[openstack-dev] [nova] [all] Excessively high greenlet default + excessively low connection pool defaults leads to connection pool latency, timeout errors, idle database connections / workers

Mike Bayer mbayer at redhat.com
Wed Jan 6 22:02:49 UTC 2016

On 01/06/2016 09:11 AM, Roman Podoliaka wrote:
> Hi Mike,
> Thank you for this brilliant analysis! We've been seeing such timeout
> errors in downstream periodically and this is the first time someone
> has analysed the root cause thoroughly.
> On Fri, Dec 18, 2015 at 10:33 PM, Mike Bayer <mbayer at redhat.com> wrote:
>> But if we only have a super low number of greenlets and only a few dozen
>> workers, what happens if we have more than 240 requests come in at once,
>> aren't those connections going to get rejected?  No way!  eventlet's
>> networking system is better than that, those connection requests just
>> get queued up in any case, waiting for a greenlet to be available.  Play
>> with the script and its settings to see.
> Right, it must be controlled by the backlog argument value here:
> https://github.com/openstack/oslo.service/blob/master/oslo_service/wsgi.py#L80

oh wow, totally missed that!  But, how does backlog here interact with
multiple processes?   E.g. all workers are saturated, it will place a
waiting connection onto a random greenlet which then has to wait?  It
would be better if the "backlog" were pushed up to the parent process,
not sure if that's possible?

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