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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta name="generator" content="HTML Tidy, see www.w3.org" />
<title>Apache Performance Notes</title>
</head>
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
vlink="#000080" alink="#FF0000">
<!--#include virtual="header.html" -->
<blockquote>
<strong>Warning:</strong> This document has not been 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.
</blockquote>
<h1 align="center">Apache Performance Notes</h1>
<p>Author: Dean Gaudet</p>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#hardware">Hardware and Operating System
Issues</a></li>
<li><a href="#runtime">Run-Time Configuration Issues</a></li>
<li><a href="#compiletime">Compile-Time Configuration
Issues</a></li>
<li>
Appendixes
<ul>
<li><a href="#trace">Detailed Analysis of a
Trace</a></li>
<li><a href="#patches">Patches Available</a></li>
<li><a href="#preforking">The Pre-Forking Model</a></li>
</ul>
</li>
</ul>
<hr />
<table border="1">
<tr>
<td valign="top"><strong>Related Modules</strong><br />
<br />
<a href="../mod/mod_dir.html">mod_dir</a><br />
<a href="../mod/mpm_common.html">Multi-Processing
module</a><br />
<a href="../mod/mod_status.html">mod_status</a><br />
</td>
<td valign="top"><strong>Related Directives</strong><br />
<br />
<a
href="../mod/core.html#allowoverride">AllowOverride</a><br />
<a
href="../mod/mod_dir.html#directoryindex">DirectoryIndex</a><br />
<a
href="../mod/core.html#hostnamelookups">HostnameLookups</a><br />
<a
href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a><br />
<a
href="../mod/prefork.html#maxspareservers">MaxSpareServers</a><br />
<a
href="../mod/prefork.html#mixspareservers">MinSpareServers</a><br />
<a href="../mod/core.html#options">Options</a>
(FollowSymLinks and FollowIfOwnerMatch)<br />
<a
href="../mod/mpm_common.html#startservers">StartServers</a><br />
</td>
</tr>
</table>
<h3><a id="introduction"
name="introduction">Introduction</a></h3>
<p>Apache is a general webserver, which is designed to be
correct first, and fast second. Even so, its performance is
quite satisfactory. Most sites have less than 10Mbits of
outgoing bandwidth, which Apache can fill using only a low end
Pentium-based webserver. In practice sites with more bandwidth
require more than one machine to fill the bandwidth due to
other constraints (such as CGI or database transaction
overhead). For these reasons the development focus has been
mostly on correctness and configurability.</p>
<p>Unfortunately many folks overlook these facts and cite raw
performance numbers as if they are some indication of the
quality of a web server product. There is a bare minimum
performance that is acceptable, beyond that extra speed only
caters to a much smaller segment of the market. But in order to
avoid this hurdle to the acceptance of Apache in some markets,
effort was put into Apache 1.3 to bring performance up to a
point where the difference with other high-end webservers is
minimal.</p>
<p>Finally there are the folks who just plain want to see how
fast something can go. The author falls into this category. The
rest of this document is dedicated to these folks who want to
squeeze every last bit of performance out of Apache's current
model, and want to understand why it does some things which
slow it down.</p>
<p>Note that this is tailored towards Apache 1.3 on Unix. Some
of it applies to Apache on NT. Apache on NT has not been tuned
for performance yet; in fact it probably performs very poorly
because NT performance requires a different programming
model.</p>
<hr />
<h3><a id="hardware" name="hardware">Hardware and Operating
System Issues</a></h3>
<p>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 <code>MaxClients</code> setting so that
your server does not spawn so many children it starts
swapping.</p>
<p>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.</p>
<p>Operating system choice is largely a matter of local
concerns. But a general guideline is to always apply the latest
vendor TCP/IP patches. HTTP serving completely breaks many of
the assumptions built into Unix kernels up through 1994 and
even 1995. Good choices include recent FreeBSD, and Linux.</p>
<hr />
<h3><a id="runtime" name="runtime">Run-Time Configuration
Issues</a></h3>
<h4>HostnameLookups</h4>
<p>Prior to Apache 1.3, <code>HostnameLookups</code> 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 <code>Allow from domain</code> or
<code>Deny from domain</code> 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).</p>
<p>Note that it's possible to scope the directives, such as
within a <code>&lt;Location /server-status&gt;</code> 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:</p>
<blockquote>
<pre>
HostnameLookups off
&lt;Files ~ "\.(html|cgi)$"&gt;
HostnameLookups on
&lt;/Files&gt;
</pre>
</blockquote>
But even still, if you just need DNS names in some CGIs you
could consider doing the <code>gethostbyname</code> call in the
specific CGIs that need it.
<p>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 <a
href="../programs/logresolve.html">logresolve</a>, 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.</p>
<h4>FollowSymLinks and SymLinksIfOwnerMatch</h4>
<p>Wherever in your URL-space you do not have an <code>Options
FollowSymLinks</code>, or you do have an <code>Options
SymLinksIfOwnerMatch</code> Apache will have to issue extra
system calls to check up on symlinks. One extra call per
filename component. For example, if you had:</p>
<blockquote>
<pre>
DocumentRoot /www/htdocs
&lt;Directory /&gt;
Options SymLinksIfOwnerMatch
&lt;/Directory&gt;
</pre>
</blockquote>
and a request is made for the URI <code>/index.html</code>.
Then Apache will perform <code>lstat(2)</code> on
<code>/www</code>, <code>/www/htdocs</code>, and
<code>/www/htdocs/index.html</code>. The results of these
<code>lstats</code> 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:
<blockquote>
<pre>
DocumentRoot /www/htdocs
&lt;Directory /&gt;
Options FollowSymLinks
&lt;/Directory&gt;
&lt;Directory /www/htdocs&gt;
Options -FollowSymLinks +SymLinksIfOwnerMatch
&lt;/Directory&gt;
</pre>
</blockquote>
This at least avoids the extra checks for the
<code>DocumentRoot</code> path. Note that you'll need to add
similar sections if you have any <code>Alias</code> or
<code>RewriteRule</code> paths outside of your document root.
For highest performance, and no symlink protection, set
<code>FollowSymLinks</code> everywhere, and never set
<code>SymLinksIfOwnerMatch</code>.
<h4>AllowOverride</h4>
<p>Wherever in your URL-space you allow overrides (typically
<code>.htaccess</code> files) Apache will attempt to open
<code>.htaccess</code> for each filename component. For
example,</p>
<blockquote>
<pre>
DocumentRoot /www/htdocs
&lt;Directory /&gt;
AllowOverride all
&lt;/Directory&gt;
</pre>
</blockquote>
and a request is made for the URI <code>/index.html</code>.
Then Apache will attempt to open <code>/.htaccess</code>,
<code>/www/.htaccess</code>, and
<code>/www/htdocs/.htaccess</code>. The solutions are similar
to the previous case of <code>Options FollowSymLinks</code>.
For highest performance use <code>AllowOverride None</code>
everywhere in your filesystem.
<h4>Negotiation</h4>
<p>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:</p>
<blockquote>
<pre>
DirectoryIndex index
</pre>
</blockquote>
Use a complete list of options:
<blockquote>
<pre>
DirectoryIndex index.cgi index.pl index.shtml index.html
</pre>
</blockquote>
where you list the most common choice first.
<p>Also note that explicitly creating a <code>type-map</code>
file provides better performance than using
<code>MultiViews</code>, as the necessary information can be
determined by reading this single file, rather than having to
scan the directory for files.</p>
<h4>Process Creation</h4>
<p>Prior to Apache 1.3 the <code>MinSpareServers</code>,
<code>MaxSpareServers</code>, and <code>StartServers</code>
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
<code>StartServers</code> children, only one child per second
would be created to satisfy the <code>MinSpareServers</code>
setting. So a server being accessed by 100 simultaneous
clients, using the default <code>StartServers</code> 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.</p>
<p>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
<code>MinSpareServers</code> setting.</p>
<p>This appears to be responsive enough that it's almost
unnecessary to twiddle the <code>MinSpareServers</code>,
<code>MaxSpareServers</code> and <code>StartServers</code>
knobs. When more than 4 children are spawned per second, a
message will be emitted to the <code>ErrorLog</code>. If you
see a lot of these errors then consider tuning these settings.
Use the <code>mod_status</code> output as a guide.</p>
<p>Related to process creation is process death induced by the
<code>MaxRequestsPerChild</code> 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.</p>
<p>When keep-alives are in use, children will be kept busy
doing nothing waiting for more requests on the already open
connection. The default <code>KeepAliveTimeout</code> 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 <a
href="http://www.research.digital.com/wrl/techreports/abstracts/95.4.html">
most of the benefits are lost</a>.</p>
<hr />
<h3><a id="compiletime" name="compiletime">Compile-Time
Configuration Issues</a></h3>
<h4>mod_status and ExtendedStatus On</h4>
<p>If you include <code>mod_status</code> and you also set
<code>ExtendedStatus On</code> when building and running
Apache, then on every request Apache will perform two calls to
<code>gettimeofday(2)</code> (or <code>times(2)</code>
depending on your operating system), and (pre-1.3) several
extra calls to <code>time(2)</code>. This is all done so that
the status report contains timing indications. For highest
performance, set <code>ExtendedStatus off</code> (which is the
default).</p>
<h4>accept Serialization - multiple sockets</h4>
<p>This discusses a shortcoming in the Unix socket API. Suppose
your web server uses multiple <code>Listen</code> 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
<code>select(2)</code>. <code>select(2)</code> indicates that a
socket has <em>zero</em> or <em>at least one</em> 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):</p>
<blockquote>
<pre>
for (;;) {
for (;;) {
fd_set accept_fds;
FD_ZERO (&amp;accept_fds);
for (i = first_socket; i &lt;= last_socket; ++i) {
FD_SET (i, &amp;accept_fds);
}
rc = select (last_socket+1, &amp;accept_fds, NULL, NULL, NULL);
if (rc &lt; 1) continue;
new_connection = -1;
for (i = first_socket; i &lt;= last_socket; ++i) {
if (FD_ISSET (i, &amp;accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
process the new_connection;
}
</pre>
</blockquote>
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
<code>select</code> when they are in between requests. All
those blocked children will awaken and return from
<code>select</code> 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 <code>accept</code> the
connection. But only one will succeed (assuming there's still
only one connection ready), the rest will be <em>blocked</em>
in <code>accept</code>. 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 <a
href="http://bugs.apache.org/index/full/467">PR#467</a>. There
are at least two solutions.
<p>One solution is to make the sockets non-blocking. In this
case the <code>accept</code> 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
<code>select</code>, and one connection arrives. Then nine of
those children will wake up, try to <code>accept</code> the
connection, fail, and loop back into <code>select</code>,
accomplishing nothing. Meanwhile none of those children are
servicing requests that occurred on other sockets until they
get back up to the <code>select</code> 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.</p>
<p>Another solution, the one used by Apache, is to serialize
entry into the inner loop. The loop looks like this
(differences highlighted):</p>
<blockquote>
<pre>
for (;;) {
<strong>accept_mutex_on ();</strong>
for (;;) {
fd_set accept_fds;
FD_ZERO (&amp;accept_fds);
for (i = first_socket; i &lt;= last_socket; ++i) {
FD_SET (i, &amp;accept_fds);
}
rc = select (last_socket+1, &amp;accept_fds, NULL, NULL, NULL);
if (rc &lt; 1) continue;
new_connection = -1;
for (i = first_socket; i &lt;= last_socket; ++i) {
if (FD_ISSET (i, &amp;accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
<strong>accept_mutex_off ();</strong>
process the new_connection;
}
</pre>
</blockquote>
<a id="serialize" name="serialize">The functions</a>
<code>accept_mutex_on</code> and <code>accept_mutex_off</code>
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
<code>src/conf.h</code> (pre-1.3) or
<code>src/include/ap_config.h</code> (1.3 or later). Some
architectures do not have any locking choice made, on these
architectures it is unsafe to use multiple <code>Listen</code>
directives.
<dl>
<dt><code>USE_FLOCK_SERIALIZED_ACCEPT</code></dt>
<dd>This method uses the <code>flock(2)</code> system call to
lock a lock file (located by the <code>LockFile</code>
directive).</dd>
<dt><code>USE_FCNTL_SERIALIZED_ACCEPT</code></dt>
<dd>This method uses the <code>fcntl(2)</code> system call to
lock a lock file (located by the <code>LockFile</code>
directive).</dd>
<dt><code>USE_SYSVSEM_SERIALIZED_ACCEPT</code></dt>
<dd>(1.3 or later) This method uses SysV-style semaphores to
implement the mutex. Unfortunately SysV-style semaphores have
some bad side-effects. One is that it's possible Apache will
die without cleaning up the semaphore (see the
<code>ipcs(8)</code> 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
(<em>i.e.</em>, 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).</dd>
<dt><code>USE_USLOCK_SERIALIZED_ACCEPT</code></dt>
<dd>(1.3 or later) This method is only available on IRIX, and
uses <code>usconfig(2)</code> 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
<code>-DUSE_USLOCK_SERIALIZED_ACCEPT</code> on the
<code>EXTRA_CFLAGS</code>.</dd>
<dt><code>USE_PTHREAD_SERIALIZED_ACCEPT</code></dt>
<dd>(1.3 or later) This method uses POSIX mutexes and should
work on any architecture implementing the full POSIX threads
specification, however appears to only work on Solaris (2.5
or later), and even then only in certain configurations. If
you experiment with this you should watch out for your server
hanging and not responding. Static content only servers may
work just fine.</dd>
</dl>
<p>If 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).</p>
<p>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.</p>
<p>Ideally you should run servers without multiple
<code>Listen</code> statements if you want the highest
performance. But read on.</p>
<h4>accept Serialization - single socket</h4>
<p>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 <code>accept(2)</code> 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 <code>accept</code> 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.</p>
<p>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
<code>SINGLE_LISTEN_UNSERIALIZED_ACCEPT</code> and then
single-socket servers will not serialize at all.</p>
<h4>Lingering Close</h4>
<p>As discussed in <a
href="http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-connection-00.txt">
draft-ietf-http-connection-00.txt</a> section 8, in order for
an HTTP server to <strong>reliably</strong> 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.</p>
<p>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 (<em>i.e.</em>, SunOS4 -- although folks with
a source license can patch it themselves) we have decided to
disable this feature.</p>
<p>There are two ways of accomplishing this. One is the socket
option <code>SO_LINGER</code>. 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 (<em>i.e.</em>,
Linux 2.0.31) this method proves to be more expensive (cputime)
than the next solution.</p>
<p>For the most part, Apache implements this in a function
called <code>lingering_close</code> (in
<code>http_main.c</code>). The function looks roughly like
this:</p>
<blockquote>
<pre>
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)) &lt;= 0) {
break;
}
/* just toss away whatever is here */
}
}
close (s);
}
</pre>
</blockquote>
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
<code>NO_LINGCLOSE</code>, but this is not recommended at all.
In particular, as HTTP/1.1 pipelined persistent connections
come into use <code>lingering_close</code> is an absolute
necessity (and <a
href="http://www.w3.org/Protocols/HTTP/Performance/Pipeline.html">
pipelined connections are faster</a>, so you want to support
them).
<h4>Scoreboard File</h4>
<p>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
<code>src/main/conf.h</code> file for your architecture and
look for either <code>USE_MMAP_SCOREBOARD</code> or
<code>USE_SHMGET_SCOREBOARD</code>. Defining one of those two
(as well as their companions <code>HAVE_MMAP</code> and
<code>HAVE_SHMGET</code> respectively) enables the supplied
shared memory code. If your system has another type of shared
memory, edit the file <code>src/main/http_main.c</code> and add
the hooks necessary to use it in Apache. (Send us back a patch
too please.)</p>
<p>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.</p>
<h4><code>DYNAMIC_MODULE_LIMIT</code></h4>
<p>If 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
<code>-DDYNAMIC_MODULE_LIMIT=0</code> when building your
server. This will save RAM that's allocated only for supporting
dynamically loaded modules.</p>
<hr />
<h3><a id="trace" name="trace">Appendix: Detailed Analysis of a
Trace</a></h3>
Here is a system call trace of Apache 1.3 running on Linux. The
run-time configuration file is essentially the default plus:
<blockquote>
<pre>
&lt;Directory /&gt;
AllowOverride none
Options FollowSymLinks
&lt;/Directory&gt;
</pre>
</blockquote>
The file being requested is a static 6K file of no particular
content. Traces of non-static requests or requests with content
negotiation look wildly different (and quite ugly in some
cases). First the entire trace, then we'll examine details.
(This was generated by the <code>strace</code> program, other
similar programs include <code>truss</code>,
<code>ktrace</code>, and <code>par</code>.)
<blockquote>
<pre>
accept(15, {sin_family=AF_INET, sin_port=htons(22283), sin_addr=inet_addr("127.0.0.1")}, [16]) = 3
flock(18, LOCK_UN) = 0
sigaction(SIGUSR1, {SIG_IGN}, {0x8059954, [], SA_INTERRUPT}) = 0
getsockname(3, {sin_family=AF_INET, sin_port=htons(8080), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
setsockopt(3, IPPROTO_TCP1, [1], 4) = 0
read(3, "GET /6k HTTP/1.0\r\nUser-Agent: "..., 4096) = 60
sigaction(SIGUSR1, {SIG_IGN}, {SIG_IGN}) = 0
time(NULL) = 873959960
gettimeofday({873959960, 404935}, NULL) = 0
stat("/home/dgaudet/ap/apachen/htdocs/6k", {st_mode=S_IFREG|0644, st_size=6144, ...}) = 0
open("/home/dgaudet/ap/apachen/htdocs/6k", O_RDONLY) = 4
mmap(0, 6144, PROT_READ, MAP_PRIVATE, 4, 0) = 0x400ee000
writev(3, [{"HTTP/1.1 200 OK\r\nDate: Thu, 11"..., 245}, {"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 6144}], 2) = 6389
close(4) = 0
time(NULL) = 873959960
write(17, "127.0.0.1 - - [10/Sep/1997:23:39"..., 71) = 71
gettimeofday({873959960, 417742}, NULL) = 0
times({tms_utime=5, tms_stime=0, tms_cutime=0, tms_cstime=0}) = 446747
shutdown(3, 1 /* send */) = 0
oldselect(4, [3], NULL, [3], {2, 0}) = 1 (in [3], left {2, 0})
read(3, "", 2048) = 0
close(3) = 0
sigaction(SIGUSR1, {0x8059954, [], SA_INTERRUPT}, {SIG_IGN}) = 0
munmap(0x400ee000, 6144) = 0
flock(18, LOCK_EX) = 0
</pre>
</blockquote>
<p>Notice the accept serialization:</p>
<blockquote>
<pre>
flock(18, LOCK_UN) = 0
...
flock(18, LOCK_EX) = 0
</pre>
</blockquote>
These two calls can be removed by defining
<code>SINGLE_LISTEN_UNSERIALIZED_ACCEPT</code> as described
earlier.
<p>Notice the <code>SIGUSR1</code> manipulation:</p>
<blockquote>
<pre>
sigaction(SIGUSR1, {SIG_IGN}, {0x8059954, [], SA_INTERRUPT}) = 0
...
sigaction(SIGUSR1, {SIG_IGN}, {SIG_IGN}) = 0
...
sigaction(SIGUSR1, {0x8059954, [], SA_INTERRUPT}, {SIG_IGN}) = 0
</pre>
</blockquote>
This is caused by the implementation of graceful restarts. When
the parent receives a <code>SIGUSR1</code> it sends a
<code>SIGUSR1</code> to all of its children (and it also
increments a "generation counter" in shared memory). Any
children that are idle (between connections) will immediately
die off when they receive the signal. Any children that are in
keep-alive connections, but are in between requests will die
off immediately. But any children that have a connection and
are still waiting for the first request will not die off
immediately.
<p>To see why this is necessary, consider how a browser reacts
to a closed connection. If the connection was a keep-alive
connection and the request being serviced was not the first
request then the browser will quietly reissue the request on a
new connection. It has to do this because the server is always
free to close a keep-alive connection in between requests
(<em>i.e.</em>, due to a timeout or because of a maximum number
of requests). But, if the connection is closed before the first
response has been received the typical browser will display a
"document contains no data" dialogue (or a broken image icon).
This is done on the assumption that the server is broken in
some way (or maybe too overloaded to respond at all). So Apache
tries to avoid ever deliberately closing the connection before
it has sent a single response. This is the cause of those
<code>SIGUSR1</code> manipulations.</p>
<p>Note that it is theoretically possible to eliminate all
three of these calls. But in rough tests the gain proved to be
almost unnoticeable.</p>
<p>In order to implement virtual hosts, Apache needs to know
the local socket address used to accept the connection:</p>
<blockquote>
<pre>
getsockname(3, {sin_family=AF_INET, sin_port=htons(8080), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
</pre>
</blockquote>
It is possible to eliminate this call in many situations (such
as when there are no virtual hosts, or when <code>Listen</code>
directives are used which do not have wildcard addresses). But
no effort has yet been made to do these optimizations.
<p>Apache turns off the Nagle algorithm:</p>
<blockquote>
<pre>
setsockopt(3, IPPROTO_TCP1, [1], 4) = 0
</pre>
</blockquote>
because of problems described in <a
href="http://www.isi.edu/~johnh/PAPERS/Heidemann97a.html">a
paper by John Heidemann</a>.
<p>Notice the two <code>time</code> calls:</p>
<blockquote>
<pre>
time(NULL) = 873959960
...
time(NULL) = 873959960
</pre>
</blockquote>
One of these occurs at the beginning of the request, and the
other occurs as a result of writing the log. At least one of
these is required to properly implement the HTTP protocol. The
second occurs because the Common Log Format dictates that the
log record include a timestamp of the end of the request. A
custom logging module could eliminate one of the calls. Or you
can use a method which moves the time into shared memory, see
the <a href="#patches">patches section below</a>.
<p>As described earlier, <code>ExtendedStatus On</code> causes
two <code>gettimeofday</code> calls and a call to
<code>times</code>:</p>
<blockquote>
<pre>
gettimeofday({873959960, 404935}, NULL) = 0
...
gettimeofday({873959960, 417742}, NULL) = 0
times({tms_utime=5, tms_stime=0, tms_cutime=0, tms_cstime=0}) = 446747
</pre>
</blockquote>
These can be removed by setting <code>ExtendedStatus Off</code>
(which is the default).
<p>It might seem odd to call <code>stat</code>:</p>
<blockquote>
<pre>
stat("/home/dgaudet/ap/apachen/htdocs/6k", {st_mode=S_IFREG|0644, st_size=6144, ...}) = 0
</pre>
</blockquote>
This is part of the algorithm which calculates the
<code>PATH_INFO</code> for use by CGIs. In fact if the request
had been for the URI <code>/cgi-bin/printenv/foobar</code> then
there would be two calls to <code>stat</code>. The first for
<code>/home/dgaudet/ap/apachen/cgi-bin/printenv/foobar</code>
which does not exist, and the second for
<code>/home/dgaudet/ap/apachen/cgi-bin/printenv</code>, which
does exist. Regardless, at least one <code>stat</code> call is
necessary when serving static files because the file size and
modification times are used to generate HTTP headers (such as
<code>Content-Length</code>, <code>Last-Modified</code>) and
implement protocol features (such as
<code>If-Modified-Since</code>). A somewhat more clever server
could avoid the <code>stat</code> when serving non-static
files, however doing so in Apache is very difficult given the
modular structure.
<p>All static files are served using <code>mmap</code>:</p>
<blockquote>
<pre>
mmap(0, 6144, PROT_READ, MAP_PRIVATE, 4, 0) = 0x400ee000
...
munmap(0x400ee000, 6144) = 0
</pre>
</blockquote>
On some architectures it's slower to <code>mmap</code> small
files than it is to simply <code>read</code> them. The define
<code>MMAP_THRESHOLD</code> can be set to the minimum size
required before using <code>mmap</code>. By default it's set to
0 (except on SunOS4 where experimentation has shown 8192 to be
a better value). Using a tool such as <a
href="http://www.bitmover.com/lmbench/">lmbench</a> you can
determine the optimal setting for your environment.
<p>You may also wish to experiment with
<code>MMAP_SEGMENT_SIZE</code> (default 32768) which determines
the maximum number of bytes that will be written at a time from
mmap()d files. Apache only resets the client's
<code>Timeout</code> in between write()s. So setting this large
may lock out low bandwidth clients unless you also increase the
<code>Timeout</code>.</p>
<p>It may even be the case that <code>mmap</code> isn't used on
your architecture; if so then defining
<code>USE_MMAP_FILES</code> and <code>HAVE_MMAP</code> might
work (if it works then report back to us).</p>
<p>Apache does its best to avoid copying bytes around in
memory. The first write of any request typically is turned into
a <code>writev</code> which combines both the headers and the
first hunk of data:</p>
<blockquote>
<pre>
writev(3, [{"HTTP/1.1 200 OK\r\nDate: Thu, 11"..., 245}, {"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 6144}], 2) = 6389
</pre>
</blockquote>
When doing HTTP/1.1 chunked encoding Apache will generate up to
four element <code>writev</code>s. The goal is to push the byte
copying into the kernel, where it typically has to happen
anyhow (to assemble network packets). On testing, various
Unixes (BSDI 2.x, Solaris 2.5, Linux 2.0.31+) properly combine
the elements into network packets. Pre-2.0.31 Linux will not
combine, and will create a packet for each element, so
upgrading is a good idea. Defining <code>NO_WRITEV</code> will
disable this combining, but result in very poor chunked
encoding performance.
<p>The log write:</p>
<blockquote>
<pre>
write(17, "127.0.0.1 - - [10/Sep/1997:23:39"..., 71) = 71
</pre>
</blockquote>
can be deferred by defining <code>BUFFERED_LOGS</code>. In this
case up to <code>PIPE_BUF</code> bytes (a POSIX defined
constant) of log entries are buffered before writing. At no
time does it split a log entry across a <code>PIPE_BUF</code>
boundary because those writes may not be atomic.
(<em>i.e.</em>, entries from multiple children could become
mixed together). The code does its best to flush this buffer
when a child dies.
<p>The lingering close code causes four system calls:</p>
<blockquote>
<pre>
shutdown(3, 1 /* send */) = 0
oldselect(4, [3], NULL, [3], {2, 0}) = 1 (in [3], left {2, 0})
read(3, "", 2048) = 0
close(3) = 0
</pre>
</blockquote>
which were described earlier.
<p>Let's apply some of these optimizations:
<code>-DSINGLE_LISTEN_UNSERIALIZED_ACCEPT
-DBUFFERED_LOGS</code> and <code>ExtendedStatus Off</code>.
Here's the final trace:</p>
<blockquote>
<pre>
accept(15, {sin_family=AF_INET, sin_port=htons(22286), sin_addr=inet_addr("127.0.0.1")}, [16]) = 3
sigaction(SIGUSR1, {SIG_IGN}, {0x8058c98, [], SA_INTERRUPT}) = 0
getsockname(3, {sin_family=AF_INET, sin_port=htons(8080), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
setsockopt(3, IPPROTO_TCP1, [1], 4) = 0
read(3, "GET /6k HTTP/1.0\r\nUser-Agent: "..., 4096) = 60
sigaction(SIGUSR1, {SIG_IGN}, {SIG_IGN}) = 0
time(NULL) = 873961916
stat("/home/dgaudet/ap/apachen/htdocs/6k", {st_mode=S_IFREG|0644, st_size=6144, ...}) = 0
open("/home/dgaudet/ap/apachen/htdocs/6k", O_RDONLY) = 4
mmap(0, 6144, PROT_READ, MAP_PRIVATE, 4, 0) = 0x400e3000
writev(3, [{"HTTP/1.1 200 OK\r\nDate: Thu, 11"..., 245}, {"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 6144}], 2) = 6389
close(4) = 0
time(NULL) = 873961916
shutdown(3, 1 /* send */) = 0
oldselect(4, [3], NULL, [3], {2, 0}) = 1 (in [3], left {2, 0})
read(3, "", 2048) = 0
close(3) = 0
sigaction(SIGUSR1, {0x8058c98, [], SA_INTERRUPT}, {SIG_IGN}) = 0
munmap(0x400e3000, 6144) = 0
</pre>
</blockquote>
That's 19 system calls, of which 4 remain relatively easy to
remove, but don't seem worth the effort.
<h3><a id="patches" name="patches">Appendix: Patches
Available</a></h3>
There are <a
href="http://www.arctic.org/~dgaudet/apache/1.3/">several
performance patches available for 1.3.</a> Although they may
not apply cleanly to the current version, it shouldn't be
difficult for someone with a little C knowledge to update them.
In particular:
<ul>
<li>A <a
href="http://www.arctic.org/~dgaudet/apache/1.3/shared_time.patch">
patch</a> to remove all <code>time(2)</code> system
calls.</li>
<li>A <a
href="http://www.arctic.org/~dgaudet/apache/1.3/mod_include_speedups.patch">
patch</a> to remove various system calls from
<code>mod_include</code>, these calls are used by few sites
but required for backwards compatibility.</li>
<li>A <a
href="http://www.arctic.org/~dgaudet/apache/1.3/top_fuel.patch">
patch</a> which integrates the above two plus a few other
speedups at the cost of removing some functionality.</li>
</ul>
<h3><a id="preforking" name="preforking">Appendix: The
Pre-Forking Model</a></h3>
<p>Apache (on Unix) is a <em>pre-forking</em> model server. The
<em>parent</em> process is responsible only for forking
<em>child</em> processes, it does not serve any requests or
service any network sockets. The child processes actually
process connections, they serve multiple connections (one at a
time) before dying. The parent spawns new or kills off old
children in response to changes in the load on the server (it
does so by monitoring a scoreboard which the children keep up
to date).</p>
<p>This model for servers offers a robustness that other models
do not. In particular, the parent code is very simple, and with
a high degree of confidence the parent will continue to do its
job without error. The children are complex, and when you add
in third party code via modules, you risk segmentation faults
and other forms of corruption. Even should such a thing happen,
it only affects one connection and the server continues serving
requests. The parent quickly replaces the dead child.</p>
<p>Pre-forking is also very portable across dialects of Unix.
Historically this has been an important goal for Apache, and it
continues to remain so.</p>
<p>The pre-forking model comes under criticism for various
performance aspects. Of particular concern are the overhead of
forking a process, the overhead of context switches between
processes, and the memory overhead of having multiple
processes. Furthermore it does not offer as many opportunities
for data-caching between requests (such as a pool of
<code>mmapped</code> files). Various other models exist and
extensive analysis can be found in the <a
href="http://www.cs.wustl.edu/~jxh/research/research.html">papers
of the JAWS project</a>. In practice all of these costs vary
drastically depending on the operating system.</p>
<p>Apache's core code is already multithread aware, and Apache
version 1.3 is multithreaded on NT. There have been at least
two other experimental implementations of threaded Apache, one
using the 1.3 code base on DCE, and one using a custom
user-level threads package and the 1.0 code base; neither is
publicly available. There is also an experimental port of
Apache 1.3 to <a
href="http://www.mozilla.org/docs/refList/refNSPR/">Netscape's
Portable Run Time</a>, which <a
href="http://www.arctic.org/~dgaudet/apache/2.0/">is
available</a> (but you're encouraged to join the <a
href="http://dev.apache.org/mailing-lists">new-httpd mailing
list</a> if you intend to use it). Part of our redesign for
version 2.0 of Apache will include abstractions of the server
model so that we can continue to support the pre-forking model,
and also support various threaded models.
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