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

Sean Dague sean at dague.net
Thu Jan 7 16:02:35 UTC 2016

On 01/07/2016 09:56 AM, Brant Knudson wrote:
> On Thu, Jan 7, 2016 at 6:39 AM, Clayton O'Neill <clayton at oneill.net
> <mailto:clayton at oneill.net>> wrote:
>     On Thu, Jan 7, 2016 at 2:49 AM, Roman Podoliaka
>     <rpodolyaka at mirantis.com <mailto:rpodolyaka at mirantis.com>> wrote:
>     >
>     > Linux gurus please correct me here, but my understanding is that Linux
>     > kernel queues up to $backlog number of connections *per socket*. In
>     > our case child processes inherited the FD of the socket, so they will
>     > accept() connections from the same queue in the kernel, i.e. the
>     > backlog value is for *all* child processes, not *per* process.
>     Yes, it will be shared across all children.
>     >
>     > In each child process eventlet WSGI server calls accept() in a loop to
>     > get a client socket from the kernel and then puts into a greenlet from
>     > a pool for processing:
>     It’s worse than that.  What I’ve seen (via strace) is that eventlet
>     actually
>     converts socket into a non-blocking socket, then converts that
>     accept() into a
>     epoll()/accept() pair in every child.  Then when a connection comes
>     in, every
>     child process wakes up out of poll and races to try to accept on the the
>     non-blocking socket, and all but one of them fails.
>     This means that every time there is a request, every child process
>     is woken
>     up, scheduled on CPU and then put back to sleep.  This is one of the
>     reasons we’re (slowly) moving to uWSGI.
> I just want to note that I've got a change proposed to devstack that
> adds a config option to run keystone in uwsgi (rather than under
> eventlet or in apache httpd mod_wsgi), see
> https://review.openstack.org/#/c/257571/ . It's specific to keystone
> since I didn't think other projects were moving away from eventlet, too.

I feel like this is a confused point that keeps being brought up.

The preferred long term direction of all API services is to be deployed
on a real web server platform. It's a natural fit for those services as
they are accepting HTTP requests and doing things with them.

Most OpenStack projects have worker services beyond just an HTTP server.
(Keystone is one of the very few exceptions here). Nova has nearly a
dozen of these worker services. These don't naturally fit as wsgi apps,
they are more traditional daemons, which accept requests over the
network, but also have periodic jobs internally and self initiate
actions. They are not just call / response. There is no long term
direction for these to move off of eventlet.


Sean Dague

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