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Revision 17 (KB1PYW, 2008-10-21 23:14) → Revision 18/27 (zkv, 2008-11-21 22:46)

h1. Performance Improvements 

 *Module: core* 

 {{>toc}} 

 h2. Description - Performance Issues 

 lighttpd is optimized into varying directions. The most important direction is 
 performance. The operation system has two major facilities to help lighttpd deliver its 
 best performance. 

 h2. HTTP Keep-Alive 

 Disabling keep-alive might help your server if you suffer from a large 
 number of open file descriptors. 

 
 <pre> 
 The defaults for the server are: 
 <pre> :: 

   server.max-keep-alive-requests = 16 
   server.max-keep-alive-idle = 5 
   server.max-read-idle = 60 
   server.max-write-idle = 360 

 
 </pre> 

 Handling 128 keep-alive requests in a row on a single connection, waiting 30 seconds 
 before an unused keep-alive connection gets dropped by lighttpd. 

 If you handle several connections at once under a high load (let's assume 500 connections 
 in parallel for 24h) you might run into the out-of-fd problem described below. :: 

 <pre> 

   
   server.max-keep-alive-requests = 4 
   server.max-keep-alive-idle = 4 

 
 </pre> 

 It would release the connections earlier and would free file descriptors without a 
 detrimental performance loss. 

 Disabling keep-alive completely is the last resort if you are still short on file descriptors: :: 

 <pre> 

   
   server.max-keep-alive-requests = 0 

 
 </pre> 

 h2. Event Handlers 

 The first one is the Event Handler which takes care of notifying the server 
 that one of the connections is ready to send or receive. As you can see, 
 every OS has at least the select() call which has some limitations. 

 |_.OS          |_.Method    |_.Config value    | <pre> 
 |all           |select      |select ============ ========== =============== 
 OS             Method       Config Value 
 ============ ========== =============== 
 all            | select       select 
 |Unix          |poll        |poll              | Unix           poll         poll 
 |Linux Linux 2.4+    |rt-signals|linux-rtsig       |     rt-signals linux-rtsig 
 |Linux Linux 2.6+     epoll        linux-sysepoll 
 Solaris        /dev/poll    |epoll solaris-devpoll 
 FreeBSD, ... kqueue       |linux-sysepoll    | freebsd-kqueue 
 |Solaris NetBSD         kqueue       |/dev/poll |solaris-devpoll | kqueue 
 |FreeBSD, ...|kqueue      |freebsd-kqueue    | ============ ========== =============== 
 |NetBSD        |kqueue      |kqueue            | </pre> 

 For more information on this topic take a look at http://www.kegel.com/c10k.html 

 h2. Configuration 

 The event handler can be set by specifying the 'Config Value' from above 
 in the ``server.event-handler`` variable 

 e.g.: :: 
 <pre> 

   
   server.event-handler = "linux-sysepoll" 

 
 </pre> 

 h2. Network Handlers 

 The basic network interface for all platforms at the syscalls read() and 
 write(). Every modern OS provides its own syscall to help network servers 
 transfer files as fast as possible. 

 If you want to send out a file from the webserver, it doesn't make any sense 
 to copy the file into the webserver just to write() it back into a socket 
 in the next step. 

 sendfile() minimizes the work in the application and pushes a file directly 
 into the network card (ideally). 

 lighttpd supports all major platform-specific calls: 

 |_.OS          |_.Method     |_.Config Value     | <pre> 
 |all ========== ========== ================= 
 OS           |write        |write              | Method       Config Value 
 |Unix ========== ========== ================= 
 all          |writev write        write   
 Unix         writev       |writev             | writev 
 |Linux Linux 2.4+    |sendfile sendfile     |linux-sendfile     | linux-sendfile 
 |Linux Linux 2.6+ sendfile64 linux-sendfile 
 Solaris      sendfilev    |sendfile64 |linux-sendfile solaris-sendfilev 
 FreeBSD      sendfile     | freebsd-sendfile 
 |Solaris       |sendfilev    |solaris-sendfilev| ========== ========== ================= 
 |FreeBSD       |sendfile     |freebsd-sendfile | </pre> 

 The best backend is selected at compile time. In case you want to use  
 another backend set: 
 ::  
 <pre> 
   server.network-backend = "writev" 
 </pre> 

 You can find more information about network backend in:  
 
   http://blog.lighttpd.net/articles/2005/11/11/optimizing-lighty-for-high-concurrent-large-file-downloads 


 h2. Max Connections 

 As lighttpd is a single-threaded server, its main resource limit is the 
 number of file descriptors, which is set to 1024 by default (on most systems). 

 If you are running a high-traffic site you might want to increase this limit 
 by setting: setting :: 
 <pre> 

   
   server.max-fds = 2048 

 
 </pre> 

 This only works if lighttpd is started as root. 

 h2. Out-of-fd condition 

 Since file descriptors are used for TCP/IP sockets, files and directories, 
 a simple request for a PHP page might result in using 3 file descriptors: 

 # the TCP/IP socket to the client 
 # the TCP/IP and Unix domain socket to the FastCGI process 
 # the filehandle to the file in the document root to check if it exists 

 If lighttpd runs out of file descriptors, it will stop accepting new 
 connections for awhile to use the existing file descriptors to handle the 
 currently-running requests. 

 If more than 90% of the file descriptors are used then the handling of new 
 connections is disabled. If it drops below 80% again new connections will 
 be accepted again. 

 Under some circumstances you will see :: 

 <pre> 

 
 ... accept() failed: Too many open files 

 
 </pre> 

 in the error log. This tells you there were too many new requests at once 
 and lighttpd could not disable the incoming connections soon enough. The 
 connection was dropped and the client received an error message like 'connection 
 failed'. This is very rare and might only occur in test setups. 

 Increasing the ``server.max-fds`` limit will reduce the probability of this 
 problem. 

 h2. stat() cache 

 A stat(2) can be expensive; caching it saves time and context switches. 

 Instead of using stat() every time to check for the existence of a file 
 you can stat() it once and monitor the directory the file is in for 
 modifications. As long as the directory doesn't change, the files in it 
 must all still be the same. 

 With the help of FAM or gamin you can use kernel events to assure that 
 your stat cache is up to date. Alternatively you can use the "simple" 
 method, which caches results for up to one second.  

 Enabling the stat cache can significantly improve performance on  
 heavily-loaded servers: 

 servers. :: 
 <pre> 

   
   server.stat-cache-engine = "fam"     # either fam, simple or disabled 

 
 </pre> 

 See http://oss.sgi.com/projects/fam/faq.html for information about FAM. 
 See http://www.gnome.org/~veillard/gamin/overview.html for information about gamin. 

 See also: http://trac.lighttpd.net/trac/wiki/server.stat-cache-engineDetails    


 h2. Platform-Specific Notes 

 h3. Linux 

 For Linux 2.4.x you should think about compiling lighttpd with the option 
 ``--disable-lfs`` to disable the support for files larger than 2GB. lighttpd will 
 fall back to the ``writev() + mmap()`` network calls which is ok, but not as 
 fast as possible but support files larger than 2GB. 

 Disabling the TCP options reduces the overhead of each TCP packet and might 
 help to get the last few percent of performance out of the server. Be aware that 
 disabling these options most likely decreases performance for high-latency and lossy 
 links. 

 * <pre> 
 - net.ipv4.tcp_sack = 0 
 * - net.ipv4.tcp_timestamps = 0 

 *Note:* // Be carefull with net.ipv4.tcp_timestamps. It this one,  
                               // it caused massive problems for me under benchmark load 
                               // with a high count of concurrent connections. 
 </pre> 

 Increasing the TCP send and receive buffers will increase the performance a 
 lot if (and only if) you have a lot of large files to send. 

 * <pre> 
 - net.ipv4.tcp_wmem = 4096 65536 524288 
 * - net.core.wmem_max = 1048576 
 </pre> 

 If you have a lot of large file uploads, increasing the receive buffers will help. 

 * <pre> 
 - net.ipv4.tcp_rmem = 4096 87380 524288 
 * - net.core.rmem_max = 1048576 
 </pre> 

 Some things that a high-traffic site found useful: :: 

 <pre> 

   
   # These ensure that TIME_WAIT ports either get reused or closed fast. 
   net.ipv4.tcp_fin_timeout = 1 
   net.ipv4.tcp_tw_recycle = 1 
   # TCP memory 
   net.core.rmem_max = 16777216 
   net.core.rmem_default = 16777216 
   net.core.netdev_max_backlog = 262144 
   net.core.somaxconn = 262144 
  
   net.ipv4.tcp_syncookies = 1 
   net.ipv4.tcp_max_orphans = 262144 
   net.ipv4.tcp_max_syn_backlog = 262144 
   net.ipv4.tcp_synack_retries = 2 
   net.ipv4.tcp_syn_retries = 2 
  
   # you shouldn't be using conntrack on a heavily loaded server anyway, but these are  
   # suitably high for our uses, insuring that if conntrack gets turned on, the box doesn't die 
   net.ipv4.netfilter.ip_conntrack_max = 1048576 
   net.nf_conntrack_max = 1048576 

 
 </pre> 

 Keep in mind that every TCP connection uses the configured amount of memory for socket 
 buffers. If you've got many connections this can quickly drain the available memory. 

 See http://www.acc.umu.se/~maswan/linux-netperf.txt for more information on these parameters. 

 h3. FreeBSD 

 On FreeBSD you might gain some performance by enabling accept filters. Just 
 compile your kernel with: :: 

 <pre> 

   
   options     ACCEPT_FILTER_HTTP 

 
 </pre> 

 or just load it using 

 <pre> 

   
   kldload accf_http  

  
 </pre>  

 For more ideas about tuning FreeBSD read: tuning(7) 

 Reducing the recvspace should always be ok if the server only handles HTTP 
 requests without large uploads. Increasing the sendspace would reduce the 
 system load if you have a lot of large files to be sent, but keep in mind that 
 you have to provide the memory in the kernel for each connection. 1024 * 64KB 
 would mean 64MB of kernel RAM. Keep this in mind. 

 * <pre> 
 - net.inet.tcp.recvspace = 4096 
 </pre>