Improving mod_perl Driven Site's Performance -- Part VII: Performance Tuning by Tweaking Apache Configuration Page 3

  HW: RS6000, 1Gb RAM
  SW: AIX 4.1.5 . mod_perl 1.16, apache 1.3.3
  Machine running only mysql, httpd docs and mod_perl servers.
  Machine was _completely_ unloaded during the benchmarking.

After each server restart when I changed the server's configuration, I
made sure that the scripts were preloaded by fetching a script at
least once for every child.


It is important to notice that none of the requests timed out, even if
it was kept in the server's queue for more than a minute!  That is the
way ab works, which is OK for testing purposes but will be
unacceptable in the real world - users will not wait for more than
five to ten seconds for a request to complete, and the client
(i.e. the browser) will time out in a few minutes.


Now let's take a look at some real code whose execution time is more
than a few milliseconds.  We will do some real testing and collect the
data into tables for easier viewing.


I will use the following abbreviations:
  NR    = Total Number of Request
  NC    = Concurrency
  MC    = MaxClients
  MRPC  = MaxRequestsPerChild
  RPS   = Requests per second


Running a mod_perl script with lots of mysql queries (the script under
test is mysqld limited)
(http://www.example.com/perl/access/access.cgi?do_sub=query_form),
with the configuration:
  MinSpareServers        8
  MaxSpareServers       16
  StartServers          10
  MaxClients            50
  MaxRequestsPerChild 5000


gives us:
     NR   NC    RPS     comment
  ------------------------------------------------
     10   10    3.33    # not a reliable figure
    100   10    3.94    
   1000   10    4.62    
   1000   50    4.09


Conclusions: Here I wanted to show that when the application is
slow (not due to perl loading, code compilation and execution, but
limited by some external operation) it almost does not matter what
load we place on the server.  The RPS (Requests per second) is almost
the same.  Given that all the requests have been served, you have the
ability to queue the clients, but be aware that anything that goes
into the queue means a waiting client and a client (browser) that
might time out!


Now we will benchmark the same script without using the mysql (code
limited by perl only):
(http://www.example.com/perl/access/access.cgi), it's the same
script but it just returns the HTML form, without making SQL queries.

  MinSpareServers        8
  MaxSpareServers       16
  StartServers          10
  MaxClients            50
  MaxRequestsPerChild 5000
     NR   NC      RPS   comment
  ------------------------------------------------
     10   10    26.95   # not a reliable figure
    100   10    30.88   
   1000   10    29.31
   1000   50    28.01
   1000  100    29.74
  10000  200    24.92
 100000  400    24.95


Conclusions: This time the script we executed was pure perl (not
limited by I/O or mysql), so we see that the server serves the
requests much faster.  You can see the number of requests per second
is almost the same for any load, but goes lower when the number of
concurrent clients goes beyond MaxClients.  With 25 RPS, the
machine simulating a load of 400 concurrent clients will be served in
16 seconds.  To be more realistic, assuming a maximum of 100
concurrent clients and 30 requests per second, the client will be
served in 3.5 seconds.  Pretty good for a highly loaded server.