[plt-scheme] Limiting Concurrent Connections to Web Server

I think you want one thread that just manages the limits (and perhaps
runs with the dispatchers privs). Each connection would then send a
message to that thread saying "can I go now?" and when it gets a
response, it goes. The server thread would then track if its
communication partners died or if they terminated normally and when
below the limit and there are more waiting, it would reply to them,
letting them go.

You don't seem to need the full generality of kill-safety, since you
have a custodian that you know never dies.

Robby

On Mon, Jan 12, 2009 at 10:57 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
> Perhaps, I don't think I'm clever enough though.
>
> Here's a hack that "works":
>
> (define (make-limit-dispatcher num inner)
>  (let ([sem (make-semaphore num)])
>    (lambda (conn req)
>      (parameterize ([current-custodian (current-server-custodian)])
>        (thread
>         (lambda ()
>           (call-with-semaphore
>            sem
>            (lambda ()
>              (inner conn req)))))))))
>
> Basically before going inside the limit, it changes the resource
> policy so that this connection's resources are charged to the whole
> server, so when the connection goes down, the thread doesn't get
> killed. If you do this, you won't get deadlocks, but you will get lots
> of errors, because the connection's ports will die and the thread will
> keep running.
>
> The obvious kill-safe strategy[1] has the same problem.
>
> 1. Create a "limit manager" thread at the server level, it receives
> requests from the dispatchers to do the work, but doesn't die. But if
> the connection died, I don't know how to communicate that back to the
> limit manager. This would serialize requests, so you'd need N of them,
> where N is the limit. Mz threads are cheap, but not as cheap as Erlang
> threads, so this might not be a great idea.
>
> Jay
>
> On Mon, Jan 12, 2009 at 9:52 AM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
>> Do you think it can be made kill-safe with the current mz?
>>
>> Robby
>>
>> On Mon, Jan 12, 2009 at 10:41 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
>>> I can reproduce it. Essentially the problem is that when a connection
>>> dies, the server kills the threads associated with it. In this case,
>>> the killed threads were supposed to post to the semaphore when they
>>> were done, but they never finished. Basically, the limiting is not
>>> kill-safe [1].
>>>
>>> Jay
>>>
>>> 1. http://www.cs.utah.edu/plt/kill-safe/
>>>
>>> On Sat, Jan 10, 2009 at 2:15 PM, Henk Boom <lunarc.lists at gmail.com> wrote:
>>>> Has anyone else had any luck duplicating this?
>>>>
>>>>    Henk
>>>>
>>>
>>>
>>>
>>> --
>>> Jay McCarthy <jay at cs.byu.edu>
>>> Assistant Professor / Brigham Young University
>>> http://teammccarthy.org/jay
>>>
>>> "The glory of God is Intelligence" - D&C 93
>>> _________________________________________________
>>>  For list-related administrative tasks:
>>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>>
>>>
>>
>
>
>
> --
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://teammccarthy.org/jay
>
> "The glory of God is Intelligence" - D&C 93
>