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From Aaron Bannert <aa...@clove.org>
Subject Re: [PATCH] worker MPM: reuse transaction pools
Date Mon, 10 Sep 2001 22:48:24 GMT
On Mon, Sep 10, 2001 at 03:13:48PM -0700, Brian Pane wrote:
> I suppose there are a total of four candidate designs, right?  It looks
> like you're varying two independent dimensions:
>   * condition variable strategy: one CV, or one per thread
>   * pool strategy: worker-managed, or listener-managed.
> so there are four possible permutations.  (The other two
> permutations would look like hybrids of "short and sweet" and
> "time-space tradeoff."  I don't know whether they're interesting
> algorithms or not, but it's worth noting that these hybrids are
> a possibility if you find in testing that neither of the first
> two designs stands out as the clear winner.)
> In the "pool strategy" dimension, I think a worker-managed pool design (as
> used in "time-space tradeoff") is a better choice than a 
> listener-managed one.
> With worker-managed, you can clear and re-use the pool, which is cheaper 
> than
> destroying and recreating it.  And it's very easy to eliminate locking for
> subpool creation by adding a thread-private free list (you could dig up my
> old patch for this, or Sander's patch which is a more general-purpose 
> solution).
> 
> In the "CV strategy" dimension, I'll wait for benchmark results before I try
> to guess which approach is fastest. :-)


Actually, the two variants you describe here are not entirely distinct. In
both of my above models there is a global CV for managing the worker
queue. You are correct about the pool strategy, but that is pretty much
the only variant.

In order to reuse pools, we have to have a way for the listener to
get ahold of the transaction pool before it calls the accept. The
product of accept() is the socket, which must then make it back to the
appropriate worker thread, and that worker must somehow be awakened to
begin processing of the accepted socket.

The "short and sweet" model is much more a producer-consumer: the listener
thread produces (pool, socket) pairs that are then consumed by the worker
threads.

The "time-space tradeoff" model implements a resource-pool of
waiting worker threads; the listener plucks out the next available
worker thread, calls accept() on it's behalf, and sends it on it's merry
way.


I'm beginning to realize that I will end up comparing the overhead of
creating/destroying a transaction pool once per request ("short and sweet"),
and the overhead of managing that transaction pool in the reusable case
with the added overhead of an extra CV/mutex per thread
("time-space tradeoff").

(Hope that makes more sense to you, it definately helped me :)
-aaron

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