded drivers that slow to an absolute crawl and stay there when an app or tracer constantly issues glGet()'s (just use a heuristic and automatically turn threading off!)

24. Timestamp queries can stall the pipeline on some drivers
Makes them useless for cross platform, reliable GPU profiling. GL spec should strongly define when the driver is allowed to stall on these queries. Unnecessary stalling should be redefined as a driver bug (by sometimes lazy/incompetent driver developers who don't understand how key these little API's can be).
For reference, NVidia does this stuff correctly. If you are a driver writer working on pipeline query code, please measure your implementation vs. NVidia's driver before bothering to release it.

25. GL is really X different API's (one per driver, sometimes per platform!) masquerading as a single API.
You can't/shouldn't ship a GL product until after you've thoroughly tested for correctness and performance on all drivers (in both threaded and non-threaded modes). You will be surprised at the driver differences. This came as a big shock to me after working for so long with D3D.
This indicates to me that Khronos needs to be more proactive at testing and validating the drivers. GL needs the equivalent of the WHQL process.

26. Extension hell
One of the touted advantages of GL is its support for extensions. I argue that extensions actually harm the API overall, not help it.

I've been through a large amount of major and minor GL callstreams (intricately!) over the previous ~1.5 years. (Before that I was the dev actually making togl work and be shippable on all the drivers. I can't even begin to communicate how difficult and stressful that process was 2+ years ago.) Excluding the driver devs I've probably worked with more real GL callstreams than most GL devs out there. Sadly, in many cases, some to many of the most advanced "modern" extensions barely work yet (and sometimes vendors will even admit this fact publicly). Or, if you try to use a cool-sounding extension you quickly discover that you're pushing a little-used (and tested) path down the driver and the thing is useless for all practical purposes.

From studying and working with the callstreams, it's apparent that devs do a massive MIN() operation across the functionality implemented on the available/targeted drivers. This typically means core profile v3.x, maybe also a 4.x backend with very simple/safe operations. (Or it's a small title that just uses compatiblity GL like it was still 1998 or 2003 - because that's all they need.) They don't bother with most extensions (especially the "modern" ones) because they either don't work reliably (because the driver paths that implement them are not tested on real apps at all - the classic chicken and egg problem), or they are only supported (sometimes barely) by 1 driver, or the value add just isn't there to justify expanding the product testing matrix even more.

Additionally, some of these modern extensions are very difficult to trace, which means that whatever debugging tools employed by the developer aren't compatible with them. So you need a fallback anyway, and if the devs must implement a fallback they might as well just ship the fallback (which works everywhere) and not worry about the extension (unless it adds a significant amount of value to the product).

So unless it's non-extended GL it might as well not be there to a large number of devs who just want to ship a reliable/working product.

Wednesday, May 7, 2014

Replay Divergence Hell

We've had a handful of traces in vogl that don't replay correctly hanging around in our regression test suite. One g-truc sample (gl-320-fbo-blit) was randomly failing -- turns out it wasn't clearing the backbuffer every frame. It was rendering a checkerboard of quads, so half the pixels in the backbuffer were not being written. Sometimes it would play back seemingly correctly (black pixels where quads weren't being rendered), and sometimes we would see random-looking bits in there.

Anyhow, I'm now trying to figure out why the g-truc sample gl-330-blend-rtt diverges when replayed with vogl. It's also randomly failing. Beyond Compare's image comparison mode can be pretty helpful in cases like this.


Update: OK, I found the problem. The sample uses a FBO with 3 texture attachments, but it was only clearing the first one in display(). The fix is simple:

for (int i = 0; i < 3; i++)
  glClearBufferfv(GL_COLOR, i, &glm::vec4(1.0f)[0]);