Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries

[Matthew Wilcox]

On Wed, Feb 26, 2020 at 11:13:53AM -0500, Waiman Long wrote:
> As there is no limit for negative dentries, it is possible that a sizeable
> portion of system memory can be tied up in dentry cache slabs. Dentry slabs
> are generally recalimable if the dentries are in the LRUs. Still having
> too much memory used up by dentries can be problematic:
> 
>  1) When a filesystem with too many negative dentries is being unmounted,
>     the process of draining the dentries associated with the filesystem
>     can take some time. To users, the system may seem to hang for
>     a while.  The long wait may also cause unexpected timeout error or
>     other warnings.  This can happen when a long running container with
>     many negative dentries is being destroyed, for instance.
> 
>  2) Tying up too much memory in unused negative dentries means there
>     are less memory available for other use. Even though the kernel is
>     able to reclaim unused dentries when running out of free memory,
>     it will still introduce additional latency to the application
>     reducing its performance.

There's a third problem, which is that having a lot of negative dentries
can clog the hash chains.  I tried to quantify this, and found a weird result:

root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m5.402s
user	0m4.361s
sys	0m1.230s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m5.572s
user	0m4.337s
sys	0m1.407s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m5.607s
user	0m4.522s
sys	0m1.342s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m5.599s
user	0m4.472s
sys	0m1.369s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m5.574s
user	0m4.498s
sys	0m1.300s

Pretty consistent system time, between about 1.3 and 1.4 seconds.

root@bobo-kvm:~# grep dentry /proc/slabinfo 
dentry             20394  21735    192   21    1 : tunables    0    0    0 : slabdata   1035   1035      0
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m5.515s
user	0m4.353s
sys	0m1.359s

At this point, we have 20k dentries allocated.

Now, pollute the dcache with names that don't exist:

root@bobo-kvm:~# for i in `seq 1 100000`; do cat /dev/null$i >/dev/zero; done 2>/dev/null
root@bobo-kvm:~# grep dentry /proc/slabinfo 
dentry             20605  21735    192   21    1 : tunables    0    0    0 : slabdata   1035   1035      0

Huh.  We've kept the number of dentries pretty constant.  Still, maybe the
bad dentries have pushed out the good ones.

root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m6.644s
user	0m4.921s
sys	0m1.946s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m6.676s
user	0m5.004s
sys	0m1.909s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m6.662s
user	0m4.980s
sys	0m1.916s
root@bobo-kvm:~# time for i in `seq 1 10000`; do cat /dev/null >/dev/zero; done
real	0m6.714s
user	0m4.973s
sys	0m1.986s

Well, we certainly made it suck.  Up to a pretty consistent 1.9-2.0 seconds
of kernel time, or 50% worse.  We've also made user time worse, somehow.

Anyhow, I should write a proper C program to measure this.  But I thought
I'd share this raw data with you now to demonstrate that dcache pollution
is a real problem today, even on a machine with 2GB.