在Apache2.0中新加入了ServerLimit指令，使得無須重編譯Apache就可以加大MaxClients。下面是筆者的prefork配置段。
<IfModule prefork.c>
StartServers 10
MinSpareServers 10
MaxSpareServers 15
ServerLimit 2000
MaxClients 1500
MaxRequestsPerChild 10000
</IfModule>
BTW: ServerLimit的最大值是20000，這對於大多數站點是足夠了，但如果你一定要再加大的話，那麼這個值位於源代碼樹下的server/mpm/prefork/prefork.c中。裡面的
#define DEFAULT_SERVER_LIMIT 256
#define MAX_SERVER_LIMIT 20000
這兩行就對應著MaxClients和ServerLimit的限制值。但我相信很少有人可以用到20000的並發連接數。
如果你有耐性看完了我這篇文檔，我相信您對Apache2.0 缺省的prefork的工作原理有所熟悉了，理解了它的工作過程後，就可以根據您的實際情況在FreeBSD下來配置Apache相關的核心參數以獲得最大的性能。
其實用apache2的話，./configure中應該用--with-mpm=worker參數，這樣才能發揮apache2的多線程機制。
vi ~apache_install_path/conf/httpd.conf
<IfModule worker.c>
StartServers 2
MaxClients 150
MinSpareThreads 25
MaxSpareThreads 75
ThreadsPerChild 25
MaxRequestsPerChild 0
</IfModule>
MaxClients x ThreadsPerChild =響應的請求數，可以根據自己的實際情況，更改threadsperchild數目。
對於linux下，其實不能實現多線程模式，要加大MaxClients 的數目，solaris下，就非常優秀了。。
wangmingda 說的也很正確，他是apache2編譯的時候採用了默認的 --with-mpm=prefork模式，這種模式是兼容舊的軟件版本的，不能更好的發揮apache2的線成特性。
引用:
Compile-Time Configuration Issues
Choosing an MPM
Apache 2.x supports pluggable concurrency models, called Multi-Processing Modules (MPMs). When building Apache, you must choose an MPM to use. There are platform-specific MPMs for some platforms: beos, mpm_netware, mpmt_os2, and mpm_winnt. For general Unix-type systems, there are several MPMs from which to choose. The choice of MPM can affect the speed and scalability of the httpd:
The worker MPM uses multiple child processes with many threads each. Each thread handles one connection at a time. Worker generally is a good choice for high-traffic servers because it has a smaller memory footprint than the prefork MPM.
The prefork MPM uses multiple child processes with one thread each. Each process handles one connection at a time. On many systems, prefork is comparable in speed to worker, but it uses more memory. Prefork's threadless design has advantages over worker in some situations: it can be used with non-thread-safe third-party modules, and it is easier to debug on platforms with poor thread debugging support.
For more information on these and other MPMs, please see the MPM documentation.
因為在設置的時候出現錯誤，所以把我的想法說說看。
設置
ServerLimit 8
StartServers 2
MinSpareThreads 15
MaxSpareThreads 25
ThreadsPerChild 25
MaxRequestsPerChild 1000
MaxClients 250
錯誤：
WARNING: MaxClients of 250 would require 10 servers,
and would exceed the ServerLimit value of 8.
Automatically lowering MaxClients to 200. To increase,
please see the ServerLimit directive.
經過多次計算測試，我覺得應該是
MaxClients <= ServerLimit * ThreadsPerChild
官方有教
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