On Fri, May 29, 2015 at 2:29 PM, Elliott, Robert (Server Storage) <Elliott@xxxxxx> wrote: >> -----Original Message----- >> From: Andy Lutomirski [mailto:luto@xxxxxxxxxxxxxx] >> Sent: Friday, May 29, 2015 1:35 PM > ... >> Whoa, there! Why would we use non-temporal stores to WB memory to >> access persistent memory? I can see two reasons not to: > > Data written to a block storage device (here, the NVDIMM) is unlikely > to be read or written again any time soon. It's not like the code > and data that a program has in memory, where there might be a loop > accessing the location every CPU clock; it's storage I/O to > historically very slow (relative to the CPU clock speed) devices. > The source buffer for that data might be frequently accessed, > but not the NVDIMM storage itself. > > Non-temporal stores avoid wasting cache space on these "one-time" > accesses. The same applies for reads and non-temporal loads. > Keep the CPU data cache lines free for the application. > > DAX and mmap() do change that; the application is now free to > store frequently accessed data structures directly in persistent > memory. But, that's not available if btt is used, and > application loads and stores won't go through the memcpy() > calls inside pmem anyway. The non-temporal instructions are > cache coherent, so data integrity won't get confused by them > if I/O going through pmem's block storage APIs happens > to overlap with the application's mmap() regions. > You answered the wrong question. :) I understand the point of the non-temporal stores -- I don't understand the point of using non-temporal stores to *WB memory*. I think we should be okay with having the kernel mapping use WT instead. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>