On 31/08/16 11:21 PM, Jason Ekstrand wrote: > On Aug 19, 2016 12:07 AM, "Michel Dänzer" <mic...@daenzer.net > <mailto:mic...@daenzer.net>> wrote: >> From: Michel Dänzer <michel.daen...@amd.com > <mailto:michel.daen...@amd.com>> >> >> Always use 3 buffers when flipping. With only 2 buffers, we have to wait >> for a flip to complete (which takes non-0 time even with asynchronous >> flips) before we can start working on the next frame. We were previously >> only using 2 buffers for flipping if the X server supports asynchronous >> flips, even when we're not using asynchronous flips. This could result >> in bad performance (the referenced bug report is an extreme case, where >> the inter-frame stalls were preventing the GPU from reaching its maximum >> clocks). > > Sorry for the post-push review but I don't usually pay much attention to > the window system code. In any case, I believe you're doing your > counting wrong. When flipping with swapinterval=0, you need 4 buffers: > > 1. The buffer currently being scanned out (will be released at next vblank) > 2. The buffer X has queued for scanout but is waiting on vblank
s/vblank/flip/g, since async flips may not wait for vblank, but yeah. > 3. The buffer the application has just submitted which X will queue next > of it doesn't get another before the window closes. > 4. The buffer the application is using for rendering. > > With only 3, you get a stall during that window in which X has queued > another flip but we're waiting on vblank before the flip begins. An I > missing something? Nothing, except maybe the paragraph below stating that I couldn't measure any benefit from using 4 buffers. :) I'm not exactly sure why, but I suspect it might be because even with just 3 buffers, the GPU can always work on at least one frame ahead of time. Also note that even before my change, we were only using 3 buffers when the X driver supports async flips (with swap interval 0; only 2 buffers with swap interval > 0). That said, I'd be interested in hearing about any test cases where 4 buffers provide a significant boost over 3. >> I couldn't measure any performance boost using 4 buffers with flipping. >> Performance actually seemed to go down slightly, but that might have >> been just noise. -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Mesa and X developer
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