linux-kernel - Re: [PATCH 07/12] xfs: don't preallocate a transaction for file size updates

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Message-ID: <20190625102507.GA1986@lst.de>
Date:   Tue, 25 Jun 2019 12:25:07 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/12] xfs: don't preallocate a transaction for file
 size updates

On Tue, Jun 25, 2019 at 09:15:23AM +1000, Dave Chinner wrote:
> > So, uh, how much of a hit do we take for having to allocate a
> > transaction for a file size extension?  Particularly since we can
> > combine those things now?
> 
> Unless we are out of log space, the transaction allocation and free
> should be largely uncontended and so it's just a small amount of CPU
> usage. i.e it's a slab allocation/free and then lockless space
> reservation/free. If we are out of log space, then we sleep waiting
> for space - the issue really comes down to where it is better to
> sleep in that case....

I see the general point, but we'll still have the same issue with
unwritten extent conversion and cow completions, and I don't remember
seeing any issue in that regard.  And we'd hit exactly that case
with random writes to preallocated or COW files, i.e. the typical image
file workload.