Re: Write through caching on NVDIMM

["Kani, Toshi" <toshi.kani@hpe.com>]

On Thu, 2018-04-19 at 16:12 -0700, Brian Stark wrote:
> I was comparing different caching techniques using MTRR entries and I
> found something strange.  Note the output is from a basic utility that
> uses mmap.
> 
> NV Memory:
> 
> Write-back
> 
> Writes took 2075.993408 Megabytes per second
> Reads took 2130.842529 Megabytes per second No Errors Found
> 
> Write-Through
> 
> Writes took 55.332256 Megabytes per second
> Reads took 92.310310 Megabytes per second No Errors Found  !!!I was
> expecting this number to be the same as Write-back
> 
> Uncached
> 
> Writes took 55.331089 Megabytes per second
> Reads took 92.315132 Megabytes per second No Errors Found
> 
> Regular memory:
> 
> Write-back
> 
> Writes took 1875.560791 Megabytes per second
> Reads took 2070.452637 Megabytes per second No Errors Found
> 
> Write-Through
> 
> Writes took 54.903713 Megabytes per second
> Reads took 2106.244629 Megabytes per second No Errors Found  !!!This
> is what I expected to see for NV Memory
> 
> Uncached
> 
> Writes took 54.903923 Megabytes per second
> Reads took 90.150986 Megabytes per second No Errors Found
> 
> I am using the same driver (which adds MTRR entries to change caching
> type) there is no physical reason why write through should behave
> differently when using NV Memory.  Does anybody on this mailing list
> who may be more familiar with caching techniques know why this might
> me the case?

I reran my old test and did not see this problem.  So, I think there is
an issue in your setup that it somehow ended up with UC...

My test driver calls ioremap_wt to an NVDIMM range and access it from
the driver itself.  It does not modify MTRR entries.

-Toshi
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