linux-ext4 - Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL

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Date:   Tue, 18 Apr 2023 14:42:03 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bernd Schubert <bschubert@....com>, io-uring@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, dsingh@....com
Subject: Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag

On Sat, 15 Apr 2023 at 15:15, Jens Axboe <axboe@...nel.dk> wrote:

> Yep, that is pretty much it. If all writes to that inode are serialized
> by a lock on the fs side, then we'll get a lot of contention on that
> mutex. And since, originally, nothing supported async writes, everything
> would get punted to the io-wq workers. io_uring added per-inode hashing
> for this, so that any punt to io-wq of a write would get serialized.
>
> IOW, it's an efficiency thing, not a correctness thing.

We could still get a performance regression if the majority of writes
still trigger the exclusive locking.  The questions are:

 - how often does that happen in real life?
 - how bad the performance regression would be?

Without first attempting to answer those questions, I'd be reluctant
to add  FMODE_DIO_PARALLEL_WRITE to fuse.

Thanks,
Miklos