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

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