Debianhelp.co.uk


Optimizing Performance in Lighttpd

Performance Issues

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

HTTP Keep-Alive

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

The defaults for the server are:

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

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.

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

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:

server.max-keep-alive-requests = 0

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
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

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(). 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
all write
Unix writev
Linux 2.4+ sendfile
Linux 2.6+ sendfile64
Solaris sendfilev
FreeBSD sendfile

The best backend is selected at compile time. In case you want to use another backend set:

server.network-backend = "writev"

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

server.max-fds = 2048

This only works if lighttpd is started as root.

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

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

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.

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.

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

Configuring Performance in 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.

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 of large files to send.

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

If you have a lot of 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 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.