Re: [RFC] what to do with IOCB_DSYNC?

[Matthew Wilcox <willy@infradead.org>]

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@zn.tnic/