linux-kernel - Re: [PATCH v2] fs: try an opportunistic lookup for O

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Message-ID: <CAGudoHHu42+VP6snbtg9gXog0UYaMv68eekxYt+2=5arrhZffg@mail.gmail.com>
Date: Tue, 6 Aug 2024 21:22:09 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Andi Kleen <ak@...ux.intel.com>
Cc: Jeff Layton <jlayton@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Andrew Morton <akpm@...ux-foundation.org>, Josef Bacik <josef@...icpanda.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs: try an opportunistic lookup for O_CREAT opens too

On Tue, Aug 6, 2024 at 9:11 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
> Mateusz Guzik <mjguzik@...il.com> writes:
> >
> > I would bench with that myself, but I temporarily don't have handy
> > access to bigger hw. Even so, the below is completely optional and
> > perhaps more of a suggestion for the future :)
> >
> > I hacked up the test case based on tests/open1.c.
>
> Don't you need two test cases? One where the file exists and one
> where it doesn't. Because the "doesn't exist" will likely be slower
> than before because it will do the lookups twice,
> and it will likely even slow single threaded.
>
> I assume the penalty will also depend on the number of entries
> in the path.
>
> That all seem to be an important considerations in judging the benefits
> of the patch.
>

This is why I suggested separately running "unlink1" which is
guaranteed to create a file every time -- all iterations will fail the
proposed fast path.

Unless you meant a mixed variant where only some of the threads create
files. Perhaps worthwhile to add, not hard to do (one can switch the
mode based on passed worker number).

-- 
Mateusz Guzik <mjguzik gmail.com>