Warning: This document has not been fully updated to take into account changes made in the 2.0 version of the Apache HTTP Server. Some of the information may still be relevant, but please use it with care.
Author: Dean Gaudet
| Related Modules mod_dir Multi-Processing module mod_status |
Related Directives AllowOverride DirectoryIndex HostnameLookups EnableMMAP KeepAliveTimeout MaxSpareServers MinSpareServers Options (FollowSymLinks and FollowIfOwnerMatch) StartServers |
Apache 2.0 is a general-purpose webserver, designed to provide a balance of flexibility, portability, and performance. Although it has not been designed specifically to set benchmark records, Apache 2.0 is capable of high performance in many real-world situations.
Compared to Apache 1.3, release 2.0 contains many additional optimizations to increase throughput and scalability. Most of these improvements are enabled by default. However, there are compile-time and run-time configuration choices that can significantly affect performance. This document describes the options that a server administrator can configure to tune the performance of an Apache 2.0 installation. Some of these configuration options enable the httpd to better take advantage of the capabilities of the hardware and OS, while others allow the administrator to trade functionality for speed.
The single biggest hardware issue affecting webserver
performance is RAM. A webserver should never ever have to swap,
swapping increases the latency of each request beyond a point
that users consider "fast enough". This causes users to hit
stop and reload, further increasing the load. You can, and
should, control the MaxClients setting so that
your server does not spawn so many children it starts
swapping.
Beyond that the rest is mundane: get a fast enough CPU, a fast enough network card, and fast enough disks, where "fast enough" is something that needs to be determined by experimentation.
Operating system choice is largely a matter of local concerns. But some guidelines that have proven generally useful are:
Prior to Apache 1.3, HostnameLookups defaulted
to On. This adds latency to every request because it requires a
DNS lookup to complete before the request is finished. In
Apache 1.3 this setting defaults to Off. However (1.3 or
later), if you use any Allow from domain or
Deny from domain directives then you will pay for
a double reverse DNS lookup (a reverse, followed by a forward
to make sure that the reverse is not being spoofed). So for the
highest performance avoid using these directives (it's fine to
use IP addresses rather than domain names).
Note that it's possible to scope the directives, such as
within a <Location /server-status> section.
In this case the DNS lookups are only performed on requests
matching the criteria. Here's an example which disables lookups
except for .html and .cgi files:
HostnameLookups off
<Files ~ "\.(html|cgi)$">
HostnameLookups on
</Files>
But even still, if you just need DNS names in some CGIs you
could consider doing the gethostbyname call in the
specific CGIs that need it.
Similarly, if you need to have hostname information in your server logs in order to generate reports of this information, you can postprocess your log file with logresolve, so that these lookups can be done without making the client wait. It is recommended that you do this postprocessing, and any other statistical analysis of the log file, somewhere other than your production web server machine, in order that this activity does not adversely affect server performance.
Wherever in your URL-space you do not have an Options
FollowSymLinks, or you do have an Options
SymLinksIfOwnerMatch Apache will have to issue extra
system calls to check up on symlinks. One extra call per
filename component. For example, if you had:
DocumentRoot /www/htdocs
<Directory />
Options SymLinksIfOwnerMatch
</Directory>
and a request is made for the URI /index.html.
Then Apache will perform lstat(2) on
/www, /www/htdocs, and
/www/htdocs/index.html. The results of these
lstats are never cached, so they will occur on
every single request. If you really desire the symlinks
security checking you can do something like this:
DocumentRoot /www/htdocs
<Directory />
Options FollowSymLinks
</Directory>
<Directory /www/htdocs>
Options -FollowSymLinks +SymLinksIfOwnerMatch
</Directory>
This at least avoids the extra checks for the
DocumentRoot path. Note that you'll need to add
similar sections if you have any Alias or
RewriteRule paths outside of your document root.
For highest performance, and no symlink protection, set
FollowSymLinks everywhere, and never set
SymLinksIfOwnerMatch.
Wherever in your URL-space you allow overrides (typically
.htaccess files) Apache will attempt to open
.htaccess for each filename component. For
example,
DocumentRoot /www/htdocs
<Directory />
AllowOverride all
</Directory>
and a request is made for the URI /index.html.
Then Apache will attempt to open /.htaccess,
/www/.htaccess, and
/www/htdocs/.htaccess. The solutions are similar
to the previous case of Options FollowSymLinks.
For highest performance use AllowOverride None
everywhere in your filesystem.
If at all possible, avoid content-negotiation if you're really interested in every last ounce of performance. In practice the benefits of negotiation outweigh the performance penalties. There's one case where you can speed up the server. Instead of using a wildcard such as:
Use a complete list of options:DirectoryIndex index
where you list the most common choice first.DirectoryIndex index.cgi index.pl index.shtml index.html
Also note that explicitly creating a type-map
file provides better performance than using
MultiViews, as the necessary information can be
determined by reading this single file, rather than having to
scan the directory for files.
In situations where Apache 2.0 needs to look at the contents of a file being delivered--for example, when doing server-side-include processing--it normally memory-maps the file if the OS supports some form of mmap(2).
On some platforms, this memory-mapping improves performance. However, there are cases where memory-mapping can hurt the performance or even the stability of the httpd:
For installations where either of these factors applies, you
should use EnableMMAP off to disable the memory-mapping
of delivered files. (Note: This directive can be overridden on
a per-directory basis.)
Prior to Apache 1.3 the MinSpareServers,
MaxSpareServers, and StartServers
settings all had drastic effects on benchmark results. In
particular, Apache required a "ramp-up" period in order to
reach a number of children sufficient to serve the load being
applied. After the initial spawning of
StartServers children, only one child per second
would be created to satisfy the MinSpareServers
setting. So a server being accessed by 100 simultaneous
clients, using the default StartServers of 5 would
take on the order 95 seconds to spawn enough children to handle
the load. This works fine in practice on real-life servers,
because they aren't restarted frequently. But does really
poorly on benchmarks which might only run for ten minutes.
The one-per-second rule was implemented in an effort to
avoid swamping the machine with the startup of new children. If
the machine is busy spawning children it can't service
requests. But it has such a drastic effect on the perceived
performance of Apache that it had to be replaced. As of Apache
1.3, the code will relax the one-per-second rule. It will spawn
one, wait a second, then spawn two, wait a second, then spawn
four, and it will continue exponentially until it is spawning
32 children per second. It will stop whenever it satisfies the
MinSpareServers setting.
This appears to be responsive enough that it's almost
unnecessary to twiddle the MinSpareServers,
MaxSpareServers and StartServers
knobs. When more than 4 children are spawned per second, a
message will be emitted to the ErrorLog. If you
see a lot of these errors then consider tuning these settings.
Use the mod_status output as a guide.
Related to process creation is process death induced by the
MaxRequestsPerChild setting. By default this is 0,
which means that there is no limit to the number of requests
handled per child. If your configuration currently has this set
to some very low number, such as 30, you may want to bump this
up significantly. If you are running SunOS or an old version of
Solaris, limit this to 10000 or so because of memory leaks.
When keep-alives are in use, children will be kept busy
doing nothing waiting for more requests on the already open
connection. The default KeepAliveTimeout of 15
seconds attempts to minimize this effect. The tradeoff here is
between network bandwidth and server resources. In no event
should you raise this above about 60 seconds, as
most of the benefits are lost.
If you include mod_status and you also set
ExtendedStatus On when building and running
Apache, then on every request Apache will perform two calls to
gettimeofday(2) (or times(2)
depending on your operating system), and (pre-1.3) several
extra calls to time(2). This is all done so that
the status report contains timing indications. For highest
performance, set ExtendedStatus off (which is the
default).
This discusses a shortcoming in the Unix socket API. Suppose
your web server uses multiple Listen statements to
listen on either multiple ports or multiple addresses. In order
to test each socket to see if a connection is ready Apache uses
select(2). select(2) indicates that a
socket has zero or at least one connection
waiting on it. Apache's model includes multiple children, and
all the idle ones test for new connections at the same time. A
naive implementation looks something like this (these examples
do not match the code, they're contrived for pedagogical
purposes):
for (;;) {
for (;;) {
fd_set accept_fds;
FD_ZERO (&accept_fds);
for (i = first_socket; i <= last_socket; ++i) {
FD_SET (i, &accept_fds);
}
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
if (rc < 1) continue;
new_connection = -1;
for (i = first_socket; i <= last_socket; ++i) {
if (FD_ISSET (i, &accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
process the new_connection;
}
But this naive implementation has a serious starvation problem.
Recall that multiple children execute this loop at the same
time, and so multiple children will block at
select when they are in between requests. All
those blocked children will awaken and return from
select when a single request appears on any socket
(the number of children which awaken varies depending on the
operating system and timing issues). They will all then fall
down into the loop and try to accept the
connection. But only one will succeed (assuming there's still
only one connection ready), the rest will be blocked
in accept. This effectively locks those children
into serving requests from that one socket and no other
sockets, and they'll be stuck there until enough new requests
appear on that socket to wake them all up. This starvation
problem was first documented in PR#467. There
are at least two solutions.
One solution is to make the sockets non-blocking. In this
case the accept won't block the children, and they
will be allowed to continue immediately. But this wastes CPU
time. Suppose you have ten idle children in
select, and one connection arrives. Then nine of
those children will wake up, try to accept the
connection, fail, and loop back into select,
accomplishing nothing. Meanwhile none of those children are
servicing requests that occurred on other sockets until they
get back up to the select again. Overall this
solution does not seem very fruitful unless you have as many
idle CPUs (in a multiprocessor box) as you have idle children,
not a very likely situation.
Another solution, the one used by Apache, is to serialize entry into the inner loop. The loop looks like this (differences highlighted):
for (;;) {
accept_mutex_on ();
for (;;) {
fd_set accept_fds;
FD_ZERO (&accept_fds);
for (i = first_socket; i <= last_socket; ++i) {
FD_SET (i, &accept_fds);
}
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
if (rc < 1) continue;
new_connection = -1;
for (i = first_socket; i <= last_socket; ++i) {
if (FD_ISSET (i, &accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
accept_mutex_off ();
process the new_connection;
}
The functions
accept_mutex_on and accept_mutex_off
implement a mutual exclusion semaphore. Only one child can have
the mutex at any time. There are several choices for
implementing these mutexes. The choice is defined in
src/conf.h (pre-1.3) or
src/include/ap_config.h (1.3 or later). Some
architectures do not have any locking choice made, on these
architectures it is unsafe to use multiple Listen
directives.
USE_FLOCK_SERIALIZED_ACCEPTflock(2) system call to
lock a lock file (located by the LockFile
directive).USE_FCNTL_SERIALIZED_ACCEPTfcntl(2) system call to
lock a lock file (located by the LockFile
directive).USE_SYSVSEM_SERIALIZED_ACCEPTipcs(8) man page). The other is that the
semaphore API allows for a denial of service attack by any
CGIs running under the same uid as the webserver
(i.e., all CGIs, unless you use something like
suexec or cgiwrapper). For these reasons this method is not
used on any architecture except IRIX (where the previous two
are prohibitively expensive on most IRIX boxes).USE_USLOCK_SERIALIZED_ACCEPTusconfig(2) to create a mutex. While this
method avoids the hassles of SysV-style semaphores, it is not
the default for IRIX. This is because on single processor
IRIX boxes (5.3 or 6.2) the uslock code is two orders of
magnitude slower than the SysV-semaphore code. On
multi-processor IRIX boxes the uslock code is an order of
magnitude faster than the SysV-semaphore code. Kind of a
messed up situation. So if you're using a multiprocessor IRIX
box then you should rebuild your webserver with
-DUSE_USLOCK_SERIALIZED_ACCEPT on the
EXTRA_CFLAGS.USE_PTHREAD_SERIALIZED_ACCEPTIf your system has another method of serialization which isn't in the above list then it may be worthwhile adding code for it (and submitting a patch back to Apache).
Another solution that has been considered but never implemented is to partially serialize the loop -- that is, let in a certain number of processes. This would only be of interest on multiprocessor boxes where it's possible multiple children could run simultaneously, and the serialization actually doesn't take advantage of the full bandwidth. This is a possible area of future investigation, but priority remains low because highly parallel web servers are not the norm.
Ideally you should run servers without multiple
Listen statements if you want the highest
performance. But read on.
The above is fine and dandy for multiple socket servers, but
what about single socket servers? In theory they shouldn't
experience any of these same problems because all children can
just block in accept(2) until a connection
arrives, and no starvation results. In practice this hides
almost the same "spinning" behaviour discussed above in the
non-blocking solution. The way that most TCP stacks are
implemented, the kernel actually wakes up all processes blocked
in accept when a single connection arrives. One of
those processes gets the connection and returns to user-space,
the rest spin in the kernel and go back to sleep when they
discover there's no connection for them. This spinning is
hidden from the user-land code, but it's there nonetheless.
This can result in the same load-spiking wasteful behaviour
that a non-blocking solution to the multiple sockets case
can.
For this reason we have found that many architectures behave
more "nicely" if we serialize even the single socket case. So
this is actually the default in almost all cases. Crude
experiments under Linux (2.0.30 on a dual Pentium pro 166
w/128Mb RAM) have shown that the serialization of the single
socket case causes less than a 3% decrease in requests per
second over unserialized single-socket. But unserialized
single-socket showed an extra 100ms latency on each request.
This latency is probably a wash on long haul lines, and only an
issue on LANs. If you want to override the single socket
serialization you can define
SINGLE_LISTEN_UNSERIALIZED_ACCEPT and then
single-socket servers will not serialize at all.
As discussed in draft-ietf-http-connection-00.txt section 8, in order for an HTTP server to reliably implement the protocol it needs to shutdown each direction of the communication independently (recall that a TCP connection is bi-directional, each half is independent of the other). This fact is often overlooked by other servers, but is correctly implemented in Apache as of 1.2.
When this feature was added to Apache it caused a flurry of problems on various versions of Unix because of a shortsightedness. The TCP specification does not state that the FIN_WAIT_2 state has a timeout, but it doesn't prohibit it. On systems without the timeout, Apache 1.2 induces many sockets stuck forever in the FIN_WAIT_2 state. In many cases this can be avoided by simply upgrading to the latest TCP/IP patches supplied by the vendor. In cases where the vendor has never released patches (i.e., SunOS4 -- although folks with a source license can patch it themselves) we have decided to disable this feature.
There are two ways of accomplishing this. One is the socket
option SO_LINGER. But as fate would have it, this
has never been implemented properly in most TCP/IP stacks. Even
on those stacks with a proper implementation (i.e.,
Linux 2.0.31) this method proves to be more expensive (cputime)
than the next solution.
For the most part, Apache implements this in a function
called lingering_close (in
http_main.c). The function looks roughly like
this:
void lingering_close (int s)
{
char junk_buffer[2048];
/* shutdown the sending side */
shutdown (s, 1);
signal (SIGALRM, lingering_death);
alarm (30);
for (;;) {
select (s for reading, 2 second timeout);
if (error) break;
if (s is ready for reading) {
if (read (s, junk_buffer, sizeof (junk_buffer)) <= 0) {
break;
}
/* just toss away whatever is here */
}
}
close (s);
}
This naturally adds some expense at the end of a connection,
but it is required for a reliable implementation. As HTTP/1.1
becomes more prevalent, and all connections are persistent,
this expense will be amortized over more requests. If you want
to play with fire and disable this feature you can define
NO_LINGCLOSE, but this is not recommended at all.
In particular, as HTTP/1.1 pipelined persistent connections
come into use lingering_close is an absolute
necessity (and
pipelined connections are faster, so you want to support
them).
Apache's parent and children communicate with each other
through something called the scoreboard. Ideally this should be
implemented in shared memory. For those operating systems that
we either have access to, or have been given detailed ports
for, it typically is implemented using shared memory. The rest
default to using an on-disk file. The on-disk file is not only
slow, but it is unreliable (and less featured). Peruse the
src/main/conf.h file for your architecture and
look for either USE_MMAP_SCOREBOARD or
USE_SHMGET_SCOREBOARD. Defining one of those two
(as well as their companions HAVE_MMAP and
HAVE_SHMGET respectively) enables the supplied
shared memory code. If your system has another type of shared
memory, edit the file src/main/http_main.c and add
the hooks necessary to use it in Apache. (Send us back a patch
too please.)
Historical note: The Linux port of Apache didn't start to use shared memory until version 1.2 of Apache. This oversight resulted in really poor and unreliable behaviour of earlier versions of Apache on Linux.
DYNAMIC_MODULE_LIMITIf you have no intention of using dynamically loaded modules
(you probably don't if you're reading this and tuning your
server for every last ounce of performance) then you should add
-DDYNAMIC_MODULE_LIMIT=0 when building your
server. This will save RAM that's allocated only for supporting
dynamically loaded modules.
Here is a system call trace of Apache 2.0.38 with the worker MPM on Solaris 8. This trace was collected using:
truss -l -p httpd_child_pid.
The -l option tells truss to log the ID of the
LWP (lightweight process--Solaris's form of kernel-level thread)
that invokes each system call.
Other systems may have different system call tracing utilities
such as strace, ktrace, or par.
They all produce similar output.
In this trace, a client has requested a 10KB static file from the httpd. Traces of non-static requests or requests with content negotiation look wildly different (and quite ugly in some cases).
/67: accept(3, 0x00200BEC, 0x00200C0C, 1) (sleeping...) /67: accept(3, 0x00200BEC, 0x00200C0C, 1) = 9
In this trace, the listener thread is running within LWP #67.
Note the lack of accept(2) serialization. On this particular platform, the worker MPM uses an unserialized accept by default unless it is listening on multiple ports.
/65: lwp_park(0x00000000, 0) = 0 /67: lwp_unpark(65, 1) = 0
Upon accepting the connection, the listener thread wakes up a worker thread to do the request processing. In this trace, the worker thread that handles the request is mapped to LWP #65.
/65: getsockname(9, 0x00200BA4, 0x00200BC4, 1) = 0
In order to implement virtual hosts, Apache needs to know the local socket address used to accept the connection. It is possible to eliminate this call in many situations (such as when there are no virtual hosts, or when
Listendirectives are used which do not have wildcard addresses). But no effort has yet been made to do these optimizations.
/65: brk(0x002170E8) = 0 /65: brk(0x002190E8) = 0
The brk(2) calls allocate memory from the heap. It is rare to see these in a system call trace, because the httpd uses custom memory allocators (
apr_poolandapr_bucket_alloc) for most request processing. In this trace, the httpd has just been started, so it must call malloc(3) to get the blocks of raw memory with which to create the custom memory allocators.
/65: fcntl(9, F_GETFL, 0x00000000) = 2 /65: fstat64(9, 0xFAF7B818) = 0 /65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B910, 2190656) = 0 /65: fstat64(9, 0xFAF7B818) = 0 /65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B914, 2190656) = 0 /65: setsockopt(9, 65535, 8192, 0xFAF7B918, 4, 2190656) = 0 /65: fcntl(9, F_SETFL, 0x00000082) = 0
Next, the worker thread puts the connection to the client (file descriptor 9) in non-blocking mode. The setsockopt(2) and getsockopt(2) calls are a side-effect of how Solaris's libc handles fcntl(2) on sockets.
/65: read(9, " G E T / 1 0 k . h t m".., 8000) = 97
The worker thread reads the request from the client.
/65: stat("/var/httpd/apache/httpd-8999/htdocs/10k.html", 0xFAF7B978) = 0
/65: open("/var/httpd/apache/httpd-8999/htdocs/10k.html", O_RDONLY) = 10
This httpd has been configured with
Options FollowSymLinksandAllowOverride None. Thus it doesn't need to lstat(2) each directory in the path leading up to the requested file, nor check for.htaccessfiles. It simply calls stat(2) to verify that the file: 1) exists, and 2) is a regular file, not a directory.
/65: sendfilev(0, 9, 0x00200F90, 2, 0xFAF7B53C) = 10269
In this example, the httpd is able to send the HTTP response header and the requested file with a single sendfilev(2) system call. Sendfile semantics vary among operating systems. On some other systems, it is necessary to do a write(2) or writev(2) call to send the headers before calling sendfile(2).
/65: write(4, " 1 2 7 . 0 . 0 . 1 - ".., 78) = 78
This write(2) call records the request in the access log. Note that one thing missing from this trace is a time(2) call. Unlike Apache 1.3, Apache 2.0 uses gettimeofday(3) to look up the time. On some operating systems, like Linux or Solaris, gettimeofday has an optimized implementation that doesn't require as much overhead as a typical system call.
/65: shutdown(9, 1, 1) = 0 /65: poll(0xFAF7B980, 1, 2000) = 1 /65: read(9, 0xFAF7BC20, 512) = 0 /65: close(9) = 0
The worker thread does a lingering close of the connection.
/65: close(10) = 0 /65: lwp_park(0x00000000, 0) (sleeping...)
Finally the worker thread closes the file that it has just delivered and blocks until the listener assigns it another connection.
/67: accept(3, 0x001FEB74, 0x001FEB94, 1) (sleeping...)
Meanwhile, the listener thread is able to accept another connection as soon as it has dispatched this connection to a worker thread (subject to some flow-control logic in the worker MPM that throttles the listener if all the available workers are busy). Though it isn't apparent from this trace, the next accept(2) can (and usually does, under high load conditions) occur in parallel with the worker thread's handling of the just-accepted connection.