Re: [PATCH] memcpy_flushcache: use cache flusing for larger lengths

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 9 Apr 2020, Mikulas Patocka wrote:

> With dm-writecache on emulated pmem (with the memmap argument), we get
> 
> With the original kernel:
> 8508 - 11378
> real    0m4.960s
> user    0m0.638s
> sys     0m4.312s
> 
> With dm-writecache hacked to use cached writes + clflushopt:
> 8505 - 11378
> real    0m4.151s
> user    0m0.560s
> sys     0m3.582s

I did some multithreaded tests: 
http://people.redhat.com/~mpatocka/testcases/pmem/microbenchmarks/pmem-multithreaded.txt

And it turns out that for singlethreaded access, write+clwb performs 
better, while for multithreaded access, non-temporal stores perform 
better.

1       sequential write-nt 8 bytes             1.3 GB/s
2       sequential write-nt 8 bytes             2.5 GB/s
3       sequential write-nt 8 bytes             2.8 GB/s
4       sequential write-nt 8 bytes             2.8 GB/s
5       sequential write-nt 8 bytes             2.5 GB/s

1       sequential write 8 bytes + clwb         1.6 GB/s
2       sequential write 8 bytes + clwb         2.4 GB/s
3       sequential write 8 bytes + clwb         1.7 GB/s
4       sequential write 8 bytes + clwb         1.2 GB/s
5       sequential write 8 bytes + clwb         0.8 GB/s

For one thread, we can see that write-nt 8 bytes has 1.3 GB/s and write 
8+clwb has 1.6 GB/s, but for multiple threads, write-nt has better 
throughput.

The dm-writecache target is singlethreaded (all the copying is done while 
holding the writecache lock), so it benefits from clwb.

Should memcpy_flushcache be changed to write+clwb? Or are there some 
multithreaded users of memcpy_flushcache that would be hurt by this 
change?

Mikulas