Bug 1283676 - net.core.netdev_max_backlog not set high enough
net.core.netdev_max_backlog not set high enough
Product: Red Hat OpenStack
Classification: Red Hat
Component: openstack-tripleo-heat-templates (Show other bugs)
7.0 (Kilo)
Unspecified Unspecified
urgent Severity unspecified
: ga
: 8.0 (Liberty)
Assigned To: Giulio Fidente
Amit Ugol
: TestOnly, Triaged
: 1299080 (view as bug list)
Depends On:
Blocks: 1299080
  Show dependency treegraph
Reported: 2015-11-19 09:44 EST by Will Foster
Modified: 2016-06-23 14:18 EDT (History)
10 users (show)

See Also:
Fixed In Version: openstack-tripleo-heat-templates-0.8.11-1.el7ost
Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of:
: 1299080 (view as bug list)
Last Closed: 2016-06-23 13:34:54 EDT
Type: Bug
Regression: ---
Mount Type: ---
Documentation: ---
Verified Versions:
Category: ---
oVirt Team: ---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---

Attachments (Terms of Use)

External Trackers
Tracker ID Priority Status Summary Last Updated
Red Hat Bugzilla 1095811 None None None Never
OpenStack gerrit 277601 None None None 2016-02-08 18:00 EST

  None (edit)
Description Will Foster 2015-11-19 09:44:59 EST
Description of problem:

/proc/sys/net/core/netdev_max_backlog default setting of 1000 is extremely low/unsuitable for medium to large cloud environments.

What happens is that things just stop working once you reach over
1,000 dhcp-agent ports in use, very common for medium to large clouds.

We first saw this on Trystack[1] and thought it was a dnsmasq bug
until this bugzilla was brought to our attention:


We've set net.core.netdev_max_backlog to around 100k which has solved
Neutron grinding to a halt, a backlog of dhcp-agent/L3 churn and the
system being brought down because of backlog queues.

I understand that setting sysctl settings from an installer
perspective might seem a little invasive, but it's something that I
think would trip up large customers and deployments as it's not immediately apparent this might be the cause.

On a recent RHOS-D OSP7 installation I don't see this default changed so figure
I'd bring it to light here.

[root@overcloud-controller-0 ~]# cat

Whether or not the installer is the best place to set this I am not sure.

Version-Release number of selected component (if applicable):

RHEL-OSP7 (or anything with Kilo)

How reproducible:

Steps to Reproduce:
1. Deploy RHEL-OSP7
2. Create over 1,000 Neutron networks with gateways set (using dhcp-agent)
3. Note things grinding to a halt.

Actual results:

Expected results:

Additional info:
Comment 3 Mike Burns 2016-01-20 12:23:08 EST
Will can you provide an appropriate default value?
Comment 4 Mike Burns 2016-01-27 10:36:05 EST
From trello

@hughbrock we use 100k for /proc/sys/net/core/netdev_max_backlog setting, best way to set this is probably a tuned profile.
Comment 5 Hugh Brock 2016-02-07 09:29:42 EST
*** Bug 1299080 has been marked as a duplicate of this bug. ***
Comment 6 Giulio Fidente 2016-02-08 17:51:56 EST
From the tuning guide it looks like this is number should be a factor of the CPU capabilities (both the number of cores and speed) and the NIC capabilities (again both the number of links and speed).

It is described as a queue within the Linux kernel where traffic is stored after reception from the NIC, but before processing by the protocol stacks.

The /proc/net/softnet_stat file contains a counter in the 2nd column that is incremented when the netdev backlog queue overflows. If this value is incrementing over time, then netdev_max_backlog needs to be increased.

The guide suggests to double it and try again until no overflows are observed. I will set a value which is 10x the default in hiera so that it can be customized further if necessary.

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