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From Tim Whittington <t...@apache.org>
Subject Re: bindOnInit and maxConnections for AJP connectors
Date Wed, 06 Apr 2011 21:51:00 GMT
On Wed, Apr 6, 2011 at 11:16 PM, Mark Thomas <markt@apache.org> wrote:
> On 05/04/2011 10:50, Tim Whittington wrote:
>> Is what's actually going on more like:
>>
>> APR: use maxConnections == pollerSize (smallest will limit, but if
>> pollerSize < maxConnections then the socket backlog effectively won't
>> be used as the poller will keep killing connections as they come in)
>>
>> NIO: use maxConnections to limit 'poller size'
>>
>> HTTP: use maxConnections. For keep alive situations, reduce
>> maxConnections to something closer to maxThreads (the default config
>> is 10,000 keepalive connections serviced by 200 threads with a 60
>> second keepalive timeout, which could lead to some large backlogs of
>> connected sockets that take 50 minutes to get serviced)
>
> This is indeed the case. There are a number of issues with the current
> BIO implementation.
>
> 1. Keep-alive timeouts
> As per the TODO comment in Http11Processor#process(), the keep-alive
> timeout needs to take account of the time spent in the queue.
>
> 2. The switch to a queue does result in the possibility of requests with
> data being delayed by requests without data in keep-alive.
>
> 3. HTTP pipe-lining is broken (this is bug 50957 [1]). The sequence is:
> - client sends 1 complete request and part of second request
> - tomcat processes first request
> - tomcat recycles the input buffer
> - client sends remainder of second request
> - tomcat sees an incomplete request and returns a 505
> There are variations of this depending on exactly how much of the second
> request has been read by Tomcat at the point the input buffer is
> recycled. Note that r1086349 [2] has protected against the worst of what
> could go wrong (mixed responses etc) but has not fixed the underlying issue.
>
> The change that triggered all of the above issues is r822234 [3].
>
> Reverting r822234 isn't an option as the async code depends on elements
> of if.
>
>
> The fix for issue 1 is simple so I do not intend to discuss it further.
>
>
> The fix for issue 2 is tricky. The fundamental issue is that to resolve
> it and to keep maxConnections >> maxThreads we need NIO like behaviour
> from a BIO socket which just isn't possible.
> Fixing 1 will reduce the maximum length of delay that any one request
> might experience which will help but that won't address the fundamental
> issue.
> For sockets in keepalive, I considered trying to fake NIO behaviour by
> using a read with a timeout of 1ms, catching the SocketTimeoutException
> and returning them to the back of the queue if there is no data. The
> overhead of that looks to be around 2-3ms for a 1ms timeout. I'm worried
> about CPU usage but for a single thread this doesn't seem to be
> noticeable. More testing with multiple threads is required. The timeout
> could be tuned by looking at the current number of active threads, size
> of the queue etc. but it is an ugly hack.
> Returning to the pre [3] approach of disabling keep-alive once
> connections > 75% of threads would fix this at the price of no longer
> being able to support maxConnections >> maxThreads.


Yeah, I went down this track as well before getting to the "Just use
APR/NIO" state of mind.
It is an ugly hack, but might be workable if the timeout is large
enough to stop it being a busy loop on the CPU.
With 200 threads, even a 100ms timeout would give you a 'reasonable' throughput.
Even if we do this, I still think maxConnections should be somewhat
closer to maxThreads than it is now if the BIO connector is being
used.


> I thought of two options for issue 3:
> a) Assign a processor (+ inputbuffer, output buffer etc.) to a socket
> and don't recycle it until the socket is closed.
> - Increases memory requirements.
> - Fixes issue 3
> - Retains current request processing order.
>
> b) Check the input buffer at the end of the loop in
> Http11Processor#process() and process the next request if there is any
> data in the input buffer.
> - No Increase in memory requirements.
> - Fixes issue 3
> - Pipelined requests will get processed earlier (before they would have
> been placed at the back of the request processing queue)
>
> I think option b) is the way to go to fix issue


+1
It's an unfair scheduling, but given issue 2 that's a fairly moot
point with the BIO connector.


>
>
> The fixes for 1 & 3 seem fairly straight forward and unless anyone
> objects, I'll go ahead (when I get a little time) and implement those. I
> think the fix for 2 needs some further discussion. What do folks think?
>
>
> Mark
>
>
>
> [1] https://issues.apache.org/bugzilla/show_bug.cgi?id=50957
> [2] http://svn.apache.org/viewvc?rev=1086349&view=rev
> [3] http://svn.apache.org/viewvc?view=rev&rev=823234
>
>>
>> cheers
>> tim
>>
>> On Tue, Apr 5, 2011 at 8:51 PM, Tim Whittington <timw@apache.org> wrote:
>>> In the AJP standard implementation docs, the following are not
>>> mentioned, although they're properties of AbstractEndpoint and
>>> probably should work:
>>> - bindOnInit
>>> - maxConnections
>>> Am I right in assuming these should be possible in the AJP connector
>>> (my reading of the code indicates they are - just wanted to check if
>>> something arcane was going on)?
>>>
>>> If so I'll update the docs.
>>>
>>> cheers
>>> tim
>>>
>>
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>
>
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