Re: [RFC][PATH 4/4] ovl: relax lock_rename when moving files between work and upper dir

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On Fri, Nov 11, 2016 at 2:27 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Nov 11, 2016 at 02:11:56AM +0200, Amir Goldstein wrote:
>> Yeah, I wrote in the cover letter that I did not generate performance
>> numbers yet,
>> which is a must for this sort of work, and that I am hoping to get some feedback
>> from testers.
>> But the serialization I am trying to avoid is between copy-ups and whiteouts of
>> different overlay mounts, all on the same fs, which is the case with
>> docker/rocket
>> containers.
>>
>> Not ruling out that I am barking up the wrong tree. The burden of
>> proof is on me.
>
> Surely, the copying of data itself is outside of that lock, isn't it?

Mmmmmm, no it isn't, but I am going to make it right.

> And renames proper, especially if there is any kind of contention going on,
> will be on the metadata hot in cache, so I would really like to see the
> actual evidence of contention-related performance issues...

I am afraid I won't find these evidence.
I did a small benchmark of 2 parallel rm -rf processes on 2 different
overlay mount
over the same fs.
In that test, every process spends 20% of the time on vfs_whiteout and 10% on
vfs_rename (of those whiteouts).  mutex_lock/unlock of s_vfs_rename_mutex
takes 4% of the time, but the lock covers both the whiteout and rename.

I estimate that after we fix the coarse grained rename_lock, it will be quite
hard to demonstrate contention-related performance issues due to
s_vfs_rename_mutex.

Thanks for taking the time to set me straight.
Amir.
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