[Bug 105256] Slow performance using glDrawElementsBaseVertex

[Bug 105256] Slow performance using glDrawElementsBaseVertex

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

            Bug ID: 105256
           Summary: Slow performance using glDrawElementsBaseVertex
           Product: Mesa
           Version: 17.3
          Hardware: Other
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/r600
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: logan@bacoosta.com
        QA Contact: dri-devel@lists.freedesktop.org

Original application bug report here:

https://github.com/gonetz/GLideN64/issues/1561

This bug happens using mupen64plus (N64 emulator) and the GLideN64 graphics
plugin.

The application uses VBO streaming
(https://www.khronos.org/opengl/wiki/Buffer_Object_Streaming) using
ARB_buffer_storage to upload the VBO data.

Multiple users that when using Mesa + r600 driver, the performance was abysmal.
It turns out that converting the glDrawElementsBaseVertex calls to glDrawArrays
fixed the problem.

You can see the commit that fixed the problem here:

https://github.com/loganmc10/GLideN64/commit/9bcfa67d9550c7f1cd4ba72f657facd66a4d27e4

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[Bug 105256] Slow performance using glDrawElementsBaseVertex

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

--- Comment #1 from Roland Scheidegger <sroland@vmware.com> ---
At a quick glance, I'd suspect the problem isn't really drawElementsBaseVertex
per se, but the use of GL_UNSIGNED_BYTE indices as the code seemed to do.
The hw doesn't support ubyte indices, and this has to be emulated by converting
the elements to ushort (and if you don't start from zero, IIRC this can get
huge buffers allocated especially if start is negative, but I don't remember
the details).
I don't know if the driver could do better, but in general you're advised to
not use ubyte indices (this is a gl only feature, d3d doesn't support it, so
it's unsurprising some hw doesn't support it natively).
(Note that even early GCN does not support ubyte indices, they are only
supported starting with Vulcanic Islands.)

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[Bug 105256] Slow performance using glDrawElementsBaseVertex

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

--- Comment #2 from Logan McNaughton <logan@bacoosta.com> ---
Interesting, is this documented somewhere? How do you know what data formats
the HW supports? Is there any other hardware besides these cards that don't
support it?

I'll have the users test using GL_UNSIGNED_SHORT

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[Bug 105256] Slow performance using glDrawElementsBaseVertex

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

Logan McNaughton <logan@bacoosta.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |NOTABUG

--- Comment #3 from Logan McNaughton <logan@bacoosta.com> ---
The user confirmed that using GL_UNSIGNED_SHORT fixed the issue, thanks. I
would still be curious if there is some easy way to figure this out via
documentation or querying the device somehow.

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[Bug 105256] Slow performance using glDrawElementsBaseVertex

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

Roland Scheidegger <sroland@vmware.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|NOTABUG                     |FIXED

--- Comment #4 from Roland Scheidegger <sroland@vmware.com> ---
(In reply to Logan McNaughton from comment #2)
> Interesting, is this documented somewhere? How do you know what data formats
> the HW supports? Is there any other hardware besides these cards that don't
> support it?
> 
> I'll have the users test using GL_UNSIGNED_SHORT

I'm not sure if AMD (or ATI) had some perf guides published where this was
stated. You can, however, see this from the published register guides -
https://developer.amd.com/resources/developer-guides-manuals/
It says VGT_INDEX_16 and VGT_INDEX_32 as valid values for VGT_DMA_INDEX_TYPE
(for evergreen, others should be similar), interestingly however it's already a
2-bit value there.
(Actually the register guides only seem to go up to Sea Islands, so you can't
even see that it is supported starting with VI.)
A look at the mesa driver is probably easier though :-). This should affect all
amd/ati hw up to Sea Islands. Of course, if you use old-school non-vbo element
data, it's probably no big deal.
At a very quick glance at the driver code, it looks like newer nvidia chips can
handle it (everything from g80). nv30/nv40 cannot, but there it looks like nv30
will emulate index buffers anyway, so the only chips which this might make a
difference is nv40 family.
Looks like all intel chips can handle it fine.

That said, even if the slowdown is due to this, I'm not convinced the driver
couldn't do better (though it does not look trivial to do better). But it's one
of the features which noone really expects to get used, therefore it's good
enough as long as it works correctly...

You can't query this afaik and just have to know... Apparently using
GL_UNSIGNED_BYTE indices was simply broken at some point with windows drivers
too - it's possible the blob implements a more sophisticated emulation
strategy: https://community.amd.com/thread/159040

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[Bug 105256] Slow performance using glDrawElements calls with GL_UNSIGNED_BYTE indices

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

Roland Scheidegger <sroland@vmware.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Slow performance using      |Slow performance using
                   |glDrawElementsBaseVertex    |glDrawElements calls with
                   |                            |GL_UNSIGNED_BYTE indices

--- Comment #5 from Roland Scheidegger <sroland@vmware.com> ---
FWIW from a driver perspective this isn't really resolved fixed, I'd consider
that a bug. Using GL_UNSIGNED_BYTE indices will never be the optimal solution,
but clearly the driver could do better, as evidenced by the driver for some
other OS (I don't know what the driver does, but it could for instance maintain
a shadow copy, translate indices with a compute shader, the problem likely is
that the driver translates the indices on the cpu for each draw call
currently).

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[Bug 105256] Slow performance using glDrawElements calls with GL_UNSIGNED_BYTE indices

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

Logan McNaughton <logan@bacoosta.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |---

--- Comment #6 from Logan McNaughton <logan@bacoosta.com> ---
Fair enough, even a warning when MESA_DEBUG is set would be good, just
something to notify the developer what is wrong

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[Bug 105256] Slow performance using glDrawElements calls with GL_UNSIGNED_BYTE indices

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

--- Comment #7 from H4nN1baL <reizemberk@gmail.com> ---
Just as a constancy and this does not remain in oblivion, this is related to
this another: https://bugs.freedesktop.org/show_bug.cgi?id=102204

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[Bug 105256] Slow performance using glDrawElements calls with GL_UNSIGNED_BYTE indices

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https://bugs.freedesktop.org/show_bug.cgi?id=105256

GitLab Migration User <gitlab-migration@fdo.invalid> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |MOVED
             Status|REOPENED                    |RESOLVED

--- Comment #8 from GitLab Migration User <gitlab-migration@fdo.invalid> ---
-- GitLab Migration Automatic Message --

This bug has been migrated to freedesktop.org's GitLab instance and has been
closed from further activity.

You can subscribe and participate further through the new bug through this link
to our GitLab instance: https://gitlab.freedesktop.org/mesa/mesa/issues/631.

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