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From Violeta Georgieva <violet...@apache.org>
Subject Re: Read events suspend/resume logic in websocket impl to achieve backpressure
Date Tue, 14 Feb 2017 21:43:27 GMT
Hi,

2017-02-10 11:07 GMT+02:00 Mark Thomas <markt@apache.org>:
>
> On 09/02/17 22:08, Violeta Georgieva wrote:
>>
>> 2017-02-08 11:25 GMT+02:00 Violeta Georgieva <violetagg@apache.org>:
>>>
>>> 2017-02-08 1:51 GMT+02:00 Mark Thomas <markt@apache.org>:
>
>
> <snip/>
>
>>>> I guess that makes me reluctantly in favour of it in principle but I'd
>>
>> very much prefer to review a patch proposal minus the reformatting.
>>>>
>>>>
>>>
>>> There is a new patch
>>> - no formatting noise
>>> - Martin's comments included
>>
>>
>> There is a new patch:
>> - With a fix for the Martin's comment (StringManager)
>> - I switched from AtomicBoolean to AtomicIntegerFieldUpdater in order to
>> minimize the memory usage
>
>
> Thanks. Much easier to read.
>
> Having reviewed the patch, I'm concerned about thread-safety on resume.
I'll use NIO terminology but I believe the same issues apply to all three
connectors.
>
> Consider the case where the client is sending data as fast as it can.
>
> On suspension, the socket will be added to the poller. More data will
arrive, the socket will be processed, no data will be read (because
processing is suspended) and the socket will be added to the poller again.
I'm fairly sure (but haven't confirmed with a test) that when more data
arrives the poller will trigger socket processing again. This loop will
continue until the network buffers are full. (Even if I am wrong on the
poller firing again immediately, there is still a problem.)
>
> On resume, the backlog of data needs to be processed. As currently
implemented, this backlog will be processed on the thread that calls
resume(). That may be undesirable for several reasons:
> - it might not be a container thread;
> - processing the backlog may take time impacting on other work the
>   thread expects to do
> - when the poller triggers socket processing again there could be
>   two threads processing the same socket (very bad)
>
> Therefore, I think resume needs to call
socketWrapper.processSocket(SocketEvent.OPEN_READ, true)
>
> That will solve the concurrent threads processing the same socket problem
but it could cause another problem. When that container thread completes,
it will add the socket to the poller again. The problem is that the socket
will already have been added to the poller. Adding a socket to the poller
more than once has caused problems in the past.
>
> That brings me to the conclusion that a different approach is needed. I
think we need a new SocketState value SUSPEND. Currently returning UPGRADED
from upgradeDispatch() registers the socket for read. SUSPEND would
essentially be a NO-OP. When resume() is called, it would trigger a call to
socketWrapper.processSocket(SocketEvent.OPEN_READ, true) during which when
upgradeDispatch() completes it would return UPGRADE, adding the socket to
the poller and allowing processing to continue.
>

A new patch is available based on the provided comments.
Can you please review it.

> This approach would mean some internal API changes but that is fine for
9.0.x and I don't see a problem with 8.5.x either. Whether this is
back-ported to 8.0.x and 7.0.x is TBD. It also opens up the possibility of
being able to suspend/resume other protocols but I haven't thought a great
deal about how that might work.

I need this functionality only for 9.0.x and 8.5.x.

Thanks,
Violeta

>
> Because I started thinking about thread-safety on resume, I haven't dug
into the patch in detail.
>
>
> Mark
>
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