UBC primary parameters

2006-10-25T03:11:33Z

Psykax: Add mention of postfix to numothersock

{{UBC toc}}

The most important parameters determining the resources available to
Virtual Environment are explained below. The meaning of the parameters
is illustrated assuming that the Virtual Environment runs some network
server applications.

== numproc ==
Maximum number of processes and kernel-level threads allowed for this Virtual Environment.

Many server applications (like Apache Web server, FTP and mail servers)
spawn a process to handle each client, so the limit on number of processes
defines how many clients the application will be able to handle in parallel.
However, the number of processes doesn't limit how “heavy” the application is
and whether the server will be able to serve heavy requests from clients.

Configuring resource control system, it is important to estimate both
the maximum number of processes and the average number of processes
(referred to as avnumproc later). Other (dependent) resource
control parameters may depend both on the limit and the average number (see [[UBC consistency check]]).

Barrier of numproc doesn't provide additional control and should be set equal to the limit.

There is a restriction on the total number of processes in the system.
More than about 16000 processes start to cause poor responsiveness of
the system, worsening when the number grows. Total number of processes
exceeding 32000 is very likely to cause hang of the system.

Note that in practice the number of processes is usually less. Each
process consumes some memory, and the available memory and the
"low memory" (see [[UBC systemwide configuration#“Low memory”|“Low memory”]]) limit the number of processes to lower
values. With typical processes, it is normal to be able to run only up
to 8000 processes in a system.

== numtcpsock ==
Maximum number of TCP sockets.

This parameter limits the
number of TCP connections and, thus, the number of clients the server
application can handle in parallel.
Barrier of this parameter should be set equal to the limit.
If each Virtual Environment has it's own set of IP addresses (which is
the only way a OpenVZ system can be configured), there are no direct
limits on the total number of TCP sockets in the system. The number
of sockes needs to be controlled because each socket needs certain
amount of memory for receive and transmit buffers (see descriptions
of [[<code>tcpsndbuf</code>]] and [[<code>tcprcvbuf</code>]]), and
the memory is a limited resource.

== numothersock ==
Maximum number of non-TCP sockets (local sockets, UDP and other types of sockets).

Local (UNIX-domain) sockets are used for communications inside the
system. Multi-tier applications (for example, a Web application with a
database server as a back-end) may need one or multiple local sockets
to serve each client. Straightforward applications (for example, most
mail servers, with the exception of Postfix) do not use local sockets.

UDP sockets are used for Domain Name Service (DNS) queries, but
the number of such sockets opened simultaneously is low. UDP and
other sockets may also be used in some very special applications
(SNMP agents and others).

<code>Barrier</code> of this parameter should be set equal to the <code>limit</code>.
The number of local sockets in a system is not limited. The number of
UDP sockets in a system, similarly to TCP sockets, is not limited in
OpenVZ systems.

Similarly to <code>numtcpsock</code> parameter discussed above, the number of
non-TCP sockets needs to be controlled because each socket needs certain
amount of memory for its buffers, and the memory is a limited
resource.

== vmguarpages ==
Memory allocation guarantee.

This parameter controls how much memory is available to the Virtual
Environment (i.e. how much memory its applications can allocate by
<code>malloc(3)</code> or other standard Linux memory allocation mechanisms).
The more clients are served or the more “heavy” the application is, the
more memory it needs.

The amount of memory that Virtual Environment's applications are
guaranteed to be able to allocate is specified as the barrier of
<code>vmguarpages</code> parameter. The current amount of allocated memory space
is accounted into <code>privvmpages</code> parameter, and <code>vmguarpages</code>
parameter does not have its own accounting. The barrier and the limit of
<code>privvmpages</code> parameter impose an upper limit on the memory allocations
(see [[privvmpages]]). The meaning of the <code>limit</code> for the
<code>vmguarpages</code> parameter is unspecified in the current version and should be set
to the maximal allowed value ([[MAX_ULONG]]).

If the current amount of allocated memory space does not exceed the
guaranteed amount (the <code>barrier</code> of <code>vmguarpages</code>),
memory allocations of Virtual Environment's applications always succeed.
If the current amount of allocated memory space exceeds the guarantee but below
the <code>barrier</code> of <code>privvmpages</code>, allocations may or may not succeed,
depending on the total amount of available memory in the system.

Starting from the <code>barrier</code> of <code>privvmpages</code>,
normal priority allocations and, starting from the <code>limit</code>
of <code>privvmpages</code>, all memory allocations
made by the applications fail.
The memory allocation guarantee (<code>vmguarpages</code>) is a primary tool for
controlling the memory available to Virtual Environments, because
it allows administrators to provide Service Level Agreements — agreements
guaranteeing certain quality of service, certain amount of resources
and general availability of the service. The unit of measurement
of vmguarpages values is memory pages (4KB on x86 processors).
The total memory allocation guarantees given to Virtual Environments
are limited by the physical resources of the computer — the size of RAM
and the swap space — as discussed in [[UBC systemwide configuration]].

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UBC primary parameters