[openstack-dev] [magnum] Magnum conductor async container operations

Joshua Harlow harlowja at fastmail.com
Thu Dec 17 07:57:33 UTC 2015

SURO wrote:
> Josh,
> Please find my reply inline.
> Regards,
> irc//freenode: suro-patz
> On 12/16/15 6:37 PM, Joshua Harlow wrote:
>> SURO wrote:
>>> Hi all,
>>> Please review and provide feedback on the following design proposal for
>>> implementing the blueprint[1] on async-container-operations -
>>> 1. Magnum-conductor would have a pool of threads for executing the
>>> container operations, viz. executor_threadpool. The size of the
>>> executor_threadpool will be configurable. [Phase0]
>>> 2. Every time, Magnum-conductor(Mcon) receives a
>>> container-operation-request from Magnum-API(Mapi), it will do the
>>> initial validation, housekeeping and then pick a thread from the
>>> executor_threadpool to execute the rest of the operations. Thus Mcon
>>> will return from the RPC request context much faster without blocking
>>> the Mapi. If the executor_threadpool is empty, Mcon will execute in a
>>> manner it does today, i.e. synchronously - this will be the
>>> rate-limiting mechanism - thus relaying the feedback of exhaustion.
>>> [Phase0]
>>> How often we are hitting this scenario, may be indicative to the
>>> operator to create more workers for Mcon.
>>> 3. Blocking class of operations - There will be a class of operations,
>>> which can not be made async, as they are supposed to return
>>> result/content inline, e.g. 'container-logs'. [Phase0]
>>> 4. Out-of-order considerations for NonBlocking class of operations -
>>> there is a possible race around condition for create followed by
>>> start/delete of a container, as things would happen in parallel. To
>>> solve this, we will maintain a map of a container and executing thread,
>>> for current execution. If we find a request for an operation for a
>>> container-in-execution, we will block till the thread completes the
>>> execution. [Phase0]
>>> This mechanism can be further refined to achieve more asynchronous
>>> behavior. [Phase2]
>>> The approach above puts a prerequisite that operations for a given
>>> container on a given Bay would go to the same Magnum-conductor instance.
>>> [Phase0]
>>> 5. The hand-off between Mcon and a thread from executor_threadpool can
>>> be reflected through new states on the 'container' object. These states
>>> can be helpful to recover/audit, in case of Mcon restart. [Phase1]
>>> Other considerations -
>>> 1. Using eventlet.greenthread instead of real threads => This approach
>>> would require further refactoring the execution code and embed yield
>>> logic, otherwise a single greenthread would block others to progress.
>>> Given, we will extend the mechanism for multiple COEs, and to keep the
>>> approach straight forward to begin with, we will use 'threading.Thread'
>>> instead of 'eventlet.greenthread'.
>> Also unsure about the above, not quite sure I connect how greenthread
>> usage requires more yield logic (I'm assuming you mean the yield
>> statement here)? Btw if magnum is running with all things monkey
>> patched (which it seems like
>> https://github.com/openstack/magnum/blob/master/magnum/common/rpc_service.py#L33
>> does) then magnum usage of 'threading.Thread' is a
>> 'eventlet.greenthread' underneath the covers, just fyi.
> SURO> Let's consider this -
> function A () {
> block B; // validation
> block C; // Blocking op
> }
> Now, if we make C a greenthread, as it is, would it not block the entire
> thread that runs through all the greenthreads? I assumed, it would and
> that's why we have to incorporate finer grain yield into C to leverage
> greenthread. If the answer is no, then we can use greenthread.
> I will validate which version of threading.Thread was getting used.

Unsure how to answer this one.

If all things are monkey patched then any time a blocking operation 
(i/o, lock acquisition...) is triggered the internals of eventlet go 
through a bunch of jumping around to then switch to another green thread 
(http://eventlet.net/doc/hubs.html). Once u start partially using 
greenthreads and mixing real threads then you have to start trying to 
reason about yielding in certain places (and at that point you might as 
well go to py3.4+ since it has syntax made just for this kind of thinking).

Pointer for the thread monkey patching btw:



Easy way to see this:

 >>> import eventlet
 >>> eventlet.monkey_patch()
 >>> import thread
 >>> thread.start_new_thread.__module__
 >>> thread.allocate_lock.__module__

> In that case, keeping the code for thread.Threading is portable, as it
> would work as desired, even if we remove monkey_patching, right?

Yes, use `thread.Threading` (if u can) so that maybe magnum could switch 
off monkey patching someday, although typically unless u are already 
testing that turning it off in unit tests/functional tests it wouldn't 
be an easy flip that will typically 'just work' (especially since afaik 
magnum is using some oslo libraries which only work under 

>>> Refs:-
>>> [1] -
>>> https://blueprints.launchpad.net/magnum/+spec/async-container-operations
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