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Date:   Fri, 17 Nov 2023 20:12:43 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Howells <dhowells@redhat.com>,
        kernel test robot <oliver.sang@intel.com>,
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        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
        Matthew Wilcox <willy@infradead.org>,
        David Laight <David.Laight@aculab.com>, ying.huang@intel.com,
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Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression
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On Fri, Nov 17, 2023 at 11:44:10AM -0500, Linus Torvalds wrote:
> So the test robot load is kind of odd.

So looking at that. IINM, its documentation says:

https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git/tree/Documentation

case-msync:
Create N sparse files, each with a size of $MemTotal. For each sparse file,
start a process to write 1/2N of the sparse file's size. After the write,
do a msync to make sure the change in memory has reached the file.

Is that something userspace usually does?

Some distributed, shared thing logging to the same file?

I obviously have no effing clue what userspace does...

> Not that I think that David's KUnit test is necessarily much of a real
> load either. so...

Which begs the question: what are our priorities here?

I wouldn't want to optimize some weird loads. Especially if you have
weird loads which perform differently depending on what uarch
"optimizations" they sport.

I guess optimizing for the majority of machines - modern FSRM ones which
can do "rep; movsb" just fine - is one way to put it. And the rest is
best effort.

Hmmm.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette