Message-ID: <9dd017ea-a0ec-47c4-b7ae-b6f441dbd5ec@linux.dev>
Date: Mon, 23 Jun 2025 12:18:08 +0800
From: Dongsheng Yang <dongsheng.yang@...ux.dev>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: agk@...hat.com, snitzer@...nel.org, axboe@...nel.dk, hch@....de,
 dan.j.williams@...el.com, Jonathan.Cameron@...wei.com,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-cxl@...r.kernel.org, nvdimm@...ts.linux.dev, dm-devel@...ts.linux.dev
Subject: Re: [RFC v2 00/11] dm-pcache – persistent-memory cache for block devices


在 6/23/2025 11:13 AM, Dongsheng Yang 写道:
>
> Hi Mikulas:
>
>      I will send dm-pcache V1 soon, below is my response to your comments.
>
> 在 6/13/2025 12:57 AM, Mikulas Patocka 写道:
>> Hi
>>
>>
>> On Thu, 5 Jun 2025, Dongsheng Yang wrote:
>>
>>> Hi Mikulas and all,
>>>
>>> This is *RFC v2* of the *pcache* series, a persistent-memory backed cache.
>>>
>>>
>>> ----------------------------------------------------------------------
>>> 1. pmem access layer
>>> ----------------------------------------------------------------------
>>>
>>> * All reads use *copy_mc_to_kernel()* so that uncorrectable media
>>>    errors are detected and reported.
>>> * All writes go through *memcpy_flushcache()* to guarantee durability
>>>    on real persistent memory.
>> You could also try to use normal write and clflushopt for big writes - I
>> found out that for larger regions it is better - see the function
>> memcpy_flushcache_optimized in dm-writecache. Test, which way is better.
>
> I did a test with fio on /dev/pmem0, with an attached patch on nd_pmem.ko:
>
> when I use memmap pmem device, I got a similar result with the comment 
> in memcpy_flushcache_optimized():
>
> Test (memmap pmem) clflushopt flushcache 
> ------------------------------------------------- test_randwrite_512 
> 200 MiB/s 228 MiB/s test_randwrite_1024 378 MiB/s 431 MiB/s 
> test_randwrite_2K 773 MiB/s 769 MiB/s test_randwrite_4K 1364 MiB/s 
> 1272 MiB/s test_randwrite_8K 2078 MiB/s 1817 MiB/s test_randwrite_16K 
> 2745 MiB/s 2098 MiB/s test_randwrite_32K 3232 MiB/s 2231 MiB/s 
> test_randwrite_64K 3660 MiB/s 2411 MiB/s test_randwrite_128K 3922 
> MiB/s 2513 MiB/s test_randwrite_1M 3824 MiB/s 2537 MiB/s 
> test_write_512 228 MiB/s 228 MiB/s test_write_1024 439 MiB/s 423 MiB/s 
> test_write_2K 841 MiB/s 800 MiB/s test_write_4K 1364 MiB/s 1308 MiB/s 
> test_write_8K 2107 MiB/s 1838 MiB/s test_write_16K 2752 MiB/s 2166 
> MiB/s test_write_32K 3213 MiB/s 2247 MiB/s test_write_64K 3661 MiB/s 
> 2415 MiB/s test_write_128K 3902 MiB/s 2514 MiB/s test_write_1M 3808 
> MiB/s 2529 MiB/s
>
> But I got a different result when I use Optane pmem100:
>
> Test (Optane pmem100) clflushopt flushcache 
> ------------------------------------------------- test_randwrite_512 
> 167 MiB/s 226 MiB/s test_randwrite_1024 301 MiB/s 420 MiB/s 
> test_randwrite_2K 615 MiB/s 639 MiB/s test_randwrite_4K 967 MiB/s 1024 
> MiB/s test_randwrite_8K 1047 MiB/s 1314 MiB/s test_randwrite_16K 1096 
> MiB/s 1377 MiB/s test_randwrite_32K 1155 MiB/s 1382 MiB/s 
> test_randwrite_64K 1184 MiB/s 1452 MiB/s test_randwrite_128K 1199 
> MiB/s 1488 MiB/s test_randwrite_1M 1178 MiB/s 1499 MiB/s 
> test_write_512 233 MiB/s 233 MiB/s test_write_1024 424 MiB/s 391 MiB/s 
> test_write_2K 706 MiB/s 760 MiB/s test_write_4K 978 MiB/s 1076 MiB/s 
> test_write_8K 1059 MiB/s 1296 MiB/s test_write_16K 1119 MiB/s 1380 
> MiB/s test_write_32K 1158 MiB/s 1387 MiB/s test_write_64K 1184 MiB/s 
> 1448 MiB/s test_write_128K 1198 MiB/s 1481 MiB/s test_write_1M 1178 
> MiB/s 1486 MiB/s
>
>
> So for now I’d rather keep using flushcache in pcache. In future, once 
> we’ve come up with a general-purpose optimization, we can switch to that.
>
Sorry for the formatting issue—the table can be checked in attachment 
<pmem_test_result>

Thanx

Dongsheng

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