Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt()

[Andy Lutomirski <luto@amacapital.net>]

On Fri, May 29, 2015 at 2:29 PM, Elliott, Robert (Server Storage)
<Elliott@hp.com> wrote:
>> -----Original Message-----
>> From: Andy Lutomirski [mailto:luto@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

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