Date:	Wed, 06 Jul 2016 23:58:54 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Phil Sutter <phil@....cc>
Cc:	Stephen Hemminger <shemming@...cade.com>, netdev@...r.kernel.org
Subject: Re: [iproute PATCH 0/2] Netns performance improvements

Phil Sutter <phil@....cc> writes:

> Hi Eric,
>
> Thanks for your quick and insightful reply rightfully pointing out the
> lack of rationale behind this change. So let me try to catch up:

Grr.  I did not get what you are trying to accomplish the first time I
skimmed this and rereading it all again closely I still don't get what
you are trying to acomplish.

What real world scenario do you have that approximates 100 mount
namespaces all sharing with each other with 1000 network namespaces
in that shared world?

I am inclined to suspect you are setting up containers that don't
contain and those 100 mount namespaces that share with each other
are your real concern.  But I don't know.

> On Tue, Jul 05, 2016 at 09:44:00AM -0500, Eric W. Biederman wrote:
>> Phil Sutter <phil@....cc> writes:
>> 
>> > Stress-testing OpenStack Neutron revealed poor performance of 'ip netns'
>> > when dealing with a high amount of namespaces. The cause of this lies in
>> > the combination of how iproute2 mounts NETNS_RUN_DIR and the netns files
>> > therein and the fact that systemd makes all mount points of the system
>> > shared.
>> 
>> So please tell me.  Given that it was clearly a deliberate choice in the
>> code to make these directories shared, and that this is not a result
>> of a systemd making all directories shared by default.  Why is it
>> better to these directories non-shared?
>
> NETNS_RUN_DIR itself is kept shared as it was intended by you (I hope).
> The only difference is that we should avoid it being in the same group
> as the parent mount point. Otherwise, all netns mount points will occur
> twice.

How do they occur twice?  Are you dealing with a system that bind mounts
/run and /var/run?  The netns mount points occurring twice sounds
correct in that scenario.  Replacing a bind mount with a symlink would
be a more appropriate fix if you are concerned with the mount overhead.

> Regarding the shared state of the netns mount points, I have actually no
> idea what's the benefit, as there won't be any child mount points and
> therefore no propagation should occur. Or am I missing something?

I think the second patch is probably ok.  I get turned around with the
finer points of mount propagation somedays as it is the parent mount
whose attributes matter when it comes to propagating the children.  

Still if the change semantically does not matter we have a missing
optimization in the kernel, and I would much rather implement that
optmization in the kernel than in every application that might possibly
hit it.  Especially given that the default on systemd systems is
"mount --make-rshared /"

>> This may be the appropriate change but saying you stress testing things
>> and have a problem but do not describe how large a scale you had a
>> problem, or anything else to make your problem reproducible by anyone
>> else makes it difficult to consider the merits of this change.
>> 
>> Sometimes things are a good default policy but have imperfect scaling on
>> extreme workloads.
>> 
>> My experience with the current situtation with ip netns is that it
>> prevents a whole lot of confusion by making the network namespace names
>> visible whichever mount namespace your processes are running in.
>
> The only functional difference I noticed was the no longer twice
> appearing netns mount points. They are still visible in all namespaces
> though, just as before.

But you are fighting the how the rest of the system is configured at
that point and that concerns me.  iproute is not the place to
reconfigure the system.

> Here's the script I wrote to benchmark 'ip netns':
>
> | #!/bin/bash
> | 
> | IP=${IP:-/usr/sbin/ip}
> | echo "using ip at $IP"
> | 
> | # make sure we start at a clean state
> | for netns in $(ls /run/netns/* 2>/dev/null); do
> |         $IP netns del ${netns##*/}
> | done
> | umount /run/netns
> | 
> | echo "creating 100 mount ns"
> | touch /tmp/stay_alive
> | for ((i = 0; i < 100; i++)); do
> |         unshare -m --propagation unchanged bash -c \
> | 		"while [[ -e /tmp/stay_alive ]]; do sleep 1; done" &
> | done
> | # give a little time for unshare to complete
> | sleep 3
> | 
> | nscount=1000
> | 
> | echo -en "\ncreating $nscount netns"
> | time (for ((i = 0; i < $nscount; i++)); do $IP netns add test$i; done)
> | 
> | echo -en "\ndeleting $nscount netns"
> | time (for ((i = 0; i < $nscount; i++)); do $IP netns del test$i; done)
> | 
> | echo "removing mount ns again"
> | rm /tmp/stay_alive
> | wait
>
> So basically it creates 100 idle mount namespaces, then times
> adding/removing 1000 network namespaces. I called it three times:
> without any patch, with just patch 1 and with both patches applied. Here
> are the results:
>
> | # IP=/tmp/base/ip /vmshare/reproducer/ip_netns_bench.sh
> | using ip at /tmp/base/ip
> | creating 100 mount ns
> | 
> | creating 1000 netns
> | real	0m8.110s
> | user	0m1.143s
> | sys	0m6.235s
> | 
> | deleting 1000 netns
> | real	0m15.347s
> | user	0m0.957s
> | sys	0m11.359s
> | removing mount ns again
>
> | # IP=/tmp/p1/ip /vmshare/reproducer/ip_netns_bench.sh
> | using ip at /tmp/p1/ip
> | creating 100 mount ns
> | 
> | creating 1000 netns
> | real	0m7.956s
> | user	0m0.987s
> | sys	0m4.896s
> | 
> | deleting 1000 netns
> | real	0m7.407s
> | user	0m1.165s
> | sys	0m3.418s
> | removing mount ns again
>
> | # IP=/tmp/p2/ip /vmshare/reproducer/ip_netns_bench.sh
> | using ip at /tmp/p2/ip
> | creating 100 mount ns
> | 
> | creating 1000 netns
> | real	0m7.843s
> | user	0m0.977s
> | sys	0m4.915s
> | 
> | deleting 1000 netns
> | real	0m6.407s
> | user	0m1.006s
> | sys	0m3.057s
> | removing mount ns again
>
> As you can see, the biggest improvement comes during deletion and from
> patch 1. Though the second patch lowers the total time to delete the
> namespaces by another second, which is still relatively much in
> comparison to the low total time.

Which all seems to be about making /run/netns and /var/run/netns not
shared with each other which appears to be semantically wrong.

Eric

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