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Message-ID: <alpine.LRH.2.02.2004160411460.7833@file01.intranet.prod.int.rdu2.redhat.com>
Date: Thu, 16 Apr 2020 04:24:20 -0400 (EDT)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Dan Williams <dan.j.williams@...el.com>
cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
device-mapper development <dm-devel@...hat.com>
Subject: Re: [PATCH] memcpy_flushcache: use cache flusing for larger
lengths
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
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