Editing UBC secondary parameters

{{UBC toc}}

'''Secondary (dependant) UBC parameters''' are directly connected
to the [[UBC primary parameters|primary ones]] and can't be configured arbitrarily.

== kmemsize ==
Size of unswappable memory in bytes, allocated by the operating system kernel.

It includes all the kernel internal data structures associated with the
Virtual Environment's processes, except the network buffers discussed below.
These data structures reside in the first gigabyte of the computer's RAM,
so called [[UBC systemwide configuration#“Low memory”|“low memory”]].

This parameter is related to the number of processes ([[numproc]]).
Each process consumes certain amount of kernel memory —
24 kilobytes at minimum, 30–60 KB typically.
Very large processes may consume much more than that.

It is important to have a certain safety gap between the <code>barrier</code> and
the <code>limit</code> of the <code>kmemsize</code> parameter
(for example, 10%, as in [[UBC configuration examples]]).  Equal <code>barrier</code> and <code>limit</code> of
the <code>kmemsize</code> parameter may lead to the situation where the kernel will
need to kill Virtual Environment's applications to keep the <code>kmemsize</code>
usage under the limit.

<code>Kmemsize</code> limits can't be set arbitrarily high.
The total amount of <code>kmemsize</code> consumable by all Virtual Environments
in the system plus the socket buffer space (see below) is limited by the
hardware resources of the system.
This total limit is discussed in [[UBC systemwide configuration#“Low memory”|“low memory”]].

== tcpsndbuf ==
The total size of buffers used to send data over TCP network connections.
These socket buffers reside in [[UBC systemwide configuration#“Low memory”|“low memory”]].

<code>Tcpsndbuf</code> parameter depends on number of TCP
sockets ([[numtcpsock]]) and should allow for some minimal amount of
socket buffer memory for each socket, as discussed in [[UBC consistency check]]:

<math>tcpsndbuf_{lim} - tcpsndbuf_{bar} \ge 2.5KB \cdot numtcpsock \rm.</math>

If this restriction is not satisfied, some network connections
may silently stall, being unable to transmit data.

Setting high values for <code>tcpsndbuf</code> parameter
may, but doesn't necessarily, increase performance of network communications.
Note that, unlike most other parameters, hitting <code>tcpsndbuf</code>
limits and failed socket buffer allocations
do not have strong negative effect on the applications, but just reduce
performance of network communications.

If you use rtorrent in a container, a low value for <code>tcpsndbuf</code> may cause rtorrent to take unusual amount of cpu. In this case, you must put a higher value. Also watch the number of failcnt in /proc/user_beancounters.

<code>Tcpsndbuf</code> limits can't be set arbitrarily high.
The total amount of <code>tcpsndbuf</code> consumable by all Virtual Environments
in the system plus the <code>kmemsize</code> and other socket buffers is limited
by the hardware resources of the system.
This total limit is discussed in [[UBC systemwide configuration#“Low memory”|“low memory”]].

== tcprcvbuf ==
The total size of buffers used to temporary store the data coming from TCP network connections.
These socket buffers also reside in [[UBC systemwide configuration#“Low memory”|“low memory”]].

<code>Tcprcvbuf</code> parameter depends on number of TCP
sockets ([[numtcpsock]]) and should allow for some minimal amount of
socket buffer memory for each socket, as discussed in [[UBC consistency check]]:

<math>tcprcvbuf_{lim} - tcprcvbuf_{bar} \ge 2.5KB \cdot numtcpsock \rm.</math>

If this restriction is not satisfied, some network connections
may stall, being unable to receive data, and will be terminated
after a couple of minutes.

Similarly to <code>tcpsndbuf</code>, setting high values for <code>tcprcvbuf</code>
parameter may, but doesn't necessarily, increase performance of network
communications.
Hitting <code>tcprcvbuf</code> limits and failed socket buffer allocations
do not have strong negative effect on the applications, but just reduce
performance of network communications.
However, staying above the <code>barrier</code> of <code>tcprcvbuf</code> parameter
for a long time is less harmless than for <code>tcpsndbuf</code>.
Long periods of exceeding the <code>barrier</code> may cause termination
of some connections.

<code>Tcprcvbuf</code> limits can't be set arbitrarily high.
The total amount of <code>tcprcvbuf</code> consumable by all Virtual Environments
in the system plus the <code>kmemsize</code> and other socket buffers is limited
by the hardware resources of the system.
This total limit is discussed in [[UBC systemwide configuration#“Low memory”|“low memory”]].

== othersockbuf ==
The total size of buffers used by local (UNIX-domain) connections between
processes inside the system (such as connections to a local database server)
and send buffers of UDP and other datagram protocols.

<code>Othersockbuf</code> parameter depends on number of non-TCP sockets (<code>[[numothersock]]</code>).

<code>Othersockbuf</code> configuration should satisfy

<math>othersockbuf_{lim} - othersockbuf_{bar} \ge 2.5KB \cdot numothersock.</math>

Increased limit for <code>othersockbuf</code> is necessary for high performance of
communications through local (UNIX-domain) sockets.
However, similarly to <code>tcpsndbuf</code>, hitting <code>othersockbuf</code> affects
the communication performance only and does not affect the functionality.

<code>Othersockbuf</code> limits can't be set arbitrarily high.
The total amount of <code>othersockbuf</code> consumable by all Virtual Environments
in the system plus the <code>kmemsize</code> and other socket buffers
is limited by the hardware resources of the system.
This total limit is discussed in [[UBC systemwide configuration#“Low memory”|“low memory”]].

== dgramrcvbuf ==
The total size of buffers used to temporary store the incoming packets of UDP and
other datagram protocols.

<code>Dgramrcvbuf</code> parameters depend on number of
non-TCP sockets (<code>[[numothersock]]</code>).

<code>Dgramrcvbuf</code> limits usually don't need to be high.
Only if the Virtual Environments needs to send and receive very large
datagrams, the <code>barrier</code>s for both <code>othersockbuf</code> and
<code>dgramrcvbuf</code> parameters should be raised.

Hitting <code>dgramrcvbuf</code> means that some datagrams are dropped, which may
or may not be important for application functionality.
UDP is a protocol with not guaranteed delivery, so even if the buffers
permit, the datagrams may be as well dropped later on any stage of the
processing, and applications should be prepared for it.

Unlike other socket buffer parameters, for <code>dgramrcvbuf</code>
the <code>barrier</code> should be set to the <code>limit</code>.

<code>Dgramrcvbuf</code> limits can't be set arbitrarily high.
The total amount of <code>dgramrcvbuf</code> consumable by all Virtual Environments
in the system plus the <code>kmemsize</code> and other socket buffers
is limited by the hardware resources of the system.
This total limit is discussed in [[UBC systemwide configuration#“Low memory”|“low memory”]].

== oomguarpages ==
The guaranteed amount of memory for the case the memory is “over-booked”
(out-of-memory kill guarantee).

<code>Oomguarpages</code> parameter is related to <code>[[vmguarpages]]</code>.
If applications start to consume more memory than the computer has,
the system faces an out-of-memory condition.
In this case the operating system will start to kill Virtual Environment's
processes to free some memory and prevent the total death
of the system.  Although it happens very rarely in typical system loads,
killing processes in out-of-memory situations is a normal reaction of the
system, and it is built into every Linux kernel<ref>The possible reasons of out-of-memory situations are the excess of total <code>[[vmguarpages]]</code> guarantees the available physical resources or high memory consumption by system processes.  Also, the kernel might allow some Virtual Environments to allocate memory above their <code>[[vmguarpages]]</code> guarantees when the system had a lot of free memory, and later, when other Virtual Environments claim their guarantees, the system will experience the memory shortage.</ref>.

<code>[[Oomguarpages]]</code> parameter accounts the total amount of
memory and swap space used by the processes of a particular
Virtual Environment.
The <code>barrier</code> of the <code>oomguarpages</code> parameter is the out-of-memory
guarantee.

If the current usage of memory and swap space
(the value of <code>oomguarpages</code>) plus the amount of used kernel memory
(<code>[[kmemsize]]</code>) and socket buffers is below the <code>barrier</code>,
processes in this Virtual Environment are guaranteed not to be killed in
out-of-memory situations.
If the system is in out-of-memory situation and there are several
Virtual Environments with <code>oomguarpages</code> excess, applications in the
Virtual Environment with the biggest excess will be killed first.
The <code>failcnt</code> counter of <code>oomguarpages</code> parameter
increases when a process in this Virtual Environment is killed because
of out-of-memory situation.

If the administrator needs to make sure that some application won't be
forcedly killed regardless of the application's behavior,
setting the <code>[[privvmpages]]</code> limit to a value not greater than the
<code>oomguarpages</code> guarantee significantly reduce the likelihood of
the application being killed,
and setting it to a half of the <code>oomguarpages</code> guarantee completely
prevents it.
Such configurations are not popular because they significantly reduce
the utilization of the hardware.

The meaning of the <code>limit</code> for the <code>oomguarpages</code> parameter is
unspecified in the current version.

The total out-of-memory guarantees given to the Virtual Environments should
not exceed the physical capacity of the computer, as discussed in [[UBC systemwide configuration#Memory and swap space]].
If guarantees are given for more than the system has, in out-of-memory
situations applications in Virtual Environments with guaranteed level of
service and system daemons may be killed.

== privvmpages ==
Memory allocation limit in [[Memory_page|pages]] (which are typically 4096 bytes in size).

<code>Privvmpages</code> parameter
allows controlling the amount of memory allocated by applications.

The <code>barrier</code> and the <code>limit</code> of <code>privvmpages</code> parameter
control the upper boundary of the total size of allocated memory.
Note that this upper boundary doesn't guarantee that the Virtual Environment
will be able to allocate that much memory, neither does it guarantee that
other Virtual Environments will be able to allocate their fair share of
memory.
The primary mechanism to control memory allocation is the <code>[[vmguarpages]]</code>
guarantee.

<code>Privvmpages</code> parameter accounts allocated (but, possibly,
not used yet) memory.
The accounted value is an estimation how much memory will be really consumed
when the Virtual Environment's applications start to use the allocated
memory.
Consumed memory is accounted into <code>[[oomguarpages]]</code> parameter.

Since the memory accounted into <code>privvmpages</code> may not be actually used,
the sum of current <code>privvmpages</code> values for all Virtual Environments
may exceed the RAM and swap size of the computer.

There should be a safety gap between the <code>barrier</code> and the <code>limit</code>
for <code>privvmpages</code> parameter to reduce the number of memory allocation
failures that the application is unable to handle.
This gap will be used for “high-priority” memory allocations, such
as process stack expansion.
Normal priority allocations will fail when the <code>barrier</code> of
<code>privvmpages</code> is reached.

Total <code>privvmpages</code> should correlate with the physical resources of the
computer.
Also, it is important not to allow any Virtual Environment to allocate a
significant portion of all system RAM to avoid serious service level
degradation for other containers.
Both these configuration requirements are discussed in [[UBC systemwide configuration#Allocated memory]].

There's also an article describing how [[user pages accounting]] works.

== System-wide limits ==
All secondary parameters are related to memory.
Total limits on memory-related parameters must not exceed the physical
resources of the computer.
The restrictions on the configuration of memory-related parameters are listed
in [[UBC systemwide configuration]].
Those restrictions are very important, because their violation may
allow any Virtual Environment cause the whole system to hang.

== Notes ==
<references/>