[openstack-dev] [mistral][oslo][messaging] Removing “blocking” executor from oslo.messaging

Ken Giusti kgiusti at gmail.com
Thu Oct 18 14:59:41 UTC 2018

Hi Renat,

The biggest issue with the blocking executor (IMHO) is that it blocks the
protocol I/O while  RPC processing is in progress.  This increases the
likelihood that protocol processing will not get done in a timely manner
and things start to fail in weird ways.  These failures are timing related
and are typically hard to reproduce or root-cause.   This isn't something
we can fix as blocking is the nature of the executor.

If we are to leave it in we'd really want to discourage its use.

However I'm ok with leaving it available if the policy for using blocking
is 'use at your own risk', meaning that bug reports may have to be marked
'won't fix' if we have reason to believe that blocking is at fault.  That
implies removing 'blocking' as the default executor value in the API and
having applications explicitly choose it.  And we keep the deprecation

We could perhaps implement time duration checks around the executor callout
and log a warning if the executor blocked for an extended amount of time

Other opinions so we can come to a consensus?

On Thu, Oct 18, 2018 at 3:24 AM Renat Akhmerov <renat.akhmerov at gmail.com>

> Hi Oslo Team,
> Can we retain “blocking” executor for now in Oslo Messaging?
> Some background..
> For a while we had to use Oslo Messaging with “blocking” executor in
> Mistral because of incompatibility of MySQL driver with green threads when
> choosing “eventlet” executor. Under certain conditions we would get
> deadlocks between green threads. Some time ago we switched to using PyMysql
> driver which is eventlet friendly and did a number of tests that showed
> that we could safely switch to “eventlet” executor (with that driver) so we
> introduced a new option in Mistral where we could choose an executor in
> Oslo Messaging. The corresponding bug is [1].
> The issue is that we recently found that not everything actually works as
> expected when using combination PyMysql + “eventlet” executor. We also
> tried “threading” executor and the system *seems* to work with it but
> surprisingly performance is much worse.
> Given all of that we’d like to ask Oslo Team not to remove “blocking”
> executor for now completely, if that’s possible. We have a strong
> motivation to switch to “eventlet” for other reasons (parallelism => better
> performance etc.) but seems like we need some time to make it smoothly.
> [1] https://bugs.launchpad.net/mistral/+bug/1696469
> Thanks
> Renat Akhmerov
> @Nokia
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Ken Giusti  (kgiusti at gmail.com)
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