Forum: Rails France Global queuing dans Passenger.

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A99870c1391c39da2089649745965bda?d=identicon&s=25 Jean-François Trân (Guest)
on 2008-10-29 13:40
(Received via mailing list)
DHH a annoncé (http://twitter.com/d2h/status/980391221) que 37signals
avaient financé la fonctionnalité de global queuing dans Phusion
Passenger.

http://www.modrails.com/documentation/Users%20guid...

Qu'est-ce que le global queueing ? me direz-vous

Extrait de la doc :

"5.4. PassengerUseGlobalQueue <on|off>

Turns the use of global queuing on or off.

This option may only occur once, in the global server
configuration. The default is off.

This feature is sponsored by 37signals.

5.4.1. What does this option do?

Recall that Phusion Passenger spawns multiple backend processes
(e.g. multiple Ruby on Rails processes), each which processes
HTTP requests serially. One of Phusion Passenger's jobs is to
forward HTTP requests to a suitable backend process. A backend
process may take an arbitrary amount of time to process a specific
HTTP request. If the websites are (temporarily) under high load,
and the backend processes cannot process the requests fast
enough, then some requests may have to be queued.

If global queuing is turned off, then Phusion Passenger will use
fair load balancing. This means that each backend process will
have its own private queue. Phusion Passenger will forward an
HTTP request to the backend process that has the least amount
of requests in its queue.

If global queuing is turned on, then Phusion Passenger will use
a global queue that's shared between all backend processes. If
an HTTP request comes in, and all the backend processes are
still busy, then Phusion Passenger will wait until at least one
backend process is done, and will then forward the request
to that process.

5.4.2. When to turn on global queuing?

You should turn on global queuing if one of your web applications
may have long-running requests.

For example suppose that:

global queuing is turned off.

we're currently in a state where all backend processes have 3
requests in their queue, except for a single backend process,
which has 1 request in its queue.

The situation looks like this:

Backend process A:  [*     ]  (1 request in queue)
Backend process B:  [***   ]  (3 requests in queue)
Backend process C:  [***   ]  (3 requests in queue)
Backend process D:  [***   ]  (3 requests in queue)
Each process is currently serving short-running requests.

Phusion Passenger will forward the next request to backend
process A. A will now have 2 items in its queue. We'll mark
this new request with an X:

Backend process A:  [*X    ]  (2 request in queue)
Backend process B:  [***   ]  (3 requests in queue)
Backend process C:  [***   ]  (3 requests in queue)
Backend process D:  [***   ]  (3 requests in queue)

Assuming that B, C and D still aren't done with their current
request, the next HTTP request - let's call this Y - will be
forwarded to backend process A as well, because it has
the least number of items in its queue:

Backend process A:  [*XY   ]  (3 requests in queue)
Backend process B:  [***   ]  (3 requests in queue)
Backend process C:  [***   ]  (3 requests in queue)
Backend process D:  [***   ]  (3 requests in queue)

But if request X happens to be a long-running request that needs
60 seconds to complete, then we'll have a problem. Y won't be
processed for at least 60 seconds. It would have been a better
idea if Y was forward to processes B, C or D instead, because
they only have short-living requests in their queues.

This problem will be avoided entirely if you turn global queuing on.
With global queuing, all backend processes will share the same
queue. The first backend process that becomes available will
take from the queue, and so this "queuing-behind-long-running-request"
problem will never occur.

Turning global queuing off will yield a minor performance improvement
(about 5%, depending on how fast/slow your web application is), which
is why it's off by default."

   -- Jean-François.

--
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