Open Stack

In our experience (running clusters of several hundred nodes) the DB performance is not generally the significant factor, so making its calls non-blocking  gives only a very small increase in processing capacity and creates other side effects in terms of slowing all eventlets down as they wait for their turn to run.

That shouldn't really be surprising given that the Nova DB is pretty small and MySQL is a pretty good DB - throw reasonable hardware at the DB server and give it a bit of TLC from a DBA (remove deleted entries from the DB, add indexes where the slow query log tells you to, etc) and it shouldn't be the bottleneck in the system for performance or scalability.

We use the python driver and have experimented with allowing the eventlet code to make the db calls non-blocking (its not the default setting), and it works, but didn't give us any significant advantage.

For example in the API server (before we made it properly multi-threaded) with blocking db calls the server was essentially a serial processing queue - each request was fully processed before the next.  With non-blocking db calls we got a lot more apparent concurrencybut only at the expense of making all of the requests equally bad.

Consider a request takes 10 seconds, where after 5 seconds there is a call to the DB which takes 1 second, and three are started at the same time:

Blocking:
0 - Request 1 starts
10 - Request 1 completes, request 2 starts
20 - Request 2 completes, request 3 starts
30 - Request 3 competes
Request 1 completes in 10 seconds
Request 2 completes in 20 seconds
Request 3 completes in 30 seconds
Ave time: 20 sec


Non-blocking
0 - Request 1 Starts
5 - Request 1 gets to db call, request 2 starts
10 - Request 2 gets to db call, request 3 starts
15 - Request 3 gets to db call, request 1 resumes
19 - Request 1 completes, request 2 resumes
23 - Request 2 completes,  request 3 resumes
27 - Request 3 completes

Request 1 completes in 19 seconds  (+ 9 seconds)
Request 2 completes in 24 seconds (+ 4 seconds)
Request 3 completes in 27 seconds (- 3 seconds)
Ave time: 20 sec
 
So instead of worrying about making db calls non-blocking we've been working to make certain eventlets non-blocking - i.e. add sleep(0) calls to long running iteration loops - which IMO has a much bigger impact on the performance of the apparent latency of the system.

Phil



-----Original Message-----
From: openstack-bounces+philip.day=hp.com at lists.launchpad.net [mailto:openstack-bounces+philip.day=hp.com at lists.launchpad.net] On Behalf Of Brian Lamar
Sent: 01 March 2012 21:31
To: openstack at lists.launchpad.net
Subject: Re: [Openstack] eventlet weirdness

>> How is MySQL access handled in eventlet? Presumably it's external C 
>> library so it's not going to be monkey patched. Does that make every 
>> db access call a blocking call? Thanks,

> Nope, it goes through a thread pool.

I feel like this might be an over-simplification. If the question is:

"How is MySQL access handled in nova?"

The answer would be that we use SQLAlchemy which can load any number of SQL-drivers. These drivers can be either pure Python or C-based drivers. In the case of pure Python drivers, monkey patching can occur and db calls are non-blocking. In the case of drivers which contain C code (or perhaps other blocking calls), db calls will most likely be blocking.

If the question is "How is MySQL access handled in eventlet?" the answer would be to use the eventlet.db_pool module to allow db access using thread pools.

B

-----Original Message-----
From: "Adam Young" <ayoung at redhat.com>
Sent: Thursday, March 1, 2012 3:27pm
To: openstack at lists.launchpad.net
Subject: Re: [Openstack] eventlet weirdness

On 03/01/2012 02:45 PM, Yun Mao wrote:
> There are plenty eventlet discussion recently but I'll stick my 
> question to this thread, although it's pretty much a separate 
> question. :)
>
> How is MySQL access handled in eventlet? Presumably it's external C 
> library so it's not going to be monkey patched. Does that make every 
> db access call a blocking call? Thanks,

Nope, it goes through a thread pool.
>
> Yun
>
> On Wed, Feb 29, 2012 at 9:18 PM, Johannes Erdfelt<johannes at erdfelt.com>  wrote:
>> On Wed, Feb 29, 2012, Yun Mao<yunmao at gmail.com>  wrote:
>>> Thanks for the explanation. Let me see if I understand this.
>>>
>>> 1. Eventlet will never have this problem if there is only 1 OS 
>>> thread
>>> -- let's call it main thread.
>> In fact, that's exactly what Python calls it :)
>>
>>> 2. In Nova, there is only 1 OS thread unless you use xenapi and/or 
>>> the virt/firewall driver.
>>> 3. The python logging module uses locks. Because of the monkey 
>>> patch, those locks are actually eventlet or "green" locks and may 
>>> trigger a green thread context switch.
>>>
>>> Based on 1-3, does it make sense to say that in the other OS threads 
>>> (i.e. not main thread), if logging (plus other pure python library 
>>> code involving locking) is never used, and we do not run a eventlet 
>>> hub at all, we should never see this problem?
>> That should be correct. I'd have to double check all of the monkey 
>> patching that eventlet does to make sure there aren't other cases 
>> where you may inadvertently use eventlet primitives across real threads.
>>
>> JE
>>
>>
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