lighttpd1.4/doc/performance.txt

========================
Performance Improvements
========================

------------
Module: core
------------

:Author: Jan Kneschke
:Date: $Date: 2004/11/03 22:26:05 $
:Revision: $Revision: 1.3 $

:abstract:
  handling performance issues in lighttpd
  
.. meta::
  :keywords: lighttpd, performance
  
.. contents:: Table of Contents

Description
===========

Performance Issues
------------------

lighttpd is optimized into various directions. The most important is
performance. The operation system has two major facalities to help lighttpd
a deliver it best performance.

HTTP Keep-Alive
---------------

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

::

  server.max-keep-alive-requests = 128
  server.max-keep-alive-idle = 30
  server.max-read-idle = 60
  server.max-write-idle = 360

Event Handlers
--------------

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

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


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

Configuration
`````````````

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

e.g.: ::

  server.event-handler = "linux-sysepoll"
  
Network Handlers
----------------

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

If you want to send out a file from the webserver it does 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 (idealy spoken). 

lighttpd supports all major platform specific calls:

========== ========== 
OS         Method     
========== ==========
all        write     
Unix       writev       
Linux 2.4+ sendfile   
Linux 2.6+ sendfile64 
Solaris    sendfilev
FreeBSD    sendfile
========== ========== 

They are selected automaticly on compile-time. If you have problems check
./src/network_backend.h and disable the corresponding USE\_... define.

Max Connections
---------------

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

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

  server.max-fds = 2048
  
This only works if lighttpd is started as root.

Out-of-fd condition
-------------------

As fds are used for tcp/ip sockets, files, directories, ... a simple request
for a PHP page might result in using 3 fds: 

1. the TCP/IP socket to the client
2. the TCP/IP and Unix domain socket to the FastCGI process
3. the filehandle to the file in the document-root to check if it is really existing

If lighttpd runs out of file-descriptors it will stop accepting new
connections for while to use the currently available fds (file-descriptors)
to handle the currently running requests. 

If more than 90% of the fds are used the the handling of new connections is
disabled, if it dropes below 80% again new connection will accepted again.

Under some circumstances you will see ::

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

in the error-log. This tells you the you had to many new requests at once 
and lighttpd could not disable the incomming connections soon enough. The
connection is drop and the client will get a 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 propability of this
problem.




Plattform Specific Notes
========================

Linux
-----

For Linux 2.4.x should 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.

- net.ipv4.tcp_sack = 0 
- net.ipv4.tcp_timestamps = 0

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

- net.ipv4.tcp_wmem = 4096 65536 524288
- net.core.wmem_max = 1048576

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

- net.ipv4.tcp_rmem = 4096 87380 524288
- net.core.rmem_max = 1048576

Keep in mind that the buffers have to multiplied by server.max-fds and be
allocated in the Kernel area. Be carefull with that.

FreeBSD
-------

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

  options   ACCEPT_FILTER_HTTP

For more ideas in 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 large files to be sent, but keep in mind that
you to provide the memory in kernel for each connection. 1024 * 64k would mean
64M of kernel-ram. Keep this in mind.

- net.inet.tcp.recvspace = 4096