<header>Child Processes</header>
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<p>
Most applications that continuously handle a large
number of requests
have some sort of subprocess mechanism.
With each request being handled by a different subprocess,
even if something unexpected
causes one of the requests to go dramatically awry,
it will just hang that one subprocess
but not the whole application.
As it happens, each subprocess of DansGuardian for handling a request
is visible to the OS as a separate "child process".
(A couple of other DansGuardian processes
will be visible to the OS too.)
<p>
In DansGuardian, there are several parameters related to
the number of "child processes" that handle individual requests.
Appropriate tuning of these parameters might affect performance.
It's roughly the case that increasing the numbers will speed
up the operation of large systems (up to a point).
But larger numbers will use more memory,
and if the total size of all DansGuardian processes plus Squid
spills over real memory into virtual memory,
speed may noticeably decrease.
So there's a balance to be struck.
<p>
You will probably want to tune <i>maxchildren</i>
to be about the same as your peak number of simultaneous users.
And you may want to tune <i>minsparechildren</i> too so it's
about the number of users that typically start
activity at the same time.
If you increase either of these parameters,
you may need to also increase the other parameters proportionally.
<p>
The parameters include:
<ul style='list-style-type: none'>
<li><u>Prefork Children</u>
<br>
Sets the minimum number of child processes to spawn all at once
whenever DansGuardian needs to spawn at least one.
Spawning several subprocesses all at once
shares some overhead and prevents the silliness
of spawing subprocesses one at a time
even though several are needed.
<br>
On large sites you might want to try 10.
<br>
Default = 6
<p>
<li><u>Minimum Spare Children</u>
<br>
Sets the minimum number of child processes to be kept
ready in reserve in case a whole bunch of users
arrive all at once.
<br>
On large sites you might want to try 8.
<br>
Default = 4
<p>
<li><u>Minimum Children</u>
<br>
Sets the minimum number of processes to keep
both to handle active requests and to by ready in reserve for future requests.
This controls how many child processes will be
spawned when DansGuardian first starts up.
It effectively controls how small the footprint
of a system that's momentarily inactive is.
<br>
On large sites you might want to try 32.
<br>
Default = 8
<p>
<li><u>Maximum Spare Children</u>
<br>
Sets the maximum number of processes to keep in reserve
for future requests even though they aren't doing anything at the moment.
When more than this many processes are inactive,
DansGuardian will cull some of them.
This is how much capability DansGuardian
should keep ready at hand after a load spike.
For optimum performance this might need to be about the same
as the peak number of users that ever start activity all at once.
<br>
On large sites you might want to try 64.
<br>
Default = 32
<p>
<li><u>Maximum Children</u>
<br>
Sets the <b>maximum</b> number of subprocesses that can exist,
This number is not simply infinite for two reasons:
<ol>
<li>A limit will <b>prevent DoS attacks</b> from
killing the server with too many spawned processes.
<li>A limit will <b>prevent RAM exhaustion</b>
so the server won't thrash itself to death
even under very high load.
</ol>
On large sites you might want to double or triple this number.
<p>
<li><u>Maximum Age of Children</u>
<br>
Sets the maximum age of a child process before DansGuardian croaks it.
This is the number of connections they handle before exiting.
Periodically getting rid of existing child processes and spawning new ones
is a safety mechanism in case there are "memory leaks"
or other bugs in the child processes
that only become visible when the process runs for a very long time.
<br>
If your system is plagued by instability,
try reducing this parameter.
If stability is not an issue but peak performance is,
try increasing this parameter.
<br>
On large sites you might want to try 10000.
<br>
Default = 500
</ul>
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