[ovs-discuss] Mempool issue for OVS 2.9

Jan Scheurich jan.scheurich at ericsson.com
Fri Jan 26 16:55:46 UTC 2018


> -----Original Message-----
> From: Stokes, Ian [mailto:ian.stokes at intel.com]
> Sent: Friday, 26 January, 2018 13:01
> To: ovs-discuss at openvswitch.org
> Cc: Kevin Traynor <ktraynor at redhat.com>; Flavio Leitner <fbl at redhat.com>; Ilya Maximets (i.maximets at samsung.com)
> <i.maximets at samsung.com>; Loftus, Ciara <ciara.loftus at intel.com>; Kavanagh, Mark B <mark.b.kavanagh at intel.com>; Jan Scheurich
> <jan.scheurich at ericsson.com>; Ben Pfaff (blp at ovn.org) <blp at ovn.org>; aconole at redhat.com; Venkatesan Pradeep
> <venkatesan.pradeep at ericsson.com>
> Subject: Mempool issue for OVS 2.9
> 
> Hi All,
> 
> Recently an issue was raised regarding the move from a single shared mempool model that was in place up to OVS 2.8, to a mempool per
> port model introduced in 2.9.
> 
> https://mail.openvswitch.org/pipermail/ovs-discuss/2018-January/046021.html
> 
> The per port mempool model was introduced in September 5th with commit d555d9bded to allow fine grain control on a per port case of
> memory usage.
> 
> In the 'common/shared mempool' model, ports sharing a similar MTU would all share the same buffer mempool (e.g. the most common
> example of this being that all ports are by default created with a 1500B MTU, and as such share the same mbuf mempool).
> 
> This approach had some drawbacks however. For example, with the shared memory pool model a user could exhaust the shared memory
> pool (for instance by requesting a large number of RXQs for a port), this would cause the vSwitch to crash as any remaining ports would
> not have the required memory to function. This bug was discovered and reported to the community in late 2016
> https://mail.openvswitch.org/pipermail/ovs-discuss/2016-September/042560.html.
> 
> The per port mempool patch aimed to avoid such issues by allocating a separate buffer mempool to each port.
> 
> An issue has been flagged on ovs-discuss, whereby memory dimensions provided for a given number of ports on OvS 2.6-2.8 may be
> insufficient to support the same number of ports in OvS 2.9, on account of the per-port mempool model without re-dimensioning extra
> memory. The effect of this is use case dependent (number of ports, RXQs, MTU settings, number of PMDs etc.) The previous 'common-
> pool' model was rudimentary in estimating the mempool size and was marked as something that was to be improved upon. The memory
> allocation calculation for per port model was modified to take the possible configuration factors mentioned into account.
> 
> It's unfortunate that this came to light so close to the release code freeze - but better late than never as it is a valid problem to be
> resolved.
> 
> I wanted to highlight some options to the community as I don't think the next steps should be taken in isolation due to the impact this
> feature has.
> 
> There are a number of possibilities for the 2.9 release.
> 
> (i) Revert the mempool per port patches and return to the shared mempool model. There are a number of features and refactoring in
> place on top of the change so this will not be a simple revert. I'm investigating what exactly is involved with this currently.

The shared mempool concept has been working fairly well in our NFVI systems for a long time. One can argue that it is too simplistic (as it does not at all take the number of ports and queue lengths into account) and may be insufficient in specific scenarios but at least it can operate with a reasonable amount of memory. 

Out of the very many vhostuser ports only very few actually carry significant amount of traffic. To reserve the theoretical worst case amount of buffers for every port separately is complete overkill.

> (ii) Leave the per port mempool implementation as is, flag to users that memory requirements have increased. Extra memory may have
> to be provided on a per use case basis.

[Jan] From my point of view this a blocker for roll-out of OVS 2.9 in existing NFVI Cloud system. On these systems the amount of huge pages allocated to OVS is fixed by zero day configuration and there is no easy way to change that memory allocation as part of a SW upgrade procedure. 

As such servers host a significant number of vhostuser ports and rx queues (in the order of 100 or more) the new per-port mempool scheme will likely cause OVS to fail after SW upgrade.

> (iii) Reduce the amount of memory allocated per mempool per port. An RFC to this effect was submitted by Kevin but on follow up the
> feeling is that it does not resolve the issue adequately.

If we can't get the shared mempool back into 2.9. from day one, this might be an emergency measure to support a reasonable number of ports with typically deployed huge-page assignments. 

> (iv) Introduce a feature to allow users to configure mempool as shared or on a per port basis: This would be the best of both worlds but
> given the proximity to the 2.9 freeze I don't think it's feasible by the end of January.

Yes, too late now. But something that should be backported to 2.9 as soon as we have it on master.

I think that we should aim for the combination of adaptive shared mempools as default and explicitly configurable per-port mempools, when needed.

Regards, Jan


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