I've discovered a setting in mod_proxy_balancer that prevents the
Mongrel/Rails request queuing vs. accept/close problem from ever being
reached.
For each BalancerMember
- max=1 -- this caps the maximum number of connections Apache will
open a BalancerMember to '1'
- acquire=N max amount of time (N seconds) to wait to acquire a
connection to a balancer member
So, at a minimum:
BalancerMember http://foo max=1 acquire=1
and I'm using
BalancerMember http://127.0.0.1:9000 max=1 keepalive=on acquire=1 timeout=1
=====
I experimented with three mongrel servers, and tied one up for 60
seconds at a time calling "sleep" in a handler.
Without the "acquire" parameter mod_proxy_balancer's simple round-robin
scheme blocked waiting when it reached a busy BalancerManager,
effectively queuing the request. With "acquire" set the balancer
stepped over the busy BalancerMember and continue searching through it's
round-robin cycle.
So, whether or not Mongrel's accept/close and request queuing are
issues, there is a setting in mod_proxy_balancer that prevents either
problem from being triggered.
At a bare minimum, for a single-threaded process running in Mongrel
BalancerMember http://127.0.0.1:9000 max=1 acquire=1
BalancerMember http://127.0.0.1:9001 max=1 acquire=1
...
With all BalancerMembers busy Apache returns a 503 Server Busy, which is
a heck of a lot more appropriate than 502 proxy error.
======
It turns out that having Mongrel reap threads before calling accept both
queueing in Mongrel and prevents Mongrel's accept/close behavior.
But BalancerMembers in mod_proxy_balancer will still need "acquire" to
be set -- otherwise proxy client threads will sit around waiting for
Mongrel to call accept -- effectively queuing requests in Apache.
Since max=1 acquire=1 steps around the queuing problem altogether, the
reap-before-accept fix, though more correct, is of no practical benefit.
====
With the current Mongrel code, BalancerMember max > 1 and Mongrel
num_processors > 1 triggers the accept/close bug.
Likewise, BalancerMember max >1 with Mongrel num_processors > 1 runs
into Mongrel's request queuing....
====
Conclusion ---
I'd like to see Mongrel return a 503 Server Busy when an incoming
request hits the num_processor limit.
For practical use, the fix to the problems is in configuring
mod_proxy_balancer such that it shields against encountering either issue.
begin:vcard
fn:Robert Mela
n:Mela;Robert
email;internet:[EMAIL PROTECTED]
x-mozilla-html:FALSE
version:2.1
end:vcard
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