![[APACHE DOCUMENTATION]](../images/sub.gif)
Author: Dean Gaudet
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.
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.
Finally there are the folks who just 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.
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.
The single biggest hardware issue affecting webserver performance is
RAM. A webserver should never ever have to swap, as 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. The procedure for doing this is simple: determine the size of
your average Apache process, by looking at your process list via a tool
such as top, and divide this into your total available
memory, leaving some room for other processes.
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 a general guideline is to always apply the latest vendor TCP/IP patches.
HostnameLookups and other DNS considerationsPrior 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. If
you need to have addresses in your log files resolved to hostnames, use
the logresolve program that
comes with Apache, or one of the numerous log reporting packages which
are available.
It is recommended that you do this sort of postprocessing of your log files on some machine other than the production web server machine, in order that this activity not adversely affect server performance.
If you use any Allow from domain or
Deny from domain
directives (i.e., using a hostname, or a domain name, rather than an IP
address) 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). For best performance, therefore, use IP addresses, rather
than names, when using these directives, if possible.
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.
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 /. Then
Apache will perform lstat(2) on /www,
/www/htdocs, and /www/htdocs/. 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 /. 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.
See also the .htaccess tutorial for further discussion of this.
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:
DirectoryIndex index
Use a complete list of options:
DirectoryIndex index.cgi index.pl index.shtml index.html
where you list the most common choice first.
If your site needs content negotiation, consider using
type-map files rather than the Options
MultiViews directive to accomplish the negotiation. See the Content Negotiation
documentation for a full discussion of the methods of negotiation, and
instructions for creating type-map files.
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 results in poor performance 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
adjust the MinSpareServers, MaxSpareServers
and StartServers settings. 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.
In particular, you may need to set MinSpareServers
higher if traffic on your site is extremely bursty - that is, if the
number of connections to your site fluctuates radically in short
periods of time. This may be the case, for example, if traffic to your
site is highly event-driven, such as sites for major sports events, or
other sites where users are encouraged to visit the site at a
particular time.
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.
Since memory usage is such an important consideration in performance, you should attempt to eliminate modules that you are not actually using. If you have built the modules as DSOs, eliminating modules is a simple matter of commenting out the associated AddModule and LoadModule directives for that module. This allows you to experiment with removing modules, and seeing if your site still functions in their absence.
If, on the other hand, you have modules statically linked into your Apache binary, you will need to recompile Apache in order to remove unwanted modules.
An associated question that arises here is, of course, what modules
you need, and which ones you don't. The answer here will, of course,
vary from one web site to another. However, the minimal list of
modules which you can get by with tends to include mod_mime, mod_dir, and mod_log_config.
mod_log_config is, of course, optional, as you can run a
web site without log files. This is, however, not recommended.
Apache comes with a module, mod_mmap_static, which is not enabled by default, which allows you to map files into RAM, and serve them directly from memory rather than from the disc, which should result in substantial performance improvement for frequently-requests files. Note that when files are modified, you will need to restart your server in order to serve the latest version of the file, so this is not appropriate for files which change frequently. See the documentation for this module for more complete details.
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).
mod_status should probably be configured to allow
access by only a few users, rather than to the general public, so this
will likely have very low impact on your overall performance.
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.
HAVE_FLOCK_SERIALIZED_ACCEPTflock(2) system call to lock a
lock file (located by the LockFile directive).HAVE_FCNTL_SERIALIZED_ACCEPTfcntl(2) system call to lock a
lock file (located by the LockFile directive).HAVE_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).HAVE_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
-DHAVE_USLOCK_SERIALIZED_ACCEPT on the
EXTRA_CFLAGS.HAVE_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). The above
HAVE_METHOD_SERIALIZED_ACCEPT defines specify which method
is available and works on the platform (you can have more than one);
USE_METHOD_SERIALIZED_ACCEPT is used to specify the
default method (see the AcceptMutex directive).
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" behavior
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 behavior 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 read */
}
}
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 behavior 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.
<Directory />
AllowOverride none
Options FollowSymLinks
</Directory>
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
strace program, other similar programs include
truss, ktrace, and par.)
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, "", 2024) = 0
close(3) = 0
sigaction(SIGUSR1, {0x8059954, [], SA_INTERRUPT}, {SIG_IGN}) = 0
munmap(0x400ee000, 6144) = 0
flock(18, LOCK_EX) = 0
Notice the accept serialization:
These two calls can be removed by definingflock(18, LOCK_UN) = 0 ... flock(18, LOCK_EX) = 0
SINGLE_LISTEN_UNSERIALIZED_ACCEPT as described earlier.
Notice the SIGUSR1 manipulation:
sigaction(SIGUSR1, {SIG_IGN}, {0x8059954, [], SA_INTERRUPT}) = 0
...
sigaction(SIGUSR1, {SIG_IGN}, {SIG_IGN}) = 0
...
sigaction(SIGUSR1, {0x8059954, [], SA_INTERRUPT}, {SIG_IGN}) = 0
This is caused by the implementation of graceful restarts. When the
parent receives a SIGUSR1 it sends a SIGUSR1
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.
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 (i.e., 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 SIGUSR1
manipulations.
Note that it is theoretically possible to eliminate all three of these calls. But in rough tests the gain proved to be almost unnoticeable.
In order to implement virtual hosts, Apache needs to know the local socket address used to accept the connection:
getsockname(3, {sin_family=AF_INET, sin_port=htons(8080), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
It is possible to eliminate this call in many situations (such as when
there are no virtual hosts, or when Listen directives are
used which do not have wildcard addresses). But no effort has yet been
made to do these optimizations.
Apache turns off the Nagle algorithm:
because of problems described in a paper by John Heidemann.setsockopt(3, IPPROTO_TCP1, [1], 4) = 0
Notice the two time calls:
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 patches section below.time(NULL) = 873959960 ... time(NULL) = 873959960
As described earlier, ExtendedStatus On causes two
gettimeofday calls and a call to times:
gettimeofday({873959960, 404935}, NULL) = 0
...
gettimeofday({873959960, 417742}, NULL) = 0
times({tms_utime=5, tms_stime=0, tms_cutime=0, tms_cstime=0}) = 446747
These can be removed by setting ExtendedStatus Off (which
is the default).
It might seem odd to call stat:
stat("/home/dgaudet/ap/apachen/htdocs/6k", {st_mode=S_IFREG|0644, st_size=6144, ...}) = 0
This is part of the algorithm which calculates the
PATH_INFO for use by CGIs. In fact if the request had been
for the URI /cgi-bin/printenv/foobar then there would be
two calls to stat. The first for
/home/dgaudet/ap/apachen/cgi-bin/printenv/foobar which
does not exist, and the second for
/home/dgaudet/ap/apachen/cgi-bin/printenv, which does
exist. Regardless, at least one stat call is necessary
when serving static files because the file size and modification times
are used to generate HTTP headers (such as Content-Length,
Last-Modified) and implement protocol features (such as
If-Modified-Since). A somewhat more clever server could
avoid the stat when serving non-static files, however
doing so in Apache is very difficult given the modular structure.
All static files are served using mmap:
On some architectures it's slower tommap(0, 6144, PROT_READ, MAP_PRIVATE, 4, 0) = 0x400ee000 ... munmap(0x400ee000, 6144) = 0
mmap small files than
it is to simply read them. The define
MMAP_THRESHOLD can be set to the minimum size required
before using mmap. 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 lmbench you can determine
the optimal setting for your environment.
You may also wish to experiment with MMAP_SEGMENT_SIZE
(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 Timeout in between write()s. So setting this
large may lock out low bandwidth clients unless you also increase the
Timeout.
It may even be the case that mmap isn't used on your
architecture; if so then defining USE_MMAP_FILES and
HAVE_MMAP might work (if it works then report back to
us).
Apache does its best to avoid copying bytes around in memory. The
first write of any request typically is turned into a
writev which combines both the headers and the first hunk
of data:
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
When doing HTTP/1.1 chunked encoding Apache will generate up to four
element writevs. 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 NO_WRITEV
will disable this combining, but result in very poor chunked encoding
performance.
The log write:
can be deferred by definingwrite(17, "127.0.0.1 - - [10/Sep/1997:23:39"..., 71) = 71
BUFFERED_LOGS. In this case up
to PIPE_BUF bytes (a POSIX defined constant) of log
entries are buffered before writing. At no time does it split a log
entry across a PIPE_BUF boundary because those writes may
not be atomic. (i.e., entries from multiple children could
become mixed together). The code does its best to flush this buffer
when a child dies.
The lingering close code causes four system calls:
shutdown(3, 1 /* send */) = 0
oldselect(4, [3], NULL, [3], {2, 0}) = 1 (in [3], left {2, 0})
read(3, "", 2024) = 0
close(3) = 0
which were described earlier.
Let's apply some of these optimizations:
-DSINGLE_LISTEN_UNSERIALIZED_ACCEPT -DBUFFERED_LOGS and
ExtendedStatus Off. Here's the final trace:
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, "", 2024) = 0
close(3) = 0
sigaction(SIGUSR1, {0x8058c98, [], SA_INTERRUPT}, {SIG_IGN}) = 0
munmap(0x400e3000, 6144) = 0
That's 19 system calls, of which 4 remain relatively easy to remove,
but don't seem worth the effort.
time(2) system calls.mod_include, these calls are used by few sites but
required for backwards compatibility.Apache (on Unix) is a pre-forking model server. The parent process is responsible only for forking child 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).
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.
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.
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
mmapped files). Various other models exist and extensive
analysis can be found in the papers of
the JAWS project. In practice all of these costs vary drastically
depending on the operating system.
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 Netscape's Portable Run Time, which is available (but you're encouraged to join the new-httpd mailing list if you intend to use it). Part of our redesign for version 2.0 of Apache includes abstractions of the server model so that we can continue to support the pre-forking model, and also support various threaded models.
