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Before you start to optimize your code and server configuration, you need to consider the demands that will be placed on the hardware and the operating system. There is no point in investing a lot of time and money in configuration tuning and code optimizing only to find that your server's performance is poor because you did not choose a suitable platform in the first place.
Because hardware platforms and operating systems are developing rapidly, the following advisory discussion must be in general terms, without mentioning specific vendors' names.
This section discusses the characteristics and features you should be looking for to support a mod_perl-enabled Apache server. When you know what you want from your OS, you can go out and find it. Visit the web sites of the operating systems that interest you. You can gauge users' opinions by searching the relevant discussions in newsgroup and mailing-list archives. Deja (http://deja.com/) and eGroups (http://egroups.com/) are good examples. However, your best shot is probably to ask other mod_perl users.
Clearly, before choosing an OS, you will want to make sure that mod_perl even runs on it! As you will have noticed throughout this book, mod_perl 1.x is traditionally a Unix-centric solution. Although it also runs on Windows, there are several limitations related to its implementation.
The problem is that Apache on Windows uses a multithreaded implementation, due to the fact that Windows can't use the multi-process scheme deployed on Unix platforms. However, when mod_perl (and thereby the Perl runtime) is built into the Apache process, it cannot run multithreaded, because before Version 5.8.0 the Perl runtime wasn't thread-safe.
What does this mean for you? Well, essentially it means that your Apache process will be able to serve only one request at a time, just like when using httpd -X. Of course, this becomes a severe performance hit, making you unable to have more than one user receiving a page at a time. The situation is resolved in mod_perl 2.0, however, thanks to advances in both Apache and Perl, as described in Chapter 24. Furthermore, you can still use mod_perl on Windows for development, although you should follow the considerations below when choosing the production OS.
Probably the most important features in an OS are stability and robustness. You are in an Internet business. You do not keep normal 9 A.M. to 5 P.M. working hours like many conventional businesses you know. You are open 24 hours a day. You cannot afford to be offline, because your customers will go shop at another service like yours (unless you have a monopoly). If the OS of your choice crashes every day, first do a little investigation. There might be a simple reason that you can find and fix. However, there are OSes that won't work unless you reboot them twice a day. You don't want to use an OS of this kind, no matter how good the OS's vendor sales department is. Do not follow flush advertisements—follow developers' advice instead.
Generally, people who have used an OS for some time can tell you a lot about its stability. Ask them. Try to find people who are doing similar things to what you are planning to do; they may even be using the same software. There are often compatibility issues to resolve, and you may need to become familiar with patching and compiling your OS.
You want an OS with a good memory-management implementation. Some OSes are well known as memory hogs. The same code can use twice as much memory on one OS compared to another. If the size of the mod_perl process is 10 MB and you have tens of these processes running, it definitely adds up!
Some OSes and/or their libraries (e.g., C runtime libraries) suffer from memory leaks. A leak is when some process requests a chunk of memory for temporary storage but then does not subsequently release it. The chunk of memory then won't be available for any purpose until the process that requested it dies. You cannot afford such leaks. A single mod_perl process sometimes serves thousands of requests before it terminates; if a leak occurs on every request, the memory demands could become huge. Of course, your code can be the cause of the memory leaks as well, but that's easy to detect and solve. Certainly, you can reduce the number of requests to be served over the process's life, but that can degrade performance. When you have so many performance concerns to think about, do you really want to be using faulty code that's not under your control?
You want an OS with good memory-sharing capabilities. If you preload the Perl modules and scripts at server startup, they are shared between the spawned children (at least for part of a process's life—memory pages can become "dirty" and cease to be shared). This feature can vastly reduce memory consumption. Therefore, you don't want an OS that doesn't have memory-sharing capabilities.
If you are in a big business, you probably do not mind paying another $1,000 for some fancy OS with bundled support. But if your resources are low, you will look for cheaper or free OSes. Free does not mean bad. In fact, it can be quite the opposite—some of the free OSes have the best support available.
This is easy to understand—most people are not rich and will try to use a cheaper or free OS first if it does the work for them. If it fits their needs, they will keep using it and eventually come to know it well enough to be able to provide support for others in trouble. Why would they do this for free? One reason is the spirit of the first days of the Internet, when there was no commercial Internet and people helped each other because someone else had helped them first. We were there, we were touched by that spirit, and we are keen to keep that spirit alive.
Nevertheless, we are living in a material world, and our bosses pay us to keep the systems running. So if you feel that you cannot provide the support yourself and you do not trust the available free resources, you must pay for an OS backed by a company to which you can turn in case of problems. Insufficient support has often been characterized as an important drawback of open source products, and in the past it may have been the main reason for many companies to choose a commercial product.
Luckily, in recent years many companies have realized how good the open source products are and started to provide official support for these products. So your suggestion of using an open source operating system cannot be dismissed solely on the basis of lacking vendor support; most likely you will be able to find commercial support just like with any other commercial OS vendor!
Also remember that the less money you spend on an OS and software, the more you will be able to spend on faster and stronger hardware. Of course, for some companies money is a non-issue, but there are many companies for which it is a major concern.
You might find yourself in a position where you have invested a lot of time and money into developing some proprietary software that is bundled with the OS you chose (say, writing a mod_perl handler that takes advantage of some proprietary features of the OS and that will not run on any other OS). Things are under control, the performance is great, and you sing with happiness on your way to work. Then, one day, the company that supplies your beloved OS goes bankrupt (not unlikely nowadays), or they produce a newer, incompatible version and decide not to support the old one (it happens all the time). You are stuck with their early masterpiece, no support, and no source code! What are you going to do? Invest more money into porting the software to another OS?
The OSes in this hazard group tend to be developed by a single company or organization, so free and open source OSes are probably less susceptible to this kind of problem. Their development is usually distributed between many companies and developers, so if a person who developed a really important part of the kernel loses interest in continuing, someone else usually will pick up the work and carry on. Of course, if some better project shows up tomorrow, developers might migrate there and finally drop the development, but in practice people are often given support on older versions and helped to migrate to current versions. Development tends to be more incremental than revolutionary, so upgrades are less traumatic, and there is usually plenty of notice of the forthcoming changes so that you have time to plan for them.
Of course, with the open source OSes you have the source code, too. You can always have a go at maintaining it yourself, but do not underestimate the amount of work involved.
Actively developed OSes generally try to keep pace with the latest technology developments and continually optimize the kernel and other parts of the OS to become better and faster. Nowadays, the Internet and networking in general are the hottest topics for system developers. Sometimes a simple OS upgrade to the latest stable version can save you an expensive hardware upgrade. Also, remember that when you buy new hardware, chances are that the latest software will make the most of it.
If a new product supports an old one by virtue of backward compatibility with previous products of the same family, you might not reap all the benefits of the new product's features. You might get almost the same functionality for much less money if you were to buy an older model of the same product.
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| 7.3. References |  | 8.2. Choosing the Right Hardware |
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