Simultaneous
    connections

            |
       2000 |
            |
            |                            **********
            |                        ****          ***
       1500 |                   *****                 **
            |               ****                        ***
            |            ***                               ***
            |           *                                     **
       1000 |          *                                        **
            |         *                                           *
            |         *                                           *
            |        *                                             *
        500 |        *                                             *
            |        *                                              *
            |      **                                                *
            |   ***                                                  ***
          1 |***                                                        **
 Time of    +-------------------------------------------------------------
   day         7am  8am  9am  10am  11am  12am  1pm  2pm  3pm  4pm  5pm

Windows
[warn] Server ran out of threads to serve requests. Consider raising the ThreadsPerChild setting

Linux and Unix
[error] server reached MaxClients setting, consider raising the MaxClients setting

[Thu Aug 28 10:12:17 2008] [notice] mpmstats: rdy 0 bsy 600 rd 1 wr 70 ka 484 log 0 dns 0 cls 45

The netstat command can be used to show the state of TCP connections between clients and IBM HTTP Server. For some of these connection states, a web server thread (or child process, with 1.3.x on Unix) is consumed. For other states, no web server thread is consumed. See the following table to determine if a TCP connection in a particular state requires a web server thread.

TCP state	meaning	is a web server thread utilized?
LISTEN	no connection	no
SYN_RCVD	not ready to be processed	no
ESTABLISHED	ready for web server to accept and process requests, or already processing requests	yes, as soon as the web server realizes that connection is established; but if there aren’t enough configured web server threads (e.g., MaxClients is too small), the connection may stall until a thread becomes ready
FIN_WAIT1	web server has closed the socket; the connection remains in this state until an ACK is received from the client.	A web server thread can be utilized for up to two seconds in this state if FIN is not received from the client, after which the web server gives up and the web server thread is no longer utilized.
CLOSE_WAIT	client has closed the socket, web server hasn’t yet noticed	yes
LAST_ACK	client closed socket then web server closed socket	no
FIN_WAIT2	web server closed the socket then client ACKed; the connection remains in this state until a FIN is received from the client or an OS-specific timeout occurs; see Connections in the FIN_WAIT_2 state and Apache for more information	A web server thread can be utilized for up to two seconds in this state if FIN is not received from the client, after which the web server gives up and the web server thread is no longer utilized.
TIME_WAIT	waiting for 2*MSL timeout before allowing quad to be reused	no
CLOSING	web server and client closed at the same time	no

IBM HTTP Server on Windows has a Parent process and a single multi-threaded Child process.

On 64-bit Windows OS’es, each instance of IHS is limited to approximately 2000 ThreadsPerChild. On 32-bit Windows, this number can be closer to 4000. These numbers are not exact limits, because the real limits are the sum of the fixed startup cost of memory for each thread + the maximum runtime memory usage per thread, which varies based on configuration and workload. Raising ThreadsPerChild and approaching these limits risks child process crashes when runtime memory usage puts the process address space over the 2GB or 3GB barrier. The upper limits become even more restrictive when loading other modules such as mod_mem_cache, or when there are many RewriteCond/RewriteRule directives. When approaching the limits, the server may run for awhile before the memory limit is exceeded and causes the child process to crash. Values even higher than this would probably cause an immediate crash during startup.

No specific limits can be provided, but it is suggested that anything over a value of 2000 for ThreadsPerChild on a Windows operating system could be at risk.
It should be noted that IHS on Windows is a 32-bit application even when running on a 64-bit Windows OS.

The relevant config directives for tuning the thread values described in this section on Windows are:

• ThreadsPerChild
  The ThreadsPerChild directive places an upper limit on the number of simultaneous connections the server can handle. ThreadsPerChild should be set according to the maximum number of expected simultaneous connections, but within the restrictions described above.
• ThreadLimit (valid with IHS 2.0 and above)
  ThreadsPerChild has a built in upper limit. Use ThreadLimit to increase the upper limit of ThreadsPerChild. The value of ThreadLimit affects the size of the shared memory segment the server uses to perform inter-process communication between the parent and the single child process. Do not increase ThreadLimit beyond what is required for ThreadsPerChild.

With PI04922, it’s possible to use nearly twice as much memory (nearly twice as many threads). See PI04922.

Recommended settings:

Directive	Value
ThreadsPerChild	maximum number of simultaneous connections (within memory constraints)
ThreadLimit	same as ThreadsPerChild (2.0 and above)

On UNIX and Linux platforms, a running instance of IBM HTTP Server will consist of one single threaded Parent process which starts and maintains one or more multi-threaded Child processes. HTTP requests are received and processed by threads running in the Child processes. Each simultaneous request (TCP connection) consumes a thread. You need to use the appropriate configuration directives to control how many threads the server starts to handle requests and on UNIX and Linux, you can control how the threads are distributed amongst the Child processes.

Relevant config directives on UNIX platforms:

• StartServersThe StartServers directive controls how many Child Processes are started when the web server initializes. The recommended value is 1. Do not set this higher than MaxSpareThreads divided by ThreadsPerChild. Otherwise, processes will be started at initialization and terminated immediately thereafter.Every second, IHS checks if new child processes are needed, so generally tuning of StartServers will be moot as early as a minute after IHS has started.
• ServerLimitThere is a built-in upper limit on the number of child processes. At runtime, the actual upper limit on the number of child processes is MaxClients divided by ThreadsPerChild.This should only be changed when you have reason to change MaxClients or ThreadsPerChild, it does not directly dictate the number of child processes created at runtime.

  It is possible to see more child processes than this if some of them are gracefully stopping. If there are many of them, it probably means that MaxSpareThreads is set too small, or that MaxRequestsPerChild is non-zero and not large enough; see below for more information on both these directives.

• ThreadsPerChildUse the ThreadsPerChild directive to control how many threads each Child process starts. More information on strategies for distributing threads amongst child processes is included below.
• ThreadLimitThreadsPerChild has a built in upper limit. Use ThreadLimit to increase the upper limit of ThreadsPerChild. The value of ThreadLimit affects the size of the shared memory segment the server uses to perform inter-process communication between the parent and child processes. Do not increase ThreadLimit beyond what is required for ThreadsPerChild.
• MaxClientsThe MaxClients directive places an upper limit on the number of simultaneous connections the server can handle. MaxClients should be set according to the expected load.

The MaxSpareThreads and MinSpareThreads directives affect how the server reacts to changes in server load. You can use these directives to instruct the server to automatically increase the number of Child processes when server load increases (subject to limits imposed by ServerLimitand MaxClients) and to decrease the number of Child processes when server load is low. This feature can be a useful for managing overall system memory utilization when your server is being used for tasks other than serving HTTP requests.

• MaxSpareThreads
• MinSpareThreads

Setting MaxSpareThreads to a relatively small value has a performance penalty: Extra CPU to terminate and create child processes. During normal operation, the load on the server may vary widely (e.g., from 150 busy threads to 450 busy threads). If MaxSpareThreads is smaller than this variance (e.g., 450-150=300), then the web server will terminate and create child processes frequently, resulting in reduced performance.

Recommended settings:

Directive	Value
ThreadsPerChild	Leave at the default value, or increase to a larger proportion of MaxClients for better coordination of WebSphere Plugin processing threads (via less child processes). Larger ThreadsPerChild (and fewer processes) also results in fewer dedicated web container threads being used by the ESI invalidation feature of the WebSphere Plugin.Increasing ThreadsPerchild too high on heavily loaded SSL servers may incur more CPU and throughput issues, as there is additional contention for memory.
MaxClients	maximum number of simultaneous connections, rounded up to an even multiple of ThreadsPerChild
StartServers	2
MinSpareThreads	The greater of “25” or 10% of MaxClients integer. Since IHS checks this value approximately once per second, MinSpareThreads should safely exceed the number of new requests you might receive in a second.Setting MinSpareThreads too high with the WebSphere Plugin may trigger premature spawning of new child processes, for which AppServer requests will be distributed over, with no sharing of MaxConnections counts or markdowns. If the ESI invalidation servlet is configured in the WebSphere Plugin, each additional process results in a dedicated web container thread being consumed.Setting MinSpareThreads too low may induce delays of a few seconds if IHS runs out of processing threads.
MaxSpareThreads	There are multiple approaches to the tuning of this directive: Preallocation: The system must have enough resources to handle MaxClients anyway, so let the web server retain idle threads/processes so that they are immediately ready to serve requests when load increases again.Set MaxSpareThreads to the same value as MaxClients. This approach should be used if there are extremely long-running application requests that would keep child processes from being able to terminate gracefully. Reduce web server resource utilization during idle periods and increase the coordination between WebSphere Plugin threads: Allow the web server to clean up idle threads after load subsides so that the resources can be used for other applications. When the load increases again, it will reclaim the resources as it creates new child processes. Set MaxSpareThreads to 25-30% of MaxClients. If it is too small a fraction of MaxClients, child processes will be terminated and recreated frequently.
ServerLimit	MaxClients divided by ThreadsPerChild, or the default if that is high enough
ThreadLimit	ThreadsPerChild

Note: ThreadLimit and ServerLimit need to appear before these other directives in the configuration file.

Default settings in recent default configuration files:

<IfModule worker.c>
ThreadLimit         25
ServerLimit         64
StartServers         2
MaxClients         600
MinSpareThreads     25
MaxSpareThreads     75
ThreadsPerChild     25
MaxRequestsPerChild  0
</IfModule>

1.3.1. If memory is constrained

If there is concern about available memory on the server, some additional tuning can be done.

Increasing ThreadsPerChild (and ThreadLimit) will reduce the number of total server processes needed, reducing the per-server memory overhead. However, there are a number of possible drawbacks to increasing ThreadsPerChild. Search this document for ThreadsPerChild and consider all the warnings before changing it. Notably, this increases the per-process footprint which can be detrimental on 32-bit httpds.

On Linux, VSZ can appear very large if ulimit -s is left as a large value or unlimited. Reduce it in bin/envvars to e.g. 512 with ulimit -s if this is a concern. A high VSZ has no real cost, it does not consume memory, but it is sometimes noticed.

Setting and MaxMemFree to e.g. 512, will limit memory retained by each thread.

IBM HTTP Server 1.3 on Linux and Unix systems uses one single-threaded child process per concurrent connection.

Recommended settings:

Directive	Value
MaxClients	maximum number of simultaneous connections
MinSpareServers	1
MaxSpareServers	same value as MaxClients
StartServers	default value

---

MaxClients, ThreadsPerChild, etc.

Refer to the previous section.

cipher ordering (SSL only)

The default SSLCipherSpec ordering enables maximum strength SSL connections at a significant performance penalty. A much better performing and reasonably strong SSLCipherSpec configuration is given below.

Sendfile (non-SSL only)

With IBM HTTP Server 2.0 and above, Sendfile usage is disabled in the current default configuration files. This avoids some occasional platform-specific problems, but it may also increase CPU utilization on platforms on which sendfile is supported (Windows, AIX, Linux, HP-UX, and Solaris/x64).

If you enable sendfile usage on AIX, ensure that the nbc_limit setting displayed by the no program is not too high for your system. On many systems, the AIX system default is 768MB. We recommend setting this to a much more conservative value, such as 256MB. If the limit is too high, and the web server use of sendfile results in a large amount of network buffer cache memory utilization, a wide range of other system functions may fail. In situations like that, the best diagnostic step is to check network buffer cache utilization by running netstat -c. If it is relatively high (hundreds of megabytes), disable sendfile usage and see if the problem occurs again. Alternately, nbc_limit can be lowered significantly but sendfile still be enabled.

Some Apache users on Solaris have noted that sendfile is slower than the normal file handling, and that sendfile may not function properly on that platform with ZFS or some Ethernet drivers. IBM HTTP Server provides support for sendfile on Solaris/x64 but not Solaris/SPARC.

With IBM HTTP Server 2.0.42 and above, the default IHSROOT/bin/envvars file specifies the setting MALLOCMULTIHEAP=considersize,heaps:8. This enables a memory management scheme for the AIX heap library which is better for multithreaded applications, and configures it to try to minimize memory use and to use a moderate number of heaps. For configurations with extensive heap operations (SSL or certain third-party modules), CPU utilization can be lowered by changing this setting to the following: MALLOCMULTIHEAP=true. This may increase the memory usage slightly.

The Fast Response Cache Accelerator (FRCA, aka AFPA) is disabled in the current default configuration files because some common Windows extensions, such as Norton Antivirus, are not compatible with it. FRCA is a kernel resident micro HTTP server optimized for serving static, non-access protected files directly out of the file system. The use of FRCA can dramatically reduce CPU utilization in some configurations. FRCA cannot be used for serving content over HTTPS/SSL connections.

This directive is commonly modified as part of tuning the web server. There are advantages and disadvantages for different values of ThreadsPerChild:

• Higher values for ThreadsPerChild result in lower overall memory use for the server, as long as the value of ThreadsPerChild isn’t higher than the normal number of concurrent TCP connections handled by the server.
• Extremely high values for ThreadsPerChild may result in encountering address space limitations.
• Higher values for ThreadsPerChild often results in lower numbers of connections which the WebSphere connection maintains to the application server and better sharing of markdown information.
• Higher values for ThreadsPerChild result in higher CPU utilization for SSL processing.
• On older Linux distributions such as RedHat Advanced Server 2.1 and SuSE SLES 8 which use the linuxthreads library, higher values for ThreadsPerChild result in higher CPU utilization in the threads library.Some features may exacerbate this problem, such as RewriteMap or the following modules: mod_mem_cache, mod_ibm_ldap, or mod_ext_filter.
• Higher ThreadsPerChild results in a more effective use of the cache and connection pooling in mod_ibm_ldap.
• Higher ThreadsPerChild results in a more effective use of the cache in mod_mem_cache, because each child must fill its own cache.MaxSpareThreads = MaxClients is also beneficial for mod_mem_cache because it prevents child processes who have built up large caches from being gracefully terminated.

System tuning changes may be necessary to run with higher values for ThreadsPerChild. If IBM HTTP Server fails to start after increasing ThreadsPerChild, check the error log for any error messages. A common failure is a failed attempt to create a worker thread.

• apr_thread_create: unable to create worker thread error message and tuning hints

This directive is commonly modified as part of tuning the web server to handle a greater client load (more concurrent TCP connections).

When MaxClients is increased, the value for MaxSpareThreads should be scaled up as well. Otherwise, extra CPU will be spent terminating and creating child processes when the load changes by a relatively small amount.

This directive controls whether some important information is saved in the scoreboard for use by mod_status and diagnostic modules. When this is set to On, web server CPU usage may increase by as much as one percent. However, it can make mod_status reports and some other diagnostic tools more useful.

The use of the MaxConnections parameter in the WebSphere plug-in configuration is most effective when IBM HTTP Server 2.0 and above is used and there is a single IHS child process. However, there are other tradeoffs:

It is usually much more effective to actively prevent backend systmes from accepting more connections than they can reliably handle, performing throttling at the TCP level. When this is done at the client (HTTP Plugin) side, there is no cross-system or cross-process coordination which makes the limits ineffective.

• linuxthreads (traditional pthread library on Linux): ThreadsPerChild greater than about 100 results in high CPU overhead
• SSL on any platform: threadsPerChild greater than about 100 results in high CPU overhead
• WebSphere 5.x plug-in has a file descriptor limitation which will be encountered on Linux and Solaris if ThreadsPerChild is greater than 500

Using MaxConnections with more then 1 child processes, or across a webserver farm, introduces a number of complications. Each IHS child process must have a high enough MaxConnections value to allow each thread to be able to find a backend server, but in aggregate the child processes should not be able to overrun an individual application server.

Choosing a value for MaxConnections

• MaxConnections has no effect if it exceeds ThreadsPerChild, because no child could try to use that many connections in the first place.
• Upper limitIf you are concerned about a single HTTP Server overloading an Application server, you must first determine “N” — the maximum number of requests the single AppServer can handle.

  MaxConnections would then be = (N / (MaxClients / ThreadsPerChild)), or N divided by the maximum number of child processes based on your configuration . This represents the worst-case number of connections by IHS to a single Application Server. As the number of backends grows, the likelyhood of the worst-case scenario decreases as even the uncoordinated child processes are still distributing load with respect to session affinity and load balancing.

  For example, if you wish to restrict each Application Server to a total of 200 connections, spread out among 4 child processes, you must set the MaxConnections parameter to 50 because each child process keeps its own count.

• Lower LimitIf MaxConnections is too small, a child process may start returning errors because it has no AppServers to use.

  To prevent problems, MaxConnections * (number of usable backend servers) should exceed ThreadsPerChild.

  For example, if each child process has 128 ThreadsPerChild and MaxConnections is only 50 with two backend AppServers, a single child process may not be able to fulfill all 128 requests because only 50 * 2 connections can be made.

To use MaxConnections, IHS should be configured to use a small, fixed number of child process, and to not vary them in response to a change in load. This provides a consistent, predictable number of child processes that each have a fixed MaxConnections parameter.

• MinSpareServers and MaxSpareServers should be set to the same value as MaxClients.
• StartServers should be set to MaxClients / ThreadsPerChild.

When more then 1 child process is configured (number of child processes is MaxClients/ThreadsPerChild), setting MaxSpareServers equal to MaxClients can have the effect of keeping multiple child process alive when they aren’t strictly needed. This can be considered detrimental to the WebSphere Plugin detecting markdowns, because the threads in each child process must discover a server should be marked down. See section 6.2 below.

Only WebSphere Plugin threads in a single IHS child process share info about AppServer markdowns, so some customers wish to aggressively limit the number of child processes that are running at any given time. If a user has problems with markdowns being discovered by many different child processes in the same webserver, consider increasing ThreadsPerChild and reducing MinSpareThreads and MaxSpareThreads as detailed below.

• One approach is to use a single child process, where MaxClients and ThreadsPerChild are set to the same value. IHS will never create or destroy child processes in response to load.Cautions
  • A WebServer crash impacts 100% of the clients.
  • Some types of hangs may influence 100% of the clients.
  • CPU usage may increase if SSL is used and ThreadsPerChild exceeds a few hundred.
  • More ramifications of high ThreadsPerChild is discussed here.
• A second approach is to use a variable number of child processes, but to aggressively limit the number created by IHS in response to demand (and aggressively remove unneeded processes). This is accomplished by setting ThreadsPerChild to 25% or 50% of MaxClients, setting MinSpareThreads and MaxSpareThreads low (relative to recommendations here).Cautions:
  • MaxSpareThreads < MaxClients causes IHS to routinely kill off child processes, however it may take some time for these processes to exit while slow requests finish processing.
  • A lower MaxSpareThread can cause extra CPU usage for the creation of replacement child processes.
  • Caches for ESI and mod_mem_cache are thrown away when child processes exit.

See also

• High CPU in child processes after WebSphere plugin config is updated

As the number of child processes increases (ratio of ThreadsPerChild / MaxClients shrinks), if the ESI Invalidation Servlet is used with the WebSphere Plugin, more and more Web Container threads will be permanently consumed. Each child processes uses 1 ESI Invalidation thread (when the feature is configured), and this thread is used synchronously in the web container.

This requires careful consideration of the number of child processes per webserver, the number of webservers, and the number of configured Web Container threads.

When an SSL connection is established, the client (web browser) and the web server negotiate the cipher to use for the connection. The web server has an ordered list of ciphers, and the first cipher in that list which is supported by the client will be selected.

By default, IBM HTTP Server prefers AES and RC4 ciphers over the computationally expensive Triple-DES (3DES) cipher suite, and tuning of the order of SSL directives for performance reasons is generally not needed.

The following is a table of most of the ciphers IHS supports. The table is sorted in approximate descending cipher strength. For a full list of ciphers IHS supports, refer to the appropriate infocenter for your version of IHS.

SSLV3 and TLSV1 ciphers
shortname	longname	Meaning
Available on v8 and later, distributed
C024	TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384	Elliptic Curve DSA AES SHA2 (256-bit)
C02c	TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384	Elliptic Curve DSA AES SHA2 (256-bit)
C028	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384	Elliptic Curve RSA AES SHA2 (256-bit)
C030	TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384	Elliptic Curve RSA AES SHA2 (256-bit)
C00a	TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA	Elliptic Curve DSA AES SHA (256-bit)
C014	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA	Elliptic Curve RSA AES SHA (256-bit)
C008	TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA	Elliptic Curve DSA Triple-DES SHA2 (168-bit)
C012	TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA	Elliptic Curve RSA Triple-DES SHA2 (168-bit)
C023	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256	Elliptic Curve DSA AES SHA2 (128-bit)
C02b	TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256	Elliptic Curve DSA AES SHA2 (128-bit)
C009	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA	Elliptic Curve DSA AES SHA (128-bit)
C027	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256	Elliptic Curve RSA AES SHA2 (128-bit)
C02f	TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256	Elliptic Curve RSA AES SHA2 (128-bit)
C013	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA	Elliptic Curve RSA AES SHA (128-bit)
default on v8 and later, distributed
9D	TLS_RSA_WITH_AES_256_GCM_SHA384	RSA AES SHA2 (256-bit)
3D	TLS_RSA_WITH_AES_256_CBC_SHA256	RSA AES SHA2 (256-bit)
9C	TLS_RSA_WITH_AES_128_GCM_SHA256	RSA AES SHA2 (128-bit)
3C	TLS_RSA_WITH_AES_128_CBC_SHA256	RSA AES SHA2 (128-bit)
All versions, all systems
3A	SSL_RSA_WITH_3DES_EDE_CBC_SHA	RSA Triple-DES SHA (168 bit)
35b	TLS_RSA_WITH_AES_256_CBC_SHA	RSA AES SHA (256 bit)

The following configuration directs the server to prefer strong 128-bit RC4 ciphers first and will provide a significant performance improvement over the default configuration. This configuration does not support the weaker 40-bit, 56-bit, or NULL/Plaintext ciphers that security scanners may complain about.

The order of the SSLCipherSpec directives dictates the priority of the ciphers, so we order them in a way that will cause IHS to prefer less CPU intensive ciphers. SSLv2 is disabled implicitly by not including any SSLv2 ciphers

<VirtualHost *:443>
  SSLEnable
  Keyfile keyfile.kdb

  ## SSLv3 128 bit Ciphers 
  SSLCipherSpec SSL_RSA_WITH_RC4_128_MD5 
  SSLCipherSpec SSL_RSA_WITH_RC4_128_SHA

  ## FIPS approved SSLV3 and TLSv1 128 bit AES Cipher
  SSLCipherSpec TLS_RSA_WITH_AES_128_CBC_SHA
  
  ## FIPS approved SSLV3 and TLSv1 256 bit AES Cipher
  SSLCipherSpec TLS_RSA_WITH_AES_256_CBC_SHA

  ## Triple DES 168 bit Ciphers
  ## These can still be used, but only if the client does
  ## not support any of the ciphers listed above.
  SSLCipherSpec SSL_RSA_WITH_3DES_EDE_CBC_SHA

  ## The following block enables SSLv2. Excluding it in the presence of 
  ## the SSLv3 configuration above disables SSLv2 support.

  ## Uncomment to enable SSLv2 (with 128 bit Ciphers)
  #SSLCipherSpec SSL_RC4_128_WITH_MD5 
  #SSLCipherSpec SSL_RC4_128_WITH_SHA
  #SSLCipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5
 
</VirtualHost>

For IHS V8R0 and later, new ciphers and a new syntax for SSLCipherSpec are supported. This is a require change since new TLS protocols with a disjoint set of ciphers are supported. These same releases of IHS also favor strong ciphers, but also completely disable weak and export ciphers.

To configure weaker, but less CPU intensive ciphers, in V8R0 and later:

You can use the following LogFormat directive to view and log the SSL cipher negotiated for each connection:

LogFormat "%h %l %u %t \"%r\" %>s %b \"SSL=%{HTTPS}e\" \"%{HTTPS_CIPHER}e\" \"%{HTTPS_KEYSIZE}e\" \"%{HTTPS_SECRETKEYSIZE}e\"" ssl_common
CustomLog logs/ssl_cipher.log ssl_common

This logformat will produce an output to the ssl_cipher.log that looks something like this:

127.0.0.1 - - [18/Feb/2005:10:02:05 -0500] "GET / HTTP/1.1" 200 1582 "SSL=ON" "SSL_RSA_WITH_RC4_128_MD5" "128" "128"

Larger server certificates are also costly. Every doubling of key size costs 4-8 times more CPU for the required computation.

Unfortunately, you don’t have a lot of choice in the size of your server certificate; the industry is currently (2010) moving from 1024-bit to 2048-bit certificates to keep up with the increasing compute power available to those trying to break SSL. But there are some SSL performance tuning tips that can help.

The primary cost of the computation associated with a larger server certificate size is in the SSL handshake when a new session is created, so using keep-alive and re-using SSL sessions can make a significant difference in performance. See more about that below.

The SSL CPU utilization will be lower with lower values of ThreadsPerChild. We recommend using a maximum of 100 if your server handles a lot of SSL traffic, so that the client load is spread among multiple child processes. (Note: This optimization is not possible on Windows, which supports only a single child process.)

Set this to the value true when there is significant SSL work-load, as this will result in better performance for the heap operations used by SSL processing.

The preferred approach to improving SSL performance is to use software tuning to the greatest extent possible. Installation and maintenance of crypto cards is relatively complex and usually results in a relatively small reduction in CPU usage. We have observed many situations where the improvement is less than 10%.

HTTP keep-alive has a much larger benefit for SSL than for non-SSL. If the goal is to limit the number of worker threads utilized for keep-alive handling, performance will be much better if KeepAlive is enabled with a small timeout for SSL-enabled virtual hosts, than if keep-alive is disabled altogether.

Example:

<VirtualHost *:443>
normal configuration
# enable keepalive support, but with very small timeout 
# to minimize the use of worker threads
KeepAlive On
KeepAliveTimeout 1
</VirtualHost>

Warning! We are not recommending “KeepAliveTimeout 1” in general. We are suggesting that this is much better than setting KeepAlive Off. Larger values for KeepAliveTimeout will result in slightly better SSL session utilization at the expense of tying up a worker thread for a longer period of time in case the browser sends in another request before the timeout is over. There are diminishing returns for larger values, and the optimal values are dependent upon the interaction between your application and client browsers.

An SSL session is a logical connection between the client and web server for secure communications. During the establishment of the SSL session, public key cryptography is used to to exchange a shared secret master key between the client and the server, and other characteristics of the communication, such as the cipher, are determined. Later data transfer over the session is encrypted and decrypted with symmetric key cryptography, using the shared key created during the SSL handshake.

The generation of the shared key is very CPU intensive. In order to avoid generating the shared key for every TCP connection, there is a capability to reuse the same SSL session for multiple connections. The client must request to reuse the same SSL session in the subsequent handshake, and the server must have the SSL session identifier cached. When these requirements are met, the handshake for the subsequent TCP connection requires far less server CPU (80% less in some tests). All web browsers in general use are able to reuse the same SSL session. Custom web clients sometimes do not have the necessary support, however.

The use of load balancers between web clients and web servers presents a special problem. IBM HTTP Server cannot share a session id cache across machines. Thus, the SSL session can be reused only if a subsequent TCP connection from the same client is sent by the load balancer to the same web server. If it goes to another web server, the session cannot be reused and the shared key must be regenerated, at great CPU expense.

Because of the importance of reusing the same SSL session, load balancer products generally provide the capability of establishing affinity between a particular web client and a particular web server, as long as the web client tries to reuse an existing SSL session. Without the affinity, subsequent connections from a client will often be handled by a different web server, which will require that a new shared key be generated because a new SSL session will be required.

Some load balancer products refer to this feature as SSL Sticky or Session Affinity. Other products may use their own terminology. It is important to activate the appropriate feature to avoid unnecessary CPU usage in the web server, by increasing the frequency that SSL sessions can be reused on subsequent TCP connections.

End users will generally not be aware that SSL session is not being reused unless the overhead of continually negotiating new sessions causes excessive delay in responses. Web server administrators will generally only become aware of this situation when they observe the CPU utilization approaching 100%. The point at which this becomes noticeable will depend on the performance of the web server hardware, and whether or not a cryptographic accelerator is being used.

When SSL is being used and excessive web server CPU utilization is noticed, it is important to first confirm that Session Affinity is enabled if a load balancer is being used.

Checking the actual reuse of SSL sessions

First, get the number of new sessions and reused sessions. LogLevel must be set to info or debug.

IBM HTTP Server 2.0.42 or 2.0.47 up through cumulative fix PK07831, and IBM HTTP Server 6 up through 6.0.2 writes messages of this format for each handshake:

[Sat Jul 09 10:37:22 2005] [info] New Session ID: 0
[Sat Jul 09 10:37:22 2005] [info] New Session ID: 1

0 means that an existing SSL session was re-used. 1 means that a new SSL session was created.

Getting the number of each type of handshake:

$ grep "New Session ID: 0" logs/error_log  | wc -l
1115
$ grep "New Session ID: 1" logs/error_log  | wc -l
163

IBM HTTP Server 2.0.42 or 2.0.47 with cumulative fix PK13230 or later and IBM HTTP Server 6.0.2.1 and later writes messages of this format for each handshake:

[Sat Oct 01 15:30:17 2005] [info] [client 9.49.202.236] Session ID: YT8AAPUJ4gWir+U4v2mZFaw5KDlYWFhYyOM+QwAAAAA= (new)
[Sat Oct 01 15:30:32 2005] [info] [client 9.49.202.236] Session ID: YT8AAPUJ4gWir+U4v2mZFaw5KDlYWFhYyOM+QwAAAAA= (reused)

To get the relative stats:

$ grep "Session ID.*reused" logs/error_log  | wc -l
1115
$ grep "Session ID:.*new" logs/error_log  | wc -l
163

The percentage of expensive handshakes for this test run is 163 / (1115 + 163), or 12.8%. To confirm that the load balancer is not impeding the reuse of SSL sessions, perform a load test with and without the load balancer*, and compare the percentage of expensive handshakes in both tests.

*Alternately, use the load balancer for both tests, but for one load test have the load balancer to send all connections to a particular web server, and for the other load test have it load balance between multiple web servers.

IBM HTTP Server uses an external session ID cache with no practical limits on the number of session IDs unless the operating system is Windows or the directive SSLCacheDisable is present in the IHS configuration.

When the operating system is Windows or the SSLCacheDisable directive is present, IBM HTTP Server uses the GSKit internal session ID cache which is limited to 512 entries by default.

This limit can be increased to a maximum of 4095 (64000 for z/OS) entries by setting the environment variable GSK_V3_SIDCACHE_SIZE to the desired value.

Problem Description

Low data transfer rates handling large POST requests.

This problem can be caused by a small TCP receive buffer size being used for web server sockets. This results in the client being limited in how much data it can send before the server machine has to acknowledge it, resulting in poor network utilization.

Resolution

Some data transfer performance problems can be solved using the native operating system mechanism for increasing the default size of TCP receive buffers. IBM HTTP Server must be restarted after making the change.

Platform	Tuning parameter	Instructions
AIX	tcp_recvspace	Run no -o tcp_recvspace to display the old value. Run no -o tcp_recvspace=new_value to set a larger value.
Solaris	tcp_recv_hiwat	Run ndd /dev/tcp tcp_recv_hiwat to display the old value. Run ndd -set /dev/tcp tcp_recv_hiwat new_value to set a larger value.
HP-UX	tcp_recv_hiwater_def	Run ndd /dev/tcp tcp_recv_hiwater_def to display the old value. Run ndd -set /dev/tcp tcp_recv_hiwater_def new_value to set a larger value.
Linux	rmem_default	Run cat /proc/sys/net/core/rmem_default to display the old value. Run echo new_value > /proc/sys/net/core/rmem_default to set a larger value.

The following levels of IBM HTTP Server contain a ReceiveBufferSize directive for setting this value in a platform-independent manner, and only for the web server:

• 2.0.42.2 with cumulative e-fix PK07831 or later
• 2.0.47.1 with cumulative e-fix PK07831 or later
• 6.0.2 or later
  (6.0.2.1 or later on Windows)

Usage:

ReceiveBufferSize number-of-bytes

This directive must appear at global scope in the configuration file.

Making the adjustment

1. Check the current system default using the platform-specific command in the previous table.
2. Use either 131072 bytes, or twice the current system default, whichever is greater.
   Example ReceiveBufferSize directive:
   ReceiveBufferSize 131072
   If the ReceiveBufferSize directive is not available, use the platform-specific command in the previous table to change the system default.
3. Restart the web server, then retry the testcase.
4. If POST performance did not improve enough, double the receive buffer value and try again.

Problem Description

Low data transfer rates running on AIX 5 when handling large (multi-megabyte) POST requests from Windows machines. Network traces show large delays (~150 ms) between packet acknowledgments.

Resolution

This performance problem can be corrected by setting an AIX network tuning option and applying AIX maintenance.

For all releases of AIX, set the tcp_nodelayack network option to 1 by using the following command:

no -o tcp_nodelayack=1

For AIX 5.1, apply the fix for APAR IY53226. For more information, see: IY53226

For AIX 5.2, apply the fix for APAR IY53254. For more information, see: IY53226

Problem Description

Unexpected network latency when the application is somewhat slow. Network traces show a normal HTTP 200 OK message for the first part of the response, then AIX waits ~150ms for a delayed ACK from the client.

Resolution

This performance problem can be corrected by setting an AIX network tuning option.

Set the rfc2414 network option to 1 by using the following command:

no -o rfc2414=1

IHS threads can block due to disk filesystem sync activity even with logging disabled. This can be caused by updates to file access times for IHS static content files on an HFS. A mount parameter of SYNC(99999) prevents the metadata updates. zFS uses asynchronous disk writes and should not be affected as much by sync activity.

Currently USS has 14,400 save areas per LPAR in a cell pool. One of these is used for every Unix system call. If the LPAR supports more concurrent web connections than this, the cell pool may become depleted. Once this happens, subsequent system calls will still work, but they must first obtain the local lock and use slower methods of allocating storage. Since the cell pool is an LPAR wide resource, other Unix applications (WAS for example) running on the same LPAR are affected.

If your LPAR needs to support tens of thousands of concurrent web connections, check the mpmstats output in the error log for each IHS server. The bsy count shows the total number of busy threads. Each of the busy threads will typically need a USS save area. The ka count shows the number of threads that are busy due to keepalive reads. With IHS 8.5 and later, the ka count will always be zero because keepalive reads are handled asynchronously and do not tie up a thread. With IHS 8.0 and earlier, you may be able to reduce the KeepaliveTimeout value to lower the number of busy threads. Disabling Keepalive altogether is not recommended due to increased CPU overhead needed to re-establish a connection if the client requests another web object. The additional CPU overhead will be much greater for re-establishing SSL connections.

IHS 8.5.5.0 introduced an optional 31-bit IHS for compatibility with 31-bit only libraries required by some DGW Plugins. If this 31-bit IHS is used, the default stack size should be increased to e.g. STACK(1M,128K,ANY,KEEP,1M,128K) in CEE_RUNOPTS. Otherwise, high CPU can be observed in CEE@HDPSO (xplink stack overflow).

Compressing large files on the fly with mod_deflate can use significant CPU. IHS 8.5.5.4 (PI24424) and later includes an alternate mod_deflate.so module, mod_deflate_z.so, that can use zEnterprise Data Compression (zEDC) when configured. In future releases, the default mod_deflate.so will contain zEDC support.

In IHS benchmarks, more than 10x CPU can be saved in configurations that use high CPU due to mod_deflate.

zEDC FAQ