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Practical mod_perl by Eric Cholet, Stas Bekman

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Chapter 11. Tuning Performance by Tweaking Apache’s Configuration

When you implement mod_perl on your system, it’s very important to go through the default configuration file (httpd.conf), because most of the default settings were designed without mod_perl in mind. Some variables (such as MaxClients) should be adapted to the capabilities of your system, while some (such as KeepAlive, in many cases) should be disabled, because although they can improve performance for a plain Apache server, they can reduce performance for a mod_perl server.

Correct configuration of the MinSpareServers, MaxSpareServers, StartServers, MaxClients, and MaxRequestsPerChild parameters is very important. If they are too low, you will under-use the system’s capabilities. If they are too high, it is likely that the server will bring the machine to its knees.

The KeepAlive directive improves the performance of a plain Apache server by saving the TCP handshake if the client requests more than one object from your server. But you don’t want this option to be enabled under mod_perl, since it will keep a large mod_perl process tied to the client and do nothing while waiting for the timeout to occur.

We will talk about these and other issues in the following sections.

Setting the MaxClients Directive

It’s important to specify MaxClients on the basis of the resources your machine has. The MaxClients directive sets the limit on the number of simultaneous requests that can be supported. No more than this number of child server processes will be created. To configure more than 256 clients, you must edit the HARD_SERVER_LIMIT entry in httpd.h and recompile Apache.

With a plain Apache server, it doesn’t matter much if you run many child processes—the processes are about 1 MB each (most of it shared), so they don’t eat a lot of RAM. The situation is different with mod_perl, where the processes can easily grow to 10 MB and more. For example, if you have MaxClients set to 50, the memory usage becomes 50 × 10 MB = 500 MB.[1] Do you have 500 MB of RAM dedicated to the mod_perl server?

With a high MaxClients, if you get a high load the server will try to serve all requests immediately. Your CPU will have a hard time keeping up, and if the child size multiplied by the number of running children is larger than the total available RAM, your server will start swapping. The swapping will slow down everything, which will lead to more swapping, slowing down everything even more, until eventually the machine will die. It’s important that you take pains to ensure that swapping does not normally happen. Swap space is an emergency pool, not a resource to be used routinely. If you are low on memory and you badly need it, buy it. Memory is cheap.

We want the value of MaxClients to be as small as possible, because in this way we can limit the resources used by the server’s children. Since we can restrict each child’s process size, as discussed later, the calculation of MaxClients is straightforward:

Equation 11-0. 

Setting the MaxClients Directive

So if we have 400 MB for the mod_perl server to use, we can set MaxClients to 40 if we know that each child is limited to 10 MB of memory.

You may be wondering what will happen to your server if there are more concurrent users than MaxClients. This situation is pointed out by the following warning message in the error_log file:

[Sat May 18 13:40:35 2002] [error] server reached MaxClients setting,
consider raising the MaxClients setting

Technically there is no problem—any connection attempts over the MaxClients limit will normally be queued, up to a number based on the ListenBacklog directive. When a child process is freed at the end of a different request, the next waiting connection will be served.

But it is an error, because clients are being put in the queue rather than getting served immediately, despite the fact that they do not get an error response. The error can be allowed to persist to balance available system resources and response time, but sooner or later you will need to get more RAM so you can start more child processes. The best approach is to prevent this situation from arising in the first place, and if it keeps on happening you should start worrying about it.

In Chapter 10 we showed that when memory sharing is available, the approximate real memory used can be calculated by adding up all the unshared memory of the client processes plus the memory of the parent process, or, if the latter is unknown, the maximum shared memory size of a single child process, which is smaller than the memory size of the parent process but good enough for our calculations. We have also devised the following formula:

Equation 11-0. 

Setting the MaxClients Directive

where Total_RAM is of course the estimated total RAM available to the web server.

Let’s perform some calculations, first with sharing in place:

Total_RAM                = 500Mb
Max_Process_Size         =  10Mb
Min_Shared_RAM_per_Child =   4Mb

Equation 11-0. 

Setting the MaxClients Directive

then with no sharing in place:

Equation 11-0. 

Setting the MaxClients Directive

With sharing in place, if your numbers are similar to the ones in our example, you can have 64% more servers without buying more RAM (82 compared to 50).

If you improve sharing and the sharing level is maintained throughout the child’s life, you might get:

Total_RAM            = 500Mb
Max_Process_Size     =  10Mb
Shared_RAM_per_Child =   8Mb

Equation 11-0. 

Setting the MaxClients Directive

Here we have 392% more servers (246 compared to 50)!

There is one more nuance to remember. The number of requests per second that your server can serve won’t grow linearly when you raise the value of MaxClients. Assuming that you have a lot of RAM available and you try to set MaxClients as high as possible, you will find that you eventually reach a point where increasing the MaxClients value will not improve performance.

The more clients that are running, the more CPU time will be required and the fewer CPU time slices each process will receive. The response latency (the time to respond to a request) will grow, so you won’t see the expected improvement. Let’s explore these issues.

The test handler that we have used is shown in Example 11-1. You can see that it does mostly CPU-intensive computations.

Example 11-1. Book/HandlerBenchmark.pm

package Book::HandlerBenchmark;
use Apache::Constants qw(:common);
sub handler {
    $r = shift;
    $r->send_http_header('text/html');
    $r->print("Hello");
    my $x = 100;
    my $y = log ($x ** 100) for (0..100);
    return OK;
}
1;

Here’s the configuration section to enable this handler:

PerlModule Book::HandlerBenchmark
<Location /benchmark_handler_middle>
    SetHandler perl-script
    PerlHandler Book::HandlerBenchmark
</Location>

Now we will run the benchmark for different values of MaxClients. The results are:

MaxClients  | avtime completed failed    rps
--------------------------------------------
       100  |    333     50000      0    755
       125  |    340     50000      0    780
       150  |    342     50000      0    791
       175  |    338     50000      0    783
       200  |    339     50000      0    785
       225  |    365     50000      0    760
       250  |    402     50000      0    741
---------------------------------------------

Non-varying sub-test parameters:
---------------------------------------------
MaxRequestsPerChild : 0
StartServers        : 100
Concurrency         : 300
Number of requests  : 50000
---------------------------------------------

Figure 11-1 depicts requests per second versus MaxClients. Looking at this figure, you can see that with a concurrency level of 300, the performance is almost identical for MaxClients values of 150 and 200, but it goes down for the value of 100 (not enough processes) and are even worse for the value of 250 (too many processes competing over CPU cycles). Note that we have kept the server fully loaded, since the number of concurrent requests was always higher than the number of available processes, which means that some requests were queued rather than responded to immediately. When the number of processes went above 200, more and more time was spent by the processes in the sleep state and context switching, enlarging the latency of response generation. On the other hand, with only 100 available processes, the CPU was not fully loaded and we had plenty of memory available. You can see that in our case, a MaxClients value of 150 is close to optimal.[2]

Requests per second as a function of MaxClients

Figure 11-1. Requests per second as a function of MaxClients

This leads us to an interesting discovery, which we can summarize in the following way: increasing your RAM might not improve the performance if your CPU is already fully loaded with the current number of processes. In fact, if you start more processes, you will get a degradation in performance. On the other hand, if you decide to upgrade your machine with a very powerful CPU but you don’t add enough memory, the machine will use swap memory or the CPU will be under-used; in any case, the performance will be poor. Whenever you opt for a more powerful CPU, you must always budget for enough extra memory to ensure that the CPU’s greater processing power is fully utilized. It is generally best to add more memory in the first place to see if that helps with performance problems (assuming you follow our tuning advice as well).

To discover the right configuration for your server, you should run benchmarks on a machine with identical hardware to the one that you are going to use in production. Try to simulate the probable loads your machine will experience. Remember that the load will be variable, and plan accordingly. Experiment with the configuration parameters under different loads to discover the optimal balance of CPU and RAM use for your machine. When you change the processor or add RAM, retest the configuration to see how to change the settings to get the best from the new hardware.

You can tune your machine using reports like the one in our example, by analyzing either the requests per second (rps) column, which shows the throughput of your server, or the average processing time (avtime) column, which can be seen as the latency of your server. Take more samples to build nicer linear graphs, and pick the value of MaxClients where the curve reaches a maximum value for a throughput graph or reaches the minimum value for a latency graph.

Setting the MaxRequestsPerChild Directive

The MaxRequestsPerChild directive sets the limit on the number of requests that an individual child process can handle during its lifetime. After MaxRequestsPerChild requests, the child process will die. If MaxRequestsPerChild is zero, the process will live until the server kills it (because it is no longer needed, which will depend on the value of MinSpareServers and the number of current requests) or until the server itself is stopped.

Setting MaxRequestsPerChild to a non-zero limit solves some memory-leakage problems caused by sloppy programming practices and bugs, whereby a child process consumes a little more memory after each request. In such cases, and where the directive is left unbounded, after a certain number of requests the children will use up all the available memory and the server will die from memory starvation. Note that sometimes standard system libraries leak memory too, especially on operating systems with bad memory management.

If this is your situation you may want to set MaxRequestsPerChild to a small number. This will allow the system to reclaim the memory that a greedy child process has consumed when it exits after MaxRequestsPerChild requests.

But beware—if you set this number too low, you will lose some of the speed bonus you get from mod_perl. Consider using Apache::PerlRun if the leakage is in the CGI script that you run. This handler flushes all the memory used by the script after each request. It does, however, reduce performance, since the script’s code will be loaded and recompiled for each request, so you may want to compare the loss in performance caused by Apache::PerlRun with the loss caused by memory leaks and accept the lesser of the evils.

Another approach is to use the memory usage-limiting modules, Apache::SizeLimit or Apache::GTopLimit. If you use either of these modules, you shouldn’t need to set MaxRequestPerChild (i.e., you can set it to 0), although for some developers, using both in combination does the job. These modules also allow you to control the maximum unshared and minimum shared memory sizes. We discuss these modules in Chapter 14.

Setting MinSpareServers, MaxSpareServers, and StartServers

With mod_perl enabled, it might take as much as 20 seconds from the time you start the server until it is ready to serve incoming requests. This delay depends on the OS, the number of preloaded modules, and the process load of the machine. It’s best to set StartServers and MinSpareServers to high numbers, so that if you get a high load just after the server has been restarted, the fresh servers will be ready to serve requests immediately.

To maximize the benefits of mod_perl, you don’t want to kill servers when they are idle; rather, you want them to stay up and available to handle new requests immediately. We think an ideal configuration is to set MinSpareServers and MaxSpareServers to similar (or even the same) values. Having MaxSpareServers close to MaxClients will completely use all of your resources (if MaxClients has been chosen to take full advantage of the resources) and make sure that at any given moment your system will be capable of responding to requests with the maximum speed (assuming that the number of concurrent requests is not higher than MaxClients—otherwise, some requests will be put on hold).

If you keep a small number of servers active most of the time, keep StartServers low. Keep it low especially if MaxSpareServers is also low, as if there is no load Apache will kill its children before they have been utilized at all. If your service is heavily loaded, make StartServers close to MaxClients, and keep MaxSpareServers equal to MaxClients.

If your server performs other work besides running the mod_perl-enabled server—for example, an SQL server—make MinSpareServers low so the memory of unused children will be freed when the load is light. If your server’s load varies (i.e., you get loads in bursts) and you want fast responses for all clients at any time, you will want to make it high, so that new children will be respawned in advance and able to handle bursts of requests.

For MaxSpareServers, the logic is the same as for MinSpareServers—low if you need the machine for other tasks, high if it’s a host dedicated to mod_perl servers and you want a minimal delay between the request and the response.

KeepAlive

If your mod_perl server’s httpd.conf file includes the following directives:

KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 15

you have a real performance penalty, since after completing the processing for each request, the process will wait for KeepAliveTimeout seconds before closing the connection and will therefore not be serving other requests during this time. With this configuration you will need many more concurrent processes on a server with high traffic.

If you use the mod_status or Apache::VMonitor server status reporting tools, you will see a process in K state when it’s in KeepAlive state.

You will probably want to switch this feature off:

KeepAlive Off

The other two directives don’t matter if KeepAlive is Off.

However, you might consider enabling KeepAlive if the client’s browser needs to request more than one object from your mod_perl server for a single HTML page. If this is the situation, by setting KeepAlive On, for every object rendered in the HTML page on the client’s browser you save the HTTP connection overhead for all requests but the first one.

For example, if the only thing your mod_perl server does is process ads, and each of your pages has 10 or more banner ads (which is not uncommon today), your server will work more efficiently if a single process serves them all during a single connection. However, your client will see a slightly slower response, since the banners will be brought one at a time and not concurrently, as is the case if each <img> tag opens a separate connection.

SSL connections benefit the most from KeepAlive if you don’t configure the server to cache session IDs. See the mod_ssl documentation for how to do this.

You have probably followed our advice to send all the requests for static objects to a plain Apache (proxy/accelerator) server. Since most pages include more than one unique static image, you should keep the default KeepAlive setting of the non-mod_perl server (i.e., keep it On). It will probably also be a good idea to reduce the KeepAliveTimeout to 1 or 2 seconds—a client is going to send a new request on the KeepAlive connection immediately, and the first bits of the request should reach the server within this limit, so wait only for the maximum latency of a modem connection plus a little bit more.

Another option is for the proxy/accelerator to keep the connection open to the client but make individual connections to the server, read the responses, buffer them for sending to the client, and close the server connection. Obviously, you would make new connections to the server as required by the client’s requests.

PerlSetupEnv

By default, PerlSetupEnv is On, but PerlSetupEnv Off is another optimization you should consider.

mod_perl modifies the environment to make it appear as if the script were being called under the CGI protocol. For example, the $ENV{QUERY_STRING} environment variable is initialized with the contents of $r->args( ), and the value returned by $r->server_hostname( ) is put into $ENV{SERVER_NAME}.

But populating %ENV is expensive. Those who have moved to the mod_perl API no longer need this duplicated data and can improve performance by turning it off. Scripts using the CGI.pm module require PerlSetupEnv On because that module relies on the environment created by mod_cgi. This is yet another reason why we recommend using the Apache::Request module in preference to CGI.pm.

Note that you can still set environment variables when PerlSetupEnv is Off. For example, say you use the following configuration:

PerlSetupEnv Off
PerlModule Apache::RegistryNG
<Location /perl>
    PerlSetEnv TEST hi
    SetHandler perl-script
    PerlHandler Apache::RegistryNG
    Options +ExecCGI
</Location>

Now issue a request for the script shown in Example 11-2.

Example 11-2. setupenvoff.pl

use Data::Dumper;
my $r = Apache->request( );
$r->send_http_header('text/plain');
print Dumper \%ENV;

You should see something like this:

$VAR1 = {
          'GATEWAY_INTERFACE' => 'CGI-Perl/1.1',
          'MOD_PERL' => 'mod_perl/1.26',
          'PATH' => '/bin:/usr/bin:/usr... snipped ...',
          'TEST' => 'hi'
        };

Note that we got the value of the TEST environment variable we set in httpd.conf.

Reducing the Number of stat( ) Calls Made by Apache

If (using truss, strace, or another tool available for your OS) you watch the system calls that your mod_perl server makes while processing a request, you will notice that a few stat( ) calls are made, and these are quite expensive. For example, if you have your DocumentRoot set to /home/httpd/docs and you fetch http://localhost/perl-status, you will see:

[snip]
stat("/home/httpd/docs/perl-status", 0xbffff8cc) = -1 
                    ENOENT (No such file or directory)
stat("/home/httpd/docs", {st_mode=S_IFDIR|0755, 
                               st_size=1024, ...}) = 0
[snip]

If you have some dynamic content and your virtual relative URI is looks like /news/perl/mod_perl/summary (i.e., there is no such directory on the web server—the path components are used only for requesting a specific report), this will generate five stat( ) calls before the DocumentRoot is reached and the search is stopped. You will see something like this:

stat("/home/httpd/docs/news/perl/mod_perl/summary", 0xbffff744) = -1 
                    ENOENT (No such file or directory)
stat("/home/httpd/docs/news/perl/mod_perl",         0xbffff744) = -1
                    ENOENT (No such file or directory)
stat("/home/httpd/docs/news/perl",                  0xbffff744) = -1
                    ENOENT (No such file or directory)
stat("/home/httpd/docs/news",                       0xbffff744) = -1
                    ENOENT (No such file or directory)
stat("/home/httpd/docs", 
                    {st_mode=S_IFDIR|0755, st_size=1024, ...})  =  0

How expensive are these calls? Let’s use the Time::HiRes module to find out.

The script in Example 11-3, which you should run on the command line, takes a time sample at the beginning, then does a million stat( ) calls to a nonexistent file, samples the time at the end, and prints the average time it took to make a single stat( ) call.

Example 11-3. stat_call_sample.pl

use Time::HiRes qw(gettimeofday tv_interval);
my $calls = 1_000_000;

my $start_time = [ gettimeofday ];

stat "/foo" for 1..$calls;

my $end_time = [ gettimeofday ];
my $avg = tv_interval($start_time,$end_time) / $calls;
print "The average execution time: $avg seconds\n";

Before we actually run the script we should distinguish between two different scenarios. When the server is idle, the time between the first and the last system call will be much shorter than the same time measured on a loaded system. This is because on an idle system, a process can use the CPU very often, whereas on a loaded system, lots of processes compete for CPU time and each process has to wait longer to get the same amount of CPU time.

So first we run the above code on an unloaded system:

panic% perl stat_call_sample.pl
The average execution time: 4.209645e-06 seconds

Here it takes about four microseconds to execute a stat( ) call. Now we’ll start a CPU-intensive process in one console (make sure to kill the process afterward!). The following code keeps the CPU busy all the time:

panic% perl -e '1 while 1'

And now we run the stat_call_sample.pl script in another console:

panic% perl stat_call_sample.pl
The average execution time: 8.777301e-06 seconds

You can see that the average time has doubled (about eight microseconds). This is intuitive, since there were two processes competing for CPU resources. Now if we run four occurrences of the above code:

panic% perl -e '1**1 while 1' &
panic% perl -e '1**1 while 1' &
panic% perl -e '1**1 while 1' &
panic% perl -e '1**1 while 1' &

and run our script in parallel with these processes, we get:

panic% perl stat_call_sample.pl
2.0853558e-05 seconds

So the average stat( ) system call is five times longer now (about 20 microseconds). Now if you have 50 mod_perl processes that keep the CPU busy all the time, the stat( ) call will be 50 times slower and it’ll take 0.2 milliseconds to complete a series of calls. If you have five redundant calls, as in the strace example above, they add up to one millisecond. If you have more processes constantly consuming CPU resources, this time adds up. Now multiply this time by the number of processes that you have and you get a few seconds lost. For some services this loss is insignificant, while for others it could be very significant.

So why does Apache do all these redundant stat( ) calls? The reason is the default installed TransHandler. One solution would be to supply our own, which would be smart enough not to look for this virtual path and would immediately return OK. In cases where you have a virtual host that serves only dynamically generated documents, you can override the default PerlTransHandler with the following one:

PerlModule Apache::Constants
<VirtualHost 10.10.10.10:80>
    ...
    PerlTransHandler Apache::Constants::OK
    ...
</VirtualHost>

The Apache::Constants::OK constant (which is actually a subroutine) is used here as a handler that does nothing but finish the translation phase by returning OK. By skipping the default translation handler, which tries to find a filesystem component that matches the given URI, you save the redundant stat( ) calls!

As you see, it affects only this specific virtual host. Remember that PerlTransHandler cannot appear inside a specific <Location> or similar section, because the request has not yet been associated with a particular file or directory.

As we will show next, Apache’s default TransHandler may perform several stat( ) calls when the request is served by a virtual resource that doesn’t reside on the filesystem. Things get worse when Apache is configured to look for .htaccess files, adding many redundant open( ) calls.

Let’s start with the following simple configuration and try to reduce the number of redundant system calls to a minimum:

DocumentRoot "/home/httpd/docs"
<Directory />
    AllowOverride All
</Directory>
<Location /foo/test>
    SetHandler perl-script
    PerlHandler Apache::Foo
</Location>

The above configuration causes the Perl handler( ) defined in Apache::Foo to be executed when we make a request to /foo/test. Notice that in the test setup there is no real file to be executed and no .htaccess file.

Using the above configuration, the system calls trace may look as follows:

stat("/home/httpd/docs/foo/test", 0xbffff8fc) = -1 ENOENT
  (No such file or directory)
stat("/home/httpd/docs/foo",      0xbffff8fc) = -1 ENOENT
  (No such file or directory)
stat("/home/httpd/docs",
  {st_mode=S_IFDIR|0755, st_size=1024, ...}) = 0
open("/.htaccess", O_RDONLY)                 = -1 ENOENT
  (No such file or directory)
open("/home/.htaccess", O_RDONLY)            = -1 ENOENT
  (No such file or directory)
open("/home/httpd/.htaccess", O_RDONLY)      = -1 ENOENT
  (No such file or directory)
open("/home/httpd/docs/.htaccess", O_RDONLY) = -1 ENOENT
  (No such file or directory)
stat("/home/httpd/docs/test", 0xbffff774)    = -1 ENOENT
  (No such file or directory)
stat("/home/httpd/docs", 
  {st_mode=S_IFDIR|0755, st_size=1024, ...}) = 0

Now we modify the <Directory> entry and add AllowOverride None, which, among other things, tells Apache not to search for .htaccess files:

<Directory />
    AllowOverride None
</Directory>

After restarting the server and issuing a request to /foo/test, we see that the four open( ) calls for .htaccess have gone. The remaining system calls are:

stat("/home/httpd/docs/foo/test", 0xbffff8fc) = -1 ENOENT 
  (No such file or directory)
stat("/home/httpd/docs/foo",      0xbffff8fc) = -1 ENOENT 
  (No such file or directory)
stat("/home/httpd/docs", 
  {st_mode=S_IFDIR|0755, st_size=1024, ...}) = 0
stat("/home/httpd/docs/test", 0xbffff774)    = -1 ENOENT 
  (No such file or directory)
stat("/home/httpd/docs", 
  {st_mode=S_IFDIR|0755, st_size=1024, ...}) = 0

Next, let’s try to shortcut the foo location with:

Alias /foo/ /

which makes Apache look for the file in the / directory and not under /home/httpd/docs/foo. Let’s restart the server and try again:

stat("/test", 0xbffff8fc) = -1 ENOENT (No such file or directory)

Now we’ve got only one stat( ) call left!

Let’s replace the Alias setting we have just added with:

PerlModule Apache::Constants
PerlTransHandler Apache::Constants::OK

as explained earlier. When we issue the request, we see no remaining stat( ) calls. This technique works if you serve content using only mod_perl handlers, since CGI scripts and other files won’t be looked for on the filesystem now. Also, since the default translation handler is now skipped, $r->filename now won’t be set.

If you want to serve both mod_perl handlers and real files, you will have to write your own PerlTransHandler to handle requests as desired. For example, the following PerlTransHandler will not look up the file on the filesystem if the URI starts with /foo—the handler will return DECLINED and the default PerlTransHandler will be used:

PerlTransHandler 'sub { return shift->uri( ) =~ m|^/foo| \
                      ? Apache::Constants::OK \
                      : Apache::Constants::DECLINED; }'

Let’s see the same configuration using a <Perl> section and a dedicated package (see Example 11-4).

Example 11-4. perl_section.conf

<Perl>
  package Book::Trans;
  use Apache::Constants qw(:common);
  sub handler {
      my $r = shift;
      return OK if $r->uri( ) =~ m|^/foo|;
      return DECLINED;
  }

  package Apache::ReadConfig;  
  $PerlTransHandler = "Book::Trans";
</Perl>

Here we have defined the Book::Trans package and implemented the handler( ) function. Then we have assigned this handler to the PerlTransHandler.

You can move the code in the module into an external file (e.g., Book/Trans.pm) and configure the PerlTransHandler with:

PerlTransHandler Book::Trans

in the normal way (no <Perl> section required).

Now we’ll run some benchmarks to test the solutions described above, both individually and in groups. To make the difference in the number of stat( ) calls more prominent, we will use a very light handler that just prints something out.

The module that we have used is shown in Example 11-5.

Example 11-5. Book/News.pm

package Book::News;
use Apache::Constants qw(:common);
sub handler {
    my $r = shift;
    my $uri = $r->uri;
    my @sections = split "/", $uri;
    # in a real handler you'd do some DB lookup and return the story:
    # my $story = get_story(@sections);
    $r->send_http_header('text/plain');
    print "Story matching @sections\n";
    return OK;
}
1;

This is the URI we have used for testing:

/news/perl/mod_perl/summary

Notice that the URI is long enough to generate many stat( ) calls with the default Apache configuration.

This is the main configuration:

<Location /news>
    SetHandler perl-script
    PerlHandler +Book::News
</Location>

Now we try different configurations and see how they influence performance. Each configuration is listed with a tag in parentheses that is used as a key in the table and explanation that follows.

  1. (default) Nothing was added:

    <Directory />
        AllowOverride All
    </Directory>
  2. (noht) Prevent .htaccess lookup:

    <Directory />
        AllowOverride None
    </Directory>
  3. (alias) Location alias shortcutting:

    Alias /news /
  4. (trans) Using a nondefault TransHandler:

    <Perl>
        package Book::Trans;
        use Apache::Constants qw(:common);
        sub handler {
            my $r = shift;
            return OK if $r->uri( ) =~ m|^/news|;
            return DECLINED;
        }
    
        package Apache::ReadConfig;
        $PerlTransHandler = "Book::Trans";
    </Perl>

The results, sorted by the requests per second (rps) rate, are:

Options    | avtime completed failed    rps
-----------|-------------------------------
noht+alias |     27      5000      0    996
noht+trans |     29      5000      0    988
trans      |     29      5000      0    975
alias      |     28      5000      0    974
noht       |     32      5000      0    885
default    |     34      5000      0    827

with static arguments:

Concurrency         : 30
Number of requests  : 5000

The concurrency and connections don’t matter here; we are concerned with the relative rather than the absolute numbers.

Figure 11-2 depicts these results.

Results of the four solutions

Figure 11-2. Results of the four solutions

Preventing .htaccess lookup (noht) improved the performance by about 8% (885 versus 827). Using alias shortcutting (alias) or a nondefault TransHandler (trans) gave even more of a performance boost: since for a long URI like the one in our example, each directory generates a few stat( ) and open( ) system calls, the speedup was around 15% compared to the standard configuration (default). Grouping the prevention of .htaccess lookup (noht) plus one of the techniques that don’t look for the nonexistent file in the filesystem (alias or trans) gave a performance boost of about 18% (996 versus 827).

As we have seen, the number of pseudo-subdirectories is in direct proportion to the number of stat( ) and open( ) system calls that are made. To prove this, let’s use the standard configuration (default) and benchmark three URIs with a different number of sections (directories), without counting the first section (/news):

Sections  URI
---------------------------------------------
1         /news/perl
3         /news/perl/mod_perl/summary
5         /news/perl/mod_perl/summary/foo/bar

The results are what we expected:

Sections  | avtime completed failed    rps 
---------------------------------------------------
       1  |     33      5000      0    849
       3  |     34      5000      0    829
       5  |     35      5000      0    801
---------------------------------------------------

Each of the two sections add an extra millisecond to the average processing and connection time, which reduces performance by about 25 requests per second.

It’s important to read the figures cautiously. Improving performance by 20% simply by adding a few configuration directives is not likely to be achieved in practice. In our test code we used a very light handler, which did nothing but send a few lines of text without doing any processing. When you use real code, whose runtime is not 30-40 milliseconds but 300-400 milliseconds, the improvement of 7 milliseconds on average (as we saw between the standard configuration (default), giving 34 ms, and the combination of noht and alias, giving 27 ms) might be insignificant. The tuning we’ve discussed here is important mostly for servers that serve millions of requests per day and where every millisecond counts.

But even if your server has a light load, you can still make it a little bit faster. Use a benchmark on the real code and see whether you win something or not.

Symbolic Links Lookup

The two options FollowSymLinks and SymLinksIfOwnerMatch are designed for the user’s security. Unless FollowSymLinks is enabled, symbolic links will not be followed by the server. If SymLinksIfOwnerMatch is enabled, the server will follow symbolic links only when the target file or directory is owned by the same user as the link. Note that the two options are ignored if set within a <Location> block.

This protection costs a little overhead for each request. Wherever in your URL-space you do not have this setting:

Options FollowSymLinks

or you do have this setting:

Options SymLinksIfOwnerMatch

Apache will have to issue an extra call to lstat( ) per directory segment in the path to the file. For example, if you have:

DocumentRoot /home/httpd/docs
<Directory />
    Options SymLinksIfOwnerMatch
</Directory>

and a request is made for the URI /index.html, Apache will perform lstat( ) on these three directories and one file:

/home
/home/httpd
/home/httpd/docs
/home/httpd/docs/index.html

The deeper the file is located in the filesystem, the more lstat( ) system calls will be made. The results of these lstat( ) calls are never cached, so they will occur for every single request. If you really want the symbolic-links security checking, you can do something like this:

DocumentRoot /home/httpd/docs
<Directory />
    Options FollowSymLinks
</Directory>
<Directory /home/httpd/docs>
    Options -FollowSymLinks +SymLinksIfOwnerMatch
</Directory>

This at least avoids the extra checks for the DocumentRoot path. Note that you’ll need to add similar sections if you have any Alias or RewriteRule paths outside of your document root. For highest performance, and no symbolic link protection, set the FollowSymLinks option everywhere, and never set the SymLinksIfOwnerMatch option.

Disabling DNS Resolution

You should make sure that your httpd.conf file has this setting:

HostnameLookups Off

This is the default.

If this directive is set to On (or even worse, Double), Apache will try to use DNS resolution to translate the client’s IP address into its hostname for every single request.

The problem is that there are many servers with broken reverse DNS, which means that resolution will never succeed, but it might take a significant time for the lookup attempt to time out. The web page will not be served before the lookup has either succeeded or timed out, because it’s assumed that if you have this feature enabled you want to know the hostname from which the request came. Consequently Apache won’t run any script or handler until the lookup attempt has concluded.

Moreover, you can end up with a hostname that is completely useless and gives you far less information than the IP address would. To avoid this problem you can enable:

HostnameLookups Double

which does a reverse lookup, then a forward lookup on what it gets to make sure that the IP address is not being spoofed. However, this double lookup makes it even slower.

If you need DNS names in some CGI script or handler, you should use gethostbyname( ) or its equivalents.

In addition to having HostnameLookups turned off, you should avoid using hostname-based access control and use IP-based access control instead. If you have a setting like this:

<Location /perl-status>
    ...
    Order deny, allow
    Deny  from all
    Allow from www.example.com
</Location>

the server will have to perform a double reverse DNS lookup for each incoming IP address to make sure it matches the domain name listed in the Allow directive and is not being spoofed. Of course, in our example this will happen only for requests for URIs starting with /perl-status.

This is another way to do the authorization based on the IP address:

<Location /perl-status>
    ...
    Order deny, allow
    Deny  from all
    Allow from 128.9.176.32
</Location>

Note that since some IP addresses map to multiple hosts (multiple CNAME records), this solution will not always do what you want.

Response Compressing

Have you ever served a huge HTML file (e.g., a file bloated with JavaScript code) and wondered how you could send it compressed, thus dramatically cutting down the download times? After all, Java applets can be compressed into a jar and benefit from faster download times. Why can’t we do the same with plain text files (HTML, JavaScript, etc.)? Plain text can often be compressed by a factor of 10.

Apache::GzipChain can help you with this task. If a client (browser) understands gzip encoding, this module compresses the output and sends it downstream. The client decompresses the data upon receiving it and renders the HTML as if it was fetching uncompressed HTML. Furthermore, this module is used as a filter, thanks to Apache::OutputChain, and can therefore compress not only static files but also dynamic content created from your handlers or scripts.

For example, to compress all HTML files on the fly, do this:

<Files *.html>
    SetHandler perl-script
    PerlHandler Apache::OutputChain Apache::GzipChain Apache::PassFile
</Files>

Browsers are supposed to declare that they can handle compressed input by setting the Accept-Encoding header. Unfortunately, many browsers cannot handle it, even if they claim that they can. Apache::GzipChain keeps a list of user agents, and also looks at the User-Agent header to check for browsers known to accept compressed output.

As an example, if you want to return compressed files that will in addition pass through the Embperl module, you would write:

<Location /test>
    SetHandler perl-script
    PerlHandler Apache::OutputChain Apache::GzipChain \
                Apache::EmbperlChain Apache::PassFile
</Location>

Watch the access_log file to see how many bytes were actually sent, and compare that with the bytes sent using a regular configuration.

Notice that the rightmost PerlHandler must be a content producer. Here we are using Apache::PassFile, but you can use any module that creates output.

Alternatively, you may want to try Apache::Compress, which is compatible with Apache::Filter and is covered in Appendix B. To compress only outgoing static files, you can look at the mod_gzip and mod_deflate modules for Apache.

The cool thing about these modules is that they don’t require any modification of the code. To enable or disable them, only httpd.conf has to be tweaked.

References



[1] Of course, you also have to take into account the shared memory usage, as described in Chapter 10.

[2] When we tried the same benchmark on different machines with a much stronger CPU and more memory, we saw different results. So we would like to stress again that the optimal configuration choices for a given application and load pattern may vary from machine to machine.

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