linux-kernel - Re: [PATCH v3 3/3] vfs: Use per-cpu list for superblock's inode list

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Message-ID: <20160225080635.GB10611@gmail.com>
Date:	Thu, 25 Feb 2016 09:06:35 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Waiman Long <Waiman.Long@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v3 3/3] vfs: Use per-cpu list for superblock's inode list


* Jan Kara <jack@...e.cz> wrote:

> > > > With an exit microbenchmark that creates a large number of threads, 
> > > > attachs many inodes to them and then exits. The runtimes of that 
> > > > microbenchmark with 1000 threads before and after the patch on a 4-socket 
> > > > Intel E7-4820 v3 system (40 cores, 80 threads) were as follows:
> > > > 
> > > >   Kernel            Elapsed Time    System Time
> > > >   ------            ------------    -----------
> > > >   Vanilla 4.5-rc4      65.29s         82m14s
> > > >   Patched 4.5-rc4      22.81s         23m03s
> > > > 
> > > > Before the patch, spinlock contention at the inode_sb_list_add() function 
> > > > at the startup phase and the inode_sb_list_del() function at the exit 
> > > > phase were about 79% and 93% of total CPU time respectively (as measured 
> > > > by perf). After the patch, the percpu_list_add() function consumed only 
> > > > about 0.04% of CPU time at startup phase. The percpu_list_del() function 
> > > > consumed about 0.4% of CPU time at exit phase. There were still some 
> > > > spinlock contention, but they happened elsewhere.
> > > 
> > > While looking through this patch, I have noticed that the 
> > > list_for_each_entry_safe() iterations in evict_inodes() and 
> > > invalidate_inodes() are actually unnecessary. So if you first apply the 
> > > attached patch, you don't have to implement safe iteration variants at all.
> > > 
> > > As a second comment, I'd note that this patch grows struct inode by 1 
> > > pointer. It is probably acceptable for large machines given the speedup but 
> > > it should be noted in the changelog. Furthermore for UP or even small SMP 
> > > systems this is IMHO undesired bloat since the speedup won't be noticeable.
> > > 
> > > So for these small systems it would be good if per-cpu list magic would just 
> > > fall back to single linked list with a spinlock. Do you think that is 
> > > reasonably doable?
> > 
> > Even many 'small' systems tend to be SMP these days.
> 
> Yes, I know. But my tablet with 4 ARM cores is unlikely to benefit from this 
> change either. [...]

I'm not sure about that at all, the above numbers are showing a 3x-4x speedup in 
system time, which ought to be noticeable on smaller SMP systems as well.

Waiman, could you please post the microbenchmark?

Thanks,

	Ingo