[openstack-dev] Moving task flow to conductor - concern about scale

Joshua Harlow harlowja at yahoo-inc.com
Fri Jul 19 20:36:18 UTC 2013

This seems to me to be a good example where a library "problem" is leaking into the openstack architecture right? That is IMHO a bad path to go down.

I like to think of a world where this isn't a problem and design the correct solution there instead and fix the eventlet problem instead. Other large applications don't fallback to rpc calls to get around a database/eventlet scaling issues afaik. 

Honestly I would almost just want to finally fix the eventlet problem (chris b. I think has been working on it) and design a system that doesn't try to work around a libraries lacking. But maybe that's to much idealism, idk...

This doesn't even touch on the synchronization issues that can happen when u start pumping db traffic over a mq. Ex, an update is now queued behind another update, the second one conflicts with the first, where does resolution happen when an async mq call is used. What about when you have X conductors doing Y reads and Z updates; I don't even want to think about the sync/races there (and so on...). Did u hit / check for any consistency issues in your tests? Consistency issues under high load using multiple conductors scare the bejezzus out of me....

Sent from my really tiny device...

On Jul 19, 2013, at 10:58 AM, "Peter Feiner" <peter at gridcentric.ca> wrote:

> On Fri, Jul 19, 2013 at 10:15 AM, Dan Smith <dms at danplanet.com> wrote:
>>> So rather than asking "what doesn't work / might not work in the
>>> future" I think the question should be "aside from them both being
>>> things that could be described as a conductor - what's the
>>> architectural reason for wanting to have these two separate groups of
>>> functionality in the same service ?"
>> IMHO, the architectural reason is "lack of proliferation of services and
>> the added complexity that comes with it." If one expects the
>> proxy workload to always overshadow the task workload, then making
>> these two things a single service makes things a lot simpler.
> I'd like to point a low-level detail that makes scaling nova-conductor
> at the process level extremely compelling: the database driver
> blocking the eventlet thread serializes nova's database access.
> Since the database connection driver is typically implemented in a
> library beyond the purview of eventlet's monkeypatching (i.e., a
> native python extension like _mysql.so), blocking database calls will
> block all eventlet coroutines. Since most of what nova-conductor does
> is access the database, a nova-conductor process's handling of
> requests is effectively serial.
> Nova-conductor is the gateway to the database for nova-compute
> processes.  So permitting a single nova-conductor process would
> effectively serialize all database queries during instance creation,
> deletion, periodic instance refreshes, etc. Since these queries are
> made frequently (i.e., easily 100 times during instance creation) and
> while other global locks are held (e.g., in the case of nova-compute's
> ResourceTracker), most of what nova-compute does becomes serialized.
> In parallel performance experiments I've done, I have found that
> running multiple nova-conductor processes is the best way to mitigate
> the serialization of blocking database calls. Say I am booting N
> instances in parallel (usually up to N=40). If I have a single
> nova-conductor process, the duration of each nova-conductor RPC
> increases linearly with N, which can add _minutes_ to instance
> creation time (i.e., dozens of RPCs, some taking several seconds).
> However, if I run N nova-conductor processes in parallel, then the
> duration of the nova-conductor RPCs do not increase with N; since each
> RPC is most likely handled by a different nova-conductor, serial
> execution of each process is moot.
> Note that there are alternative methods for preventing the eventlet
> thread from blocking during database calls. However, none of these
> alternatives performed as well as multiple nova-conductor processes:
> Instead of using the native database driver like _mysql.so, you can
> use a pure-python driver, like pymysql by setting
> sql_connection=mysql+pymysql://... in the [DEFAULT] section of
> /etc/nova/nova.conf, which eventlet will monkeypatch to avoid
> blocking. The problem with this approach is the vastly greater CPU
> demand of the pure-python driver compared to the native driver. Since
> the pure-python driver is so much more CPU intensive, the eventlet
> thread spends most of its time talking to the database, which
> effectively the problem we had before!
> Instead of making database calls from eventlet's thread, you can
> submit them to eventlet's pool of worker threads and wait for the
> results. Try this by setting dbapi_use_tpool=True in the [DEFAULT]
> section of /etc/nova/nova.conf. The problem I found with this approach
> was the overhead of synchronizing with the worker threads. In
> particular, the time elapsed between the worker thread finishing and
> the waiting coroutine being resumed was typically several times
> greater than the duration of the database call itself.
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