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

From: Ingo Molnar <mingo@kernel.org>
To: Jan Kara <jack@suse.cz>
Cc: Waiman Long <Waiman.Long@hpe.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jan Kara <jack@suse.com>, Jeff Layton <jlayton@poochiereds.net>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andi Kleen <andi@firstfloor.org>,
	Dave Chinner <dchinner@redhat.com>,
	Scott J Norton <scott.norton@hp.com>,
	Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH v3 3/3] vfs: Use per-cpu list for superblock's inode list
Date: Thu, 25 Feb 2016 09:06:35 +0100	[thread overview]
Message-ID: <20160225080635.GB10611@gmail.com> (raw)
In-Reply-To: <20160224085858.GE10096@quack.suse.cz>

* Jan Kara <jack@suse.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