I hope someone can shed some light on this. I'm running 5.0-current on an AMD64 with 4GB of physical memory.
Reading large chunks (64K or multiples) from /dev/rsd0c using the AMD chipset SATA controller and a modern 1G drive: time dd if=/dev/rsd3c of=/dev/null bs=128k count=10000 10000+0 records in 10000+0 records out 1310720000 bytes transferred in 11.348 secs (115498831 bytes/sec) 0.0u 2.0s 0:11.34 17.9% 0+0k 0+1io 0pf+0w Profiling the kernel shows that copyout() is being called from physio() via uvm_vsunlock_device() for every MAXPHYS byte chunk. On first inspection, physio calls uvm_vslock_device(..., &map) which checks to see if all pages in the request satisfy PADDR_IS_DMA_REACHABLE(). If so, it returns NULL in map. After strategy() returns, map is sent to uvm_vsunlock_device, which calls copyout() if map != NULL. There's a comment on uvm_vslock_device saying it always returns something in *retp, but the code seems to indicate otherwise. PADDR_IS_DMA_REACHABLE checks against dma_constraint, which is 0..0xffffffff which should allow all memory < 4G to be used for DMA. What have I missed? I believe that the copyout() shouldn't happen. I'm trying to run multiple 140MB/sec drives simultaneously and the copyout() is a killer - it's eating more of the system memory and CPU bandwidth than I'd like. thanks Geoff Steckel
