linux-kernel - Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput -16.9% regression

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Message-ID: <20231117191243.GHZVe7K4vN9n5M92gb@fat_crate.local>
Date:   Fri, 17 Nov 2023 20:12:43 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        kernel test robot <oliver.sang@...el.com>,
        oe-lkp@...ts.linux.dev, lkp@...el.com,
        linux-kernel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        David Laight <David.Laight@...lab.com>, ying.huang@...el.com,
        feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression

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