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

[Waiman Long]

On 02/24/2016 03:23 PM, Waiman Long wrote:
> On 02/24/2016 03:28 AM, Jan Kara wrote:
> > On Tue 23-02-16 14:04:32, Waiman Long wrote:
> > > When many threads are trying to add or delete inode to or from
> > > a superblock's s_inodes list, spinlock contention on the list can
> > > become a performance bottleneck.
> > >
> > > This patch changes the s_inodes field to become a per-cpu list with
> > > per-cpu spinlocks. As a result, the following superblock inode list
> > > (sb->s_inodes) iteration functions in vfs are also being modified:
> > >
> > >   1. iterate_bdevs()
> > >   2. drop_pagecache_sb()
> > >   3. wait_sb_inodes()
> > >   4. evict_inodes()
> > >   5. invalidate_inodes()
> > >   6. fsnotify_unmount_inodes()
> > >   7. add_dquot_ref()
> > >   8. remove_dquot_ref()
> > >
> > > 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.
>
> Thank for the patch. I will apply that in my next update. As for the 
> safe iteration variant, I think I will keep it since I had implemented 
> that already just in case it may be needed in some other places.
>
> > 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?
>
> I already have a somewhat separate code path for UP. So I can remove 
> the lock pointer for that. For small SMP system, however, the only way 
> to avoid the extra pointer is to add a config parameter to turn this 
> feature off. That can be added as a separate patch, if necessary.

I am sorry that I need to retreat from this promise for UP. Removing the 
lock pointer will require change in the list deletion API to pass in the 
lock information. So I am not going to change it for the time being.

Cheers,
Longman