Improving mod_perl Sites' Performance: Part 3

In this article we will continue the topic started in the previous article. This time we talk about tools that help us with code profiling and memory usage measuring.

Code Profiling Techniques

The profiling process helps you to determine which subroutines or just snippets of code take the longest time to execute and which subroutines are called most often. You will probably just want to optimize those.

When do you need to profile your code? You do that when you suspect that some part of your code is being called very often and so there may be a need to optimize it to significantly improve the overall performance.

For example, you might have used the diagnostics pragma, which extends the terse diagnostics normally emitted by both the Perl compiler and the Perl interpreter, augmenting them with the more verbose and endearing descriptions found in the perldiag manpage. If you've ever done so, then you know that it might slow your code down tremendously, so let's first see whether or not it actually does.

We will run a benchmark, once with diagnostics enabled and once disabled, on a subroutine called test_code.

The code inside the subroutine does an arithmetic and a numeric comparison of two strings. It assigns one string to another if the condition tests true but the condition always tests false. To demonstrate the diagnostics overhead the comparison operator is intentionally wrong. It should be a string comparison, not a numeric one.


  use Benchmark;
  use diagnostics;
  use strict;
  
  my $count = 50000;
  
  disable diagnostics;
  my $t1 = timeit($count,\&test_code);
  
  enable  diagnostics;
  my $t2 = timeit($count,\&test_code);
  
  print "Off: ",timestr($t1),"\n";
  print "On : ",timestr($t2),"\n";
  
  sub test_code{
    my ($a,$b) = qw(foo bar);
    my $c;
    if ($a == $b) {
      $c = $a;
    }
  }

For only a few lines of code we get:


  Off:  1 wallclock secs ( 0.81 usr +  0.00 sys =  0.81 CPU)
  On : 13 wallclock secs (12.54 usr +  0.01 sys = 12.55 CPU)

With diagnostics enabled, the subroutine test_code() is 16 times slower than with diagnostics disabled!

Now let's fix the comparison the way it should be, by replacing == with eq, so we get:


    my ($a,$b) = qw(foo bar);
    my $c;
    if ($a eq $b) {
      $c = $a;
    }

and run the same benchmark again:


  Off:  1 wallclock secs ( 0.57 usr +  0.00 sys =  0.57 CPU)
  On :  1 wallclock secs ( 0.56 usr +  0.00 sys =  0.56 CPU)

Now there is no overhead at all. The diagnostics pragma slows things down only when warnings are generated.

After we have verified that using the diagnostics pragma might adds a big overhead to execution runtime, let's use the code profiling to understand why this happens. We are going to use Devel::DProf to profile the code. Let's use this code:


  diagnostics.pl
  --------------
  use diagnostics;
  print "Content-type: text/html\n\n";
  test_code();
  sub test_code{
    my ($a,$b) = qw(foo bar);
    my $c;
    if ($a == $b) {
      $c = $a;
    }
  }

Run it with the profiler enabled, and then create the profiling stastics with the help of dprofpp:


  % perl -d:DProf diagnostics.pl
  % dprofpp
  
  Total Elapsed Time = 0.342236 Seconds
    User+System Time = 0.335420 Seconds
  Exclusive Times
  %Time ExclSec CumulS #Calls sec/call Csec/c  Name
   92.1   0.309  0.358      1   0.3089 0.3578  main::BEGIN
   14.9   0.050  0.039   3161   0.0000 0.0000  diagnostics::unescape
   2.98   0.010  0.010      2   0.0050 0.0050  diagnostics::BEGIN
   0.00   0.000 -0.000      2   0.0000      -  Exporter::import
   0.00   0.000 -0.000      2   0.0000      -  Exporter::export
   0.00   0.000 -0.000      1   0.0000      -  Config::BEGIN
   0.00   0.000 -0.000      1   0.0000      -  Config::TIEHASH
   0.00   0.000 -0.000      2   0.0000      -  Config::FETCH
   0.00   0.000 -0.000      1   0.0000      -  diagnostics::import
   0.00   0.000 -0.000      1   0.0000      -  main::test_code
   0.00   0.000 -0.000      2   0.0000      -  diagnostics::warn_trap
   0.00   0.000 -0.000      2   0.0000      -  diagnostics::splainthis
   0.00   0.000 -0.000      2   0.0000      -  diagnostics::transmo
   0.00   0.000 -0.000      2   0.0000      -  diagnostics::shorten
   0.00   0.000 -0.000      2   0.0000      -  diagnostics::autodescribe

It's not easy to see what is responsible for this enormous overhead, even if main::BEGIN seems to be running most of the time. To get the full picture we must see the OPs tree, which shows us who calls whom, so we run:


  % dprofpp -T

and the output is:


 main::BEGIN
   diagnostics::BEGIN
      Exporter::import
         Exporter::export
   diagnostics::BEGIN
      Config::BEGIN
      Config::TIEHASH
      Exporter::import
         Exporter::export
   Config::FETCH
   Config::FETCH
   diagnostics::unescape
   .....................
   3159 times [diagnostics::unescape] snipped
   .....................
   diagnostics::unescape
   diagnostics::import
 diagnostics::warn_trap
   diagnostics::splainthis
      diagnostics::transmo
      diagnostics::shorten
      diagnostics::autodescribe
 main::test_code
   diagnostics::warn_trap
      diagnostics::splainthis
         diagnostics::transmo
         diagnostics::shorten
         diagnostics::autodescribe
   diagnostics::warn_trap
      diagnostics::splainthis
         diagnostics::transmo
         diagnostics::shorten
        diagnostics::autodescribe

So we see that two executions of diagnostics::BEGIN and 3161 of diagnostics::unescape are responsible for most of the running overhead.

If we comment out the diagnostics module, we get:


  Total Elapsed Time = 0.079974 Seconds
    User+System Time = 0.059974 Seconds
  Exclusive Times
  %Time ExclSec CumulS #Calls sec/call Csec/c  Name
   0.00   0.000 -0.000      1   0.0000      -  main::test_code

It is possible to profile code running under mod_perl with the Devel::DProf module, available on CPAN. However, you must have apache version 1.3b3 or higher and the PerlChildExitHandler enabled during the httpd build process. When the server is started, Devel::DProf installs an END block to write the tmon.out file. This block will be called at server shutdown. Here is how to start and stop a server with the profiler enabled:


  % setenv PERL5OPT -d:DProf
  % httpd -X -d `pwd` &
  ... make some requests to the server here ...
  % kill `cat logs/httpd.pid`
  % unsetenv PERL5OPT
  % dprofpp

The Devel::DProf package is a Perl code profiler. It will collect information on the execution time of a Perl script and of the subs in that script (remember that print() and map() are just like any other subroutines you write, but they come bundled with Perl!)

Another approach is to use Apache::DProf, which hooks Devel::DProf into mod_perl. The Apache::DProf module will run a Devel::DProf profiler inside each child server and write the tmon.out file in the directory $ServerRoot/logs/dprof/$$ when the child is shutdown (where $$ is the number of the child process). All it takes is to add to httpd.conf:


  PerlModule Apache::DProf

Remember that any PerlHandler that was pulled in before Apache::DProf in the httpd.conf or startup.pl, will not have its code debugging information inserted. To run dprofpp, chdir to $ServerRoot/logs/dprof/$$ and run:


  % dprofpp

(Lookup the ServerRoot directive's value in httpd.conf to figure out what your $ServerRoot is.)

Measuring the Memory of the Process

One very important aspect of performance tuning is to make sure that your applications don't use much memory, since if they do you cannot run many servers and therefore in most cases under a heavy load the overall performance degrades.

In addition the code may not be clean and leak memory, which is even worse. In this case, the same process serves many requests and after each request more memory is used. After a while all your RAM will be used and machine will start swapping (use the swap partition) which is a very undesirable event, since it may lead to a machine crash.

The simplest way to figure out how big the processes are and see whether they grow is to watch the output of top(1) or ps(1) utilities.

For example the output of top(1):


    8:51am  up 66 days,  1:44,  1 user,  load average: 1.09, 2.27, 2.61
  95 processes: 92 sleeping, 3 running, 0 zombie, 0 stopped
  CPU states: 54.0% user,  9.4% system,  1.7% nice, 34.7% idle
  Mem:  387664K av, 309692K used,  77972K free, 111092K shrd,  70944K buff
  Swap: 128484K av,  11176K used, 117308K free                170824K cached

     PID USER PRI NI SIZE  RSS SHARE STAT LIB %CPU %MEM   TIME COMMAND
  29225 nobody 0  0  9760 9760  7132 S      0 12.5  2.5   0:00 httpd_perl
  29220 nobody 0  0  9540 9540  7136 S      0  9.0  2.4   0:00 httpd_perl
  29215 nobody 1  0  9672 9672  6884 S      0  4.6  2.4   0:01 httpd_perl
  29255 root   7  0  1036 1036   824 R      0  3.2  0.2   0:01 top
    376 squid  0  0 15920  14M   556 S      0  1.1  3.8 209:12 squid
  29227 mysql  5  5  1892 1892   956 S N    0  1.1  0.4   0:00 mysqld
  29223 mysql  5  5  1892 1892   956 S N    0  0.9  0.4   0:00 mysqld
  29234 mysql  5  5  1892 1892   956 S N    0  0.9  0.4   0:00 mysqld

Which starts with overall information of the system and then displays the most active processes at the given moment. So for example if we look at the httpd_perl processes we can see the size of the resident (RSS) and shared (SHARE) memory segments. This sample was taken on the production server running linux.

But of course we want to see all the apache/mod_perl processes, and that's where ps(1) comes to help. The options of this utility vary from one Unix flavor to another, and some flavors provide their own tools. Let's check the information about mod_perl processes:


  % ps -o pid,user,rss,vsize,%cpu,%mem,ucomm -C httpd_perl
    PID USER      RSS   VSZ %CPU %MEM COMMAND
  29213 root     8584 10264  0.0  2.2 httpd_perl
  29215 nobody   9740 11316  1.0  2.5 httpd_perl
  29216 nobody   9668 11252  0.7  2.4 httpd_perl
  29217 nobody   9824 11408  0.6  2.5 httpd_perl
  29218 nobody   9712 11292  0.6  2.5 httpd_perl
  29219 nobody   8860 10528  0.0  2.2 httpd_perl
  29220 nobody   9616 11200  0.5  2.4 httpd_perl
  29221 nobody   8860 10528  0.0  2.2 httpd_perl
  29222 nobody   8860 10528  0.0  2.2 httpd_perl
  29224 nobody   8860 10528  0.0  2.2 httpd_perl
  29225 nobody   9760 11340  0.7  2.5 httpd_perl
  29235 nobody   9524 11104  0.4  2.4 httpd_perl

Now you can see the resident (RSS) and virtual (VSZ) memory segments (and shared memory segment if you ask for it) of all mod_perl processes. Please refer to the top(1) and ps(1) man pages for more information.

You probably agree that using top(1) and ps(1) are cumbersome if we want to use memory size sampling during the benchmark test. We want to have a way to print memory sizes during the program execution at desired places. If you have GTop modules installed, which is a perl glue to the libgtop library, it's exactly what we need.

Note: GTop requires the libgtop library but is not available for all platforms. Visit http://www.home-of-linux.org/gnome/libgtop/ to check whether your platform/flavor is supported.

GTop provides an API for retrieval of information about processes and the whole system. We are only interested in memory sampling API methods. To print all the process related memory information we can execute the following code:


  use GTop;
  my $gtop = GTop->new;
  my $proc_mem = $gtop->proc_mem($$);
  for (qw(size vsize share rss)) {
      printf "   %s => %d\n", $_, $proc_mem->$_();
  }

When executed we see the following output (in bytes):


      size => 1900544
     vsize => 3108864
     share => 1392640
       rss => 1900544

So if we are interested in to print the process resident memory segment before and after some event we just do it: For example if we want to see how much extra memory was allocated after a variable creation we can write the following code:


  use GTop;
  my $gtop = GTop->new;
  my $before = $gtop->proc_mem($$)->rss;
  my $x = 'a' x 10000;
  my $after  = $gtop->proc_mem($$)->rss;
  print "diff: ",$after-$before, " bytes\n";

and the output


  diff: 20480 bytes

So we can see that Perl has allocated extra 20480 bytes to create $x (of course the creation of after needed a few bytes as well, but it's insignificant compared to a size of $x)

The Apache::VMonitor module with help of the GTop module allows you to watch all your system information using your favorite browser from anywhere in the world without a need to telnet to your machine. If you are looking into what information you can retrieve with GTop, you should examine Apache::VMonitor, as it deploys a big part of the API that GTop provides.

If you are running a true BSD system, you may use BSD::Resource::getrusage instead of GTop. For example:


  print "used memory = ".(BSD::Resource::getrusage)[2]."\n"

For more information refer to the BSD::Resource manpage.

Measuring the Memory Usage of Subroutines

With help of Apache::Status you can find out the size of each and every subroutine.

  1. Build and install mod_perl as you always do, make sure it's version 1.22 or higher.
  2. Configure /perl-status if you haven't already:
    
      <Location /perl-status>
        SetHandler perl-script
        PerlHandler Apache::Status
        order deny,allow
        #deny from all
        #allow from ...
      </Location>
  3. Add to httpd.conf
    
      PerlSetVar StatusOptionsAll On
      PerlSetVar StatusTerse On
      PerlSetVar StatusTerseSize On
      PerlSetVar StatusTerseSizeMainSummary On
    
      PerlModule B::TerseSize
  4. Start the server (best in httpd -X mode)
  5. From your favorite browser fetch http://localhost/perl-status
  6. Click on 'Loaded Modules' or 'Compiled Registry Scripts'
  7. Click on the module or script of your choice (you might need to run some script/handler before you will see it here unless it was preloaded)
  8. Click on 'Memory Usage' at the bottom
  9. You should see all the subroutines and their respective sizes.

Now you can start to optimize your code, or test which of several implementations is of the least size.

For example let's compare CGI.pm's OO vs. procedural interfaces:

As you will see below the first OO script uses about 2k bytes while the second script (procedural interface) uses about 5k.

Here are the code examples and the numbers:

  1. 
      cgi_oo.pl
      ---------
      use CGI ();
      my $q = CGI->new;
      print $q->header;
      print $q->b("Hello");
  2. 
      cgi_mtd.pl
      ---------
      use CGI qw(header b);
      print header();
      print b("Hello");

After executing each script in single server mode (-X) the results are:

  1. 
      Totals: 1966 bytes | 27 OPs
      
      handler 1514 bytes | 27 OPs
      exit     116 bytes |  0 OPs
  2. 
      Totals: 4710 bytes | 19 OPs
      
      handler  1117 bytes | 19 OPs
      basefont  120 bytes |  0 OPs
      frameset  120 bytes |  0 OPs
      caption   119 bytes |  0 OPs
      applet    118 bytes |  0 OPs
      script    118 bytes |  0 OPs
      ilayer    118 bytes |  0 OPs
      header    118 bytes |  0 OPs
      strike    118 bytes |  0 OPs
      layer     117 bytes |  0 OPs
      table     117 bytes |  0 OPs
      frame     117 bytes |  0 OPs
      style     117 bytes |  0 OPs
      Param     117 bytes |  0 OPs
      small     117 bytes |  0 OPs
      embed     117 bytes |  0 OPs
      font      116 bytes |  0 OPs
      span      116 bytes |  0 OPs
      exit      116 bytes |  0 OPs
      big       115 bytes |  0 OPs
      div       115 bytes |  0 OPs
      sup       115 bytes |  0 OPs
      Sub       115 bytes |  0 OPs
      TR        114 bytes |  0 OPs
      td        114 bytes |  0 OPs
      Tr        114 bytes |  0 OPs
      th        114 bytes |  0 OPs
      b         113 bytes |  0 OPs

Note, that the above is correct if you didn't precompile all CGI.pm's methods at server startup. Since if you did, the procedural interface in the second test will take up to 18k and not 5k as we saw. That's because the whole of CGI.pm's namespace is inherited and it already has all its methods compiled, so it doesn't really matter whether you attempt to import only the symbols that you need. So if you have:


  use CGI  qw(-compile :all);

in the server startup script. Having:


  use CGI qw(header);

or


  use CGI qw(:all);

is essentially the same. You will have all the symbols precompiled at startup imported even if you ask for only one symbol. It seems to me like a bug, but probably that's how CGI.pm works.

BTW, you can check the number of opcodes in the code by a simple command line run. For example comparing 'my %hash' vs. 'my %hash = ()'.


  % perl -MO=Terse -e 'my %hash' | wc -l
  -e syntax OK
      4

  % perl -MO=Terse -e 'my %hash = ()' | wc -l
  -e syntax OK
     10

The first one has fewer opcodes.

Note that you shouldn't use Apache::Status module on production server as it adds quite a bit of overhead to each request.


References

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