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

Brant Knudson blk at acm.org
Thu Jan 7 14:56:09 UTC 2016

On Thu, Jan 7, 2016 at 6:39 AM, Clayton O'Neill <clayton at oneill.net> wrote:

> On Thu, Jan 7, 2016 at 2:49 AM, Roman Podoliaka <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.

- Brant

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