linux-kernel - Re: [PATCH v2 3/3] vfs: Enable list batching for the superblock's inode list

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Date:	Sun, 7 Feb 2016 10:51:40 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Waiman Long <waiman.long@....com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2 3/3] vfs: Enable list batching for the superblock's
 inode list

On Wed, Feb 03, 2016 at 05:59:17PM -0500, Waiman Long wrote:
> On 02/01/2016 05:03 PM, Waiman Long wrote:
> >On 02/01/2016 12:45 PM, Andi Kleen wrote:
> >>>I'm wondering, why are inode_sb_list_add()/del() even called
> >>>for a presumably
> >>>reasonably well cached benchmark running on a system with
> >>>enough RAM? Are these
> >>>perhaps thousands of temporary files, already deleted, and
> >>>released when all the
> >>>file descriptors are closed as part of sys_exit()?
> >>>
> >>>If that's the case then I suspect an even bigger win would be
> >>>not just to batch
> >>>the (sb-)global list fiddling, but to potentially turn the sb
> >>>list into a
> >>>percpu_alloc() managed set of per CPU lists? It's a bigger
> >>>change, but it could
> >>We had such a patch in the lock elision patchkit (It avoided a lot
> >>of cache line bouncing leading to aborts)
> >>
> >>https://git.kernel.org/cgit/linux/kernel/git/ak/linux-misc.git/commit/?h=hle315/combined&id=f1cf9e715a40f44086662ae3b29f123cf059cbf4
> >>
> >>
> >>-Andi
> >>
> >>
> >
> >I like your patch though it cannot be applied cleanly for the
> >current upstream kernel. I will port it to the current kernel and
> >run my microbenchmark to see what performance gain I can get.
> >
> 
> Unfortunately, using per-cpu list didn't have the performance
> benefit that I expected. I saw maybe 1 or 2% of performance
> increase, but nothing significant. I guess the bulk of the
> performance improvement in my patch is in the elimination of most of
> the cacheline transfer latencies when the lock ownership is passed
> from one CPU to another. Those latencies are still there even if we
> use the per-cpu list.

Now that I look at the above patch, it doesn't get rid of the global
list lock. hence it won't change any of the existing global lock
cacheline contention. The list structure change to per-cpu is
completely irrelevant because it doesn't address the problem being
seen.

A proper per-cpu list implementation will provide either per-cpu
locks or some other mechanism to protect each list and eliminate a
large amount of global cacheline bouncing. Given this, I would not
use the above patch and results as a reason for saying this approach
will not work, or be a better solution....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com