Eric Anholt (anholt) wrote,
Eric Anholt

This week in vc4 2016-09-13: glmark2 performance work

Last week I spent working on the glmark2 performance issues.  I now have a NIR patch out for the pathological conditionals test (it's now faster than on the old driver), and a branch for job shuffling (+17% and +27% on the two desktop tests).

Here's the basic idea of job shuffling:

We're a tiled renderer, and tiled renderers get their wins from having a Clear at the start of the frame (indicating we don't need to load any previous contents into the tile buffer).  When your frame is done, we flush each tile out to memory.  If you do your clear, start rendering some primitives, and then switch to some other FBO (because you're rendering to a texture that you're planning on texturing from in your next draw to the main FBO), we have to flush out all of those tiles, start rendering to the new FBO, and flush its rendering, and then when you come back to the main FBO and we have to reload your old cleared-and-a-few-draws tiles.

Job shuffling deals with this by separating the single GL command stream into separate jobs per FBO.  When you switch to your temporary FBO, we don't flush the old job, we just set it aside.  To make this work we have to add tracking for which buffers have jobs writing into them (so that if you try to read those from another job, we can go flush the job that wrote it), and which buffers have jobs reading from them (so that if you try to write to them, they can get flushed so that they don't get incorrectly updated contents).

This felt like it should have been harder than it was, and there's a spot where I'm using a really bad data structure I had laying around, but that data structure has been bad news since the driver was imported and it hasn't been high in any profiles yet.  The other tests don't seem to have any problem with the possible increased CPU overhead.

The shuffling branch also unearthed a few bugs related to clearing and blitting in the multisample tests.  Some of the piglit cases involved are fixed, but some will be reporting new piglit "regressions" because the tests are now closer to working correctly (sigh, reftests).

I also started writing documentation for updating the system's X and Mesa stack on Raspbian for one of the Foundation's developers.  It's not polished, and if I was rewriting it I would use modular's instead of some of what I did there.  But it's there for reference.
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