Wednesday, 27 August 2014

egpu mk ii.5

Well that took a bit longer than I wanted; and all i've done is rejigged all the comms around but that's enough for today.

I made a bunch of changes to address some of the problems; i'm still not sure it will fix the performance but it's some stuff I wanted to look at anyway. The big performance issue remaining is the rasteriser to fragment processor stream; I have a new communication protocol that addresses it as much as possible and have changed the fragment processor to use it but I haven't written the rasteriser to feed it yet. I was going to do a quick-and-dirty but that would just be wasted work and working toward the current target goal ended up ballooning out into a big pile of changes.

  • I've changed the 4xtile geometry to 1x4 = 64x32 rather than 4x1 = 256x8. It made sense at the time. I'm hoping this simplifies the work of generating fragments in the right order but if nothing else it should divvy up the work more evenly.

  • I'm probably going to interleave the 4x64x8 tiles; the whole-row rasteriser showed that this distributes the work-load more effectively than other approaches, and maybe it will simplify the rasterisation loop as it needs to interleave output for effective streaming.

  • I'm creating a 2D index based on 64x64 tiles - this can be tuned a bit but very quickly chews memory depending on what limits i set (then again, that 32MB of shared ram isn't doing anything else atm). I'm just using the bounding box but it is quite simple and efficient to use the edge equations to make it exact to the index resolution.

  • The controller now assigns tiles to rasterisers and dynamically schedules across all of them. This is probably the most interesting change and once I had the 2D index (which is trivial) wasn't much more work than the static assignment.

    It scans across the playfield and assigns tiles to each rasteriser (3 in the current design) in turn. It then tries to keep each full of work by feeding them commands and primitives loaded from main memory using round-robin scheduling and non-blocking writes (i.e. it skips that rasteriser if it would block). If any rasteriser runs out of primitives it immediately flushes that one off and re-assigns it a new tile if there are any left - and then goes back to the stuffing loop.

    I took the opportunity to batch, double-buffer, and dma everything where appropriate, and ensure that nothing can block anything else. So if this is still a performance problem I'm out of ideas. This same controller can obviously be used if the tile topology changes or if i return to unified rasterisers+fragment processors if i am ultimately unable to improve the performance of the split one sufficiently.

    I've got a feeling that's where i'll end up.

  • I decided to create an async dma mechanism that just stores a pointer to the dma record rather than storing dma records in-line in the queue. Only needed two changes to the assembly (shift by 2 rather than 5, change an add to ldr). I haven't tested this yet but once I have confirmed it works it's likely to replace the current async dma implementation because it can add a lot of flexibility (chained, pre-calculated, scatter-gather, etc) while retaining interoperability the current api.

    I also needed a routine I could call in-line otherwise trying to call the current api from inside the rasteriser y-loop was going to spill all the registers I just spent all that code filling up with useful numbers. Since I can pre-calculate much of the DMA header into reusable blocks this should save some runtime overhead as well as I just need to update the addresses and go.

  • I changed the fragment processor protocol to take specific batched blocks rather than trying to batch smaller units across a cyclic buffer. The latter is more space efficient but gets messy when using DMA to copy the blocks in. This new design simplifies some of the logic and since I need to use DMA for any non-trivial copies makes this practical to implement.

    But I'm not entirely happy with the design so far though because to support asynchronous DMA operation I had to add a sentinal word which is written using chained DMA to notify the caller that it is ready. I use an eport to arbitrate the target location but it still needs to poll this ready indicator. This was the initial reason I needed the new async dma capability. Given that eports are probably going to need DMA more than I thought they would I might look at creating a combined "equeue" that hides some of these details and removes the need for the ready indicator (it can just update the remote head value).

Hmm, so what was again going to be a short little poke turned into a whole afternoon and now the sun is rapidly leaving this hemisphere to a crisp but cold evening. This stuff is just too interesting to put down and i've just spent another hour and a half writing this and tweaking a few things I found while writing it. Might keep going now ...

Update: Hacked into the later evening ... did some profiling. It's about half the speed of the combined by-line processor at this point. Whilst this is a very large improvement as to where it was, it's obviously not enough.

From some numbers I think the bottleneck is the rasteriser. The rasteriser routine is very simple and compiles quite well and the dma interface is about as minimal as possible so there is little possibility of improvement. It's probably just the 1:4 fan-out being too much.

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