> > > > 1. Why is the GTT->VRAM copy so much slower than the VRAM->GTT > > copy? > > > > 2. Why is the bus limited to 24 Gbit/sec? I would expect the > > Thunderbolt port to give me at least 32 Gbit/sec for PCIe traffic. > > That's unrealistic I'm afraid. As I said on IRC, from the GPU POV > there's an 8 GT/s x4 PCIe link, so ~29.8 Gbit/s (= 32 billion bit/s; > I > missed this nuance on IRC) is the theoretical raw bandwidth. However, > in > practice that's not achievable due to various overhead[0], and I'm > only > seeing up to ~90% utilization of the theoretical bandwidth with a > "normal" x16 link as well. I wouldn't expect higher utilization > without > seeing some evidence to suggest it's possible. > > > [0] According to > https://www.tested.com/tech/457440-theoretical-vs-actual-bandwidth-pci-express-and-thunderbolt/ > , PCIe 3.0 uses 1.54% of the raw bandwidth for its internal encoding. > Also keep in mind all CPU<->GPU communication has to go through the > PCIe > link, e.g. for programming the transfers, in-band signalling from the > GPU to the PCIe port where the data is being transferred to/from, ... Good point, I used 1024 and not 1000. My mistake. There is something else: In the same benchmark there is a "fill->GTT ,SDMA" row which has a 4035 MB/s number. If that traffic goes through the TB3 interface then we just found our 32 Gbit/sec. Now the question is, if I understand this correctly and the SDMA can indeed do 32 Gbit/sec for "fill->GTT", then why can't it do the same with other kinds of transfers? Not sure if there is a good answer to that question though. Also I still don't fully understand why GTT->VRAM is slower than VRAM- >GTT, when the bandwidth is clearly available. Best regards, Tim Side note: with regards to that 1.5% figure, the TB3 tech brief[0] explicitly mentions this and says that it isn't carried over: "the underlying protocol uses some data to provide encoding overhead which is not carried over the Thunderbolt 3 link reducing the consumed bandwidth by roughly 20 percent (DisplayPort) or 1.5 percent (PCI Express Gen 3)" [0] https://thunderbolttechnology.net/sites/default/files/Thunderbolt3_TechBrief_FINAL.pdf _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
