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OpenGL glTexSubImage2D - Massive Performance Drop Off

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I have a group of textures (30-40) most of which need to be updated every frame. The textures are 64x512 and RGBA internal format (with BGRA format).

Not all of the texture rectangle needs to be updated, and the size of the update rectangle is calculated from other objects and variables in the scene. It's like a dynamic texture atlas where none, some or all of the sub-textures in the atlas may be updated. And there are 30-40 of these atlasses.

Using glTexSubImage2D to update the texture causes an unacceptable performance drop off. Without updating a frame takes 2-3 milliseconds to draw, with updating it takes 5 times (or more) longer. That's just from the glTexSubImage2D calls, everything else is the same. Add in all of the other processing that goes on per frame, including sound, game logic, etc, and we're talking framerates dangerously close to single-digit.

Switching to a PBO doesn't help, and the end result is the very same - just comment out the glTexSubImage2D call and performance jumps by a factor of 5 or more. The presence or absence of a PBO is irrelevant here.

I've tried square textures, resizing the textures, checking my GL_UNPACK_ALIGNMENT, checking that formats are consistent with the glTexImage2D call that created the texture, and so on.

We can rule out "your hardware is rubbish" because the very same process in a D3D app that does the very same thing (it's actually the D3D version of the app) doesn't suffer from the falloff. So we know that the hardware is capable of doing it.

We can rule out "your OpenGL driver is rubbish" (it is Intel, after all) because I've cross-checked it with an NVIDIA card and the very same thing happens there.

We can rule out "the rest of your code is rubbish" because just commenting out a single line (the glTexSubImage2D call) is all that's needed to remove the drop-off. But then of course the textures don't get updated.

It seems pretty much definitive that glTexSubImage2D is the bottleneck here.

So what on earth could possibly be going on? What other factors could possibly cause glTexSubImage2D to perform like a tractor in 6-foot deep mud?

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glTexSubImage2D should be slow but i don't know how slow is normal. What happens is that memory from RAM is delivered to GPU ram and this information has to travel from one place to another. This takes time and is bandwidth limited. Can you post the relevant portion of the code that updates the textures both opengl version and directx version, please?

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I'm not too worried about it being slower than not doing it at all, it's how slow it is that's a problem, and compared to D3D we're deep in silly country without a map.

Anyway, the Direct3D version looks like this. Backup texture is in D3DPOOL_SYSTEMMEM, main texture is in D3DPOOL_DEFAULT.
this->d3d_BackupTexture->LockRect (0, &this->d3d_LockedRect, NULL, D3DLOCK_NO_DIRTY_UPDATE);

// fill in data

this->d3d_BackupTexture->UnlockRect (0);
this->d3d_BackupTexture->AddDirtyRect (&this->DirtyRect);
d3d_Device->UpdateTexture (this->d3d_BackupTexture, this->d3d_MainTexture);

GL version looks like this:

// map buffer and fill in data

glBindTexture (GL_TEXTURE_2D, MainTexture);

// top and bottom are the wrong way around but that's just for consistency
// glPixelStorei has been set to make this work as expected
(void *) (DirtyRect->top * 512 * 4 + DirtyRect->left * 4)


Interestingly, I've tested the standalone pboUnpack program from here: http://songho.ca/opengl/gl_pbo.html on a number of machines, with the same result (reinforcing my suspicion that it's not my code at fault), and one consistent result is that the problem with glTexSubImage2D does NOT happen on machines with downlevel hardware (older Intel chips like the 915 or 945, for example). In it's mode 0 (no PBO, standard glTexSubImage2D from system memory) the timing differences are in the order of 2ms for downlevel hardware vs 78ms for modern hardware. This uses a 1024x1024 texture and updates the full thing each frame so the difference is more extreme, of course.

I'm wondering is there some state that needs enabling or disabling on more modern hardware to bring glTexSubImage2D back to it's older performance levels?

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Why do you need a PBO? are you making a framebuffer readback and then use this information to update the texture?
PBO's are generally used fast data transfer from the GPU RAM to system RAM via DMA but even this does not come for free. You have to carefully craft your application to take advantage of the PBO's.
More, on topic, i think you should post a little more of your code :D, regarding the OpenGL version, without PBO's and also, that's running slow.


I'm wondering is there some state that needs enabling or disabling on more modern hardware to bring glTexSubImage2D back to it's older performance levels?

Nope, don't think so.

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No specific reason for a PBO but it's not really relevant to the performance problem. It's glTexSubImage2D and it's consistent whether I use a PBO or not.

It's difficult to know what other code to post; it's established that the performance of glTexSubImage2D is definitely the cause here. I have for example ripped out my entire renderer and replaced it with this:

unsigned int sidata[512*64];

void TestSubImageTransfer (void)
int i;


updates = 0;
for (i = 0; i < 256; i++)
if (!dynamic_textures) break;

glBindTexture (GL_TEXTURE_2D, dynamic_textures);

glTexSubImage2D (GL_TEXTURE_2D, 0, 0, 0, 64, 512, GL_BGRA, GL_UNSIGNED_BYTE, sidata);


Nothing else OpenGL-wise is going on per-frame, the PBO is out of the equation, this is absolutely all that happens, it's my entire render function, and the results are perfectly consistent. Running glTexSubImage2D on 6 of these textures takes 15ms per frame, running it on 33 takes 87ms per frame.

Reducing the size of the update region can help, but it's still the case that the function is too slow and that this doesn't happen with the D3D version or with the GL version on downlevel hardware.

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Original post by Fiddler
Have you enabled automatic mipmap generation?

No, I'm afraid not. Double-checked by disabling it explicitly anyway. I would have loved it if that was the cause.

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Make sure the internal texture format in glTexImage matches what you pass glTexSubImage (should be GL_RGBA in the first and GL_BGRA or GL_RGBA in the latter). If thatdoesn't help, try disabling pixel unpacking to see whether this makes a difference.

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Original post by Fiddler
Make sure the internal texture format in glTexImage matches what you pass glTexSubImage (should be GL_RGBA in the first and GL_BGRA or GL_RGBA in the latter). If thatdoesn't help, try disabling pixel unpacking to see whether this makes a difference.

Formats match, pixel unpacking already set. I've also double checked that I'm not using a compressed format. Drivers updated too with no difference; besides tests have indicated that it's hardware-neutral.

It may be relevant: another differentiating factor is Windows 7: the perf drop doesn't happen with XP (it is however definitely hardware-neutral on 7). I've already tested with the DWM switched off though.


Phew! Resolved.

Lots and lots of small glTexSubImage2D updates actually run much much faster than a few big ones. The exact opposite technique to almost everything else, in other words. Who woulda ever thought?

[Edited by - mhagain on July 20, 2010 4:41:23 PM]

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Internal Format: GL_RGBA
Format: GL_BGRA

Problem completely goes away.

I suspect that the driver was pulling the teximage data back to system memory otherwise.

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