[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
Thu Jan 7 16:55:41 UTC 2016

On 01/07/2016 11:02 AM, Sean Dague wrote:
> 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.

This is totally speaking as an outsider without taking into account all
the history of these decisions, but the notion of "Python + we're a
daemon" == "we must use eventlet" seems a little bit rigid.  Also, the
notion of "we have background tasks" == "we cant run in a web server",
also not clear.  If a service intends to serve HTTP requests, that
portion of that service should be deployed in a web server; if the
system has other "background tasks", ideally those are in a separate
daemon altogether, but also even if you're under something like
mod_wsgi, you can spawn a child process or worker thread regardless.
You always have a Python interpreter running and all the things it can do.

> 	-Sean

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