Re: [RFC] what to do with IOCB_DSYNC?

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On 5/23/22 9:49 AM, Matthew Wilcox wrote:
> On Mon, May 23, 2022 at 09:44:12AM -0600, Jens Axboe wrote:
>> On 5/23/22 9:12 AM, Jens Axboe wrote:
>>>> Current branch pushed to #new.iov_iter (at the moment; will rename
>>>> back to work.iov_iter once it gets more or less stable).
>>>
>>> Sounds good, I'll see what I need to rebase.
>>
>> On the previous branch, ran a few quick numbers. dd from /dev/zero to
>> /dev/null, with /dev/zero using ->read() as it does by default:
>>
>> 32      260MB/sec
>> 1k      6.6GB/sec
>> 4k      17.9GB/sec
>> 16k     28.8GB/sec
>>
>> now comment out ->read() so it uses ->read_iter() instead:
>>
>> 32      259MB/sec
>> 1k      6.6GB/sec
>> 4k      18.0GB/sec
>> 16k	28.6GB/sec
>>
>> which are roughly identical, all things considered. Just a sanity check,
>> but looks good from a performance POV in this basic test.
>>
>> Now let's do ->read_iter() but make iov_iter_zero() copy from the zero
>> page instead:
>>
>> 32      250MB/sec
>> 1k      7.7GB/sec
>> 4k      28.8GB/sec
>> 16k	71.2GB/sec
>>
>> Looks like it's a tad slower for 32-bytes, considerably better for 1k,
>> and massively better at page size and above. This is on an Intel 12900K,
>> so recent CPU. Let's try cacheline and above:
>>
>> Size	Method			BW		
>> 64	copy_from_zero()	508MB/sec
>> 128	copy_from_zero()	1.0GB/sec
>> 64	clear_user()		513MB/sec
>> 128	clear_user()		1.0GB/sec
> 
> See this thread-of-doom:
> 
> https://lore.kernel.org/all/Ynq1nVpu1xCpjnXm@xxxxxxx/

Yikes, will check. Also realized that my > PAGE_SIZE 16k result is
invalid as it should iterate with min(left, PAGE_SIZE). But that just
changes it from 71.2GB/sec to 68.9MB/sec, still a substantial win.
Updated diff below.

Was going to test on aarch64 next...

-- 
Jens Axboe




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