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

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Message-ID: <e7a71f770a5a29325bafee0966adcf9b72d241e3.camel@kernel.org>
Date: Tue, 06 Aug 2024 16:42:47 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>, Andi Kleen <ak@...ux.intel.com>
Cc: 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, 2024-08-06 at 21:22 +0200, Mateusz Guzik wrote:
> 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).
> 

Well...

    # ./unlink1_processes -t 70 -s 100

    average:
    v6.10:		114455
    v6.10 + patch:	149513

I suspect what's happening here is that this patch relieves contention
for the inode_lock and that allows the unlinks to proceed faster.

Running it with a single process though:

    average:
    v6.10:		200106
    v6.10 + patch:	199188

So, ~.4% degradation there? That doesn't seem too bad given the gain in
the threaded test.
-- 
Jeff Layton <jlayton@...nel.org>