Improving mod_perl Driven Site's Performance -- Part VII: Performance Tuning by Tweaking Apache Configuration Page 2

Let's raise the request load to 100 x 10 (10 users, each makes 100 requests):

  % ./ab -n 1000 -c 10  http://www.example.com/perl/access/access.cgi
  Concurrency Level:      10
  Complete requests:      1000
  Failed requests:        0
  Requests per second:    139.76

As expected, nothing changes -- we have the same 10 concurrent users. Now let's raise the number of concurrent users to 50:

  % ./ab -n 1000 -c 50  http://www.example.com/perl/access/access.cgi
  Complete requests:      1000
  Failed requests:        0
  Requests per second:    133.01

We see that the server is capable of serving 50 concurrent users at 133 requests per second! Let's find the upper limit. Using -n 10000 -c 1000 failed to get results (Broken Pipe?). Using -n 10000 -c 500 resulted in 94.82 requests per second. The server's performance went down with the high load.

The above tests were performed with the following configuration:

  MinSpareServers 8
  MaxSpareServers 6
  StartServers 10
  MaxClients 50
  MaxRequestsPerChild 1500

Now let's kill each child after it serves a single request. We will use the following configuration:

  MinSpareServers 8
  MaxSpareServers 6
  StartServers 10
  MaxClients 100
  MaxRequestsPerChild 1


Simulate 50 users each generating a total of 20 requests:
  % ./ab -n 1000 -c 50  http://www.example.com/perl/access/access.cgi


The benchmark timed out with the above configuration.... I watched the
output of ps as I ran it, the parent process just wasn't capable
of respawning the killed children at that rate.  When I raised the
MaxRequestsPerChild to 10, I got 8.34 requests per second.  Very
bad - 18 times slower!  You can't benchmark the importance of the
MinSpareServers, MaxSpareServers and StartServers with this
kind of test.


Now let's reset MaxRequestsPerChild to 1500, but reduce
MaxClients to 10 and run the same test:
  MinSpareServers 8
  MaxSpareServers 6
  StartServers 10
  MaxClients 10
  MaxRequestsPerChild 1500


I got 27.12 requests per second, which is better but still 4-5 times
slower.  (I got 133 with MaxClients set to 50.)


Summary: I have tested a few combinations of the server
configuration variables (MinSpareServers, MaxSpareServers,
StartServers, MaxClients and MaxRequestsPerChild).  The
results I got are as follows:


MinSpareServers, MaxSpareServers and StartServers are only
important for user response times.  Sometimes users will have to wait a
bit.


The important parameters are MaxClients and MaxRequestsPerChild.
MaxClients should be not too big, so it will not abuse your
machine's memory resources, and not too small, for if it is your users
will be forced to wait for the children to become free to serve them.
MaxRequestsPerChild should be as large as possible, to get the full
benefit of mod_perl, but watch your server at the beginning to make
sure your scripts are not leaking memory, thereby causing your server
(and your service) to die very fast.


Also it is important to understand that we didn't test the response
times in the tests above, but the ability of the server to respond
under a heavy load of requests. If the test script was heavier, the
numbers would be different but the conclusions very similar.


The benchmarks were run with: