[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 15:44:47 UTC 2016

On 01/07/2016 07:39 AM, Clayton O'Neill 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.

is that eventlet-specific or would we see the same thing in gevent ?

> 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.
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