Nontemporal stores to WB memory is fine in such a way that it doesn't pollute the cache. This can be done by denoting to WC or by forcing cache allocation out of only a subset of the cache.
On May 29, 2015 2:46:19 PM PDT, Andy Lutomirski <l...@amacapital.net> wrote: >On Fri, May 29, 2015 at 2:29 PM, Elliott, Robert (Server Storage) ><elli...@hp.com> wrote: >>> -----Original Message----- >>> From: Andy Lutomirski [mailto:l...@amacapital.net] >>> 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 -- Sent from my mobile phone. Please pardon brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
