Re: FCGI process handling and fine-tunning for saving CPU, under Perl code

On Fri, Dec 02, 2005 at 03:56:43PM -0200, phoenix_fly wrote:
    [...]
> > > how to handle it, I mean, how do I restart the processes
> > > when I update the script code? How do I know the number of
> > > the process??
> >
> > Two options: you can have the mod_fastcgi process manager
> > (if you use that) notice when your script file is updated
> > on disk and then automatically restart it when it is
> > (assuming you can switch that option on); this is limited
> > by the fact that it only checks your main script file, so
> > if you have modules which you also change, this won't help
> > you.
> 
> I am not aware I have a mod_cgi process manager installed, but in fact, this
> correlate updates happen just all the time. So I really will need to
> kill/restart through the shell.

OK -- in that case you're probably best off making your
fastcgi servers daemon processes listening on a specific
(UNIX or TCP) port, and have the webserver talk to them.

> Alternatively, you can check the timestamp on your
> > script files in your main loop. We have code to do this
> > here:
> >
> https://secure.mysociety.org/cvstrac/getfile/mysociety/perllib/mySociety/Wat
> chUpdate.pm
> > though it's a little ugly.
> 
> I have tried to use it, but for some reason it didn´t work, I mean, the
> script stopped working after including the code calling the module. Also, I
> fear that this verification will consume precious CPU, maybe it´s better to
> restart/kill the processes myself, as soon as I know how to do that.

Well, possibly, but checking the timestamps on the files
oughtn't to use very much CPU. E.g:

$ time perl -e ''
real    0m0.004s
user    0m0.003s
sys     0m0.001s
$ time perl -e 'for (my $i = 0; $i < 10000; ++$i) { my $s = stat "/etc/passwd"; }'
real    0m0.034s
user    0m0.013s
sys     0m0.020s


> > > My main goal with FCGI is to save precious CPU minutes,
> > > as the traffic is increasing a lot (and my host will kick
> > > me out if I get too heavy-processing, as they just dont
> > > have dedicated servers available). You guys know if the
> > > CPU savings are worth adapting the code? I mean, can I
> > > expect to use, like, 30% less CPU?
> >
> > That depends on the startup cost of your program vs the
> > marginal cost of serving one request, basically. Without
> > knowing any more detail it's impossible to comment; really
> > you need to measure it now and decide whether it's
> > worthwhile.
> 
> All scripts load 5 method libraries with more than 10 thousand lines total,
> and connect to mysql before doing anything. So FCGI it may be a good thing
> to save CPU, right? You´d say I can save 50% CPU?

Without seeing any measurements, no, I can't say that, but
the measurements are pretty easy to do!

-- 
``Some circumstantial evidence is very strong,
  as when one finds a trout in the milk.'' (Thoreau)
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