On Jan 4, 2011, at 2:19 AM, Michael Graff wrote: > On 1/4/11 2:26 AM, Matt Thomas wrote: >> Module Name: src >> Committed By: matt >> Date: Tue Jan 4 08:26:33 UTC 2011 >> >> Modified Files: >> src/sys/uvm: uvm_extern.h uvm_fault.c uvm_km.c uvm_page.c >> >> Log Message: > ... >> When allocating kernel memory pages, allow the MD to specify a preferred >> VM_FREELIST from which to choose pages. For machines with large amounts >> of memory (> 4GB), all kernel memory to come from <4GB to reduce the amount >> of bounce buffering needed with 32bit DMA devices. > > Sorry to be clueless here, but does this mean I lose my large, automatic > disk cache on my 32GB machine, or is this just for other types of memory > purposes?
Not really. A lot of device can only do 32bit DMA transfers so without some type assistance (like the alpha has) you are restricted to DMA to the first 4GB of RAM. If you are doing DMA to an address >= 4GB, the system will stage the data in a buffer < 4GB for the DMA and then move the data to its ultimate destination (for a read). For a write it copies to the "bounce" buffer, and then does a DMA. The point is if you are allocating a mbuf or a usb buffer, it makes sense to allocate from the first 4GB if you can to avoid using bounce buffers. It's just a preference.
