Difference between revisions of "UBC secondary parameters"
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Also, it is important not to allow any Virtual Environment to allocate a | Also, it is important not to allow any Virtual Environment to allocate a |
Revision as of 09:52, 5 June 2007
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Secondary (dependant) UBC parameters are directly connected to the primary ones and can't be configured arbitrarily.
Contents
kmemsize
Size of unswappable memory, 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 “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 barrier
and
the limit
of the kmemsize
parameter
(for example, 10%, as in UBC configuration examples). Equal barrier
and limit
of
the kmemsize
parameter may lead to the situation where the kernel will
need to kill Virtual Environment's applications to keep the kmemsize
usage under the limit.
Kmemsize
limits can't be set arbitrarily high.
The total amount of kmemsize
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 “low memory”.
tcpsndbuf
The total size of buffers used to send data over TCP network connections. These socket buffers reside in “low memory”.
Tcpsndbuf
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:
If this restriction is not satisfied, some network connections may silently stall, being unable to transmit data.
Setting high values for tcpsndbuf
parameter
may, but doesn't necessarily, increase performance of network communications.
Note that, unlike most other parameters, hitting tcpsndbuf
limits and failed socket buffer allocations
do not have strong negative effect on the applications, but just reduce
performance of network communications.
Tcpsndbuf
limits can't be set arbitrarily high.
The total amount of tcpsndbuf
consumable by all Virtual Environments
in the system plus the kmemsize
and other socket buffers is limited
by the hardware resources of the system.
This total limit is discussed in “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 “low memory”.
Tcprcvbuf
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:
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 tcpsndbuf
, setting high values for tcprcvbuf
parameter may, but doesn't necessarily, increase performance of network
communications.
Hitting tcprcvbuf
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 barrier
of tcprcvbuf
parameter
for a long time is less harmless than for tcpsndbuf
.
Long periods of exceeding the barrier
may cause termination
of some connections.
Tcprcvbuf
limits can't be set arbitrarily high.
The total amount of tcprcvbuf
consumable by all Virtual Environments
in the system plus the kmemsize
and other socket buffers is limited
by the hardware resources of the system.
This total limit is discussed in “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.
Othersockbuf
parameter depends on number of non-TCP sockets (numothersock
).
Othersockbuf
configuration should satisfy
Increased limit for othersockbuf
is necessary for high performance of
communications through local (UNIX-domain) sockets.
However, similarly to tcpsndbuf
, hitting othersockbuf
affects
the communication performance only and does not affect the functionality.
Othersockbuf
limits can't be set arbitrarily high.
The total amount of othersockbuf
consumable by all Virtual Environments
in the system plus the kmemsize
and other socket buffers
is limited by the hardware resources of the system.
This total limit is discussed in “low memory”.
dgramrcvbuf
The total size of buffers used to temporary store the incoming packets of UDP and other datagram protocols.
Dgramrcvbuf
parameters depend on number of
non-TCP sockets (numothersock
).
Dgramrcvbuf
limits usually don't need to be high.
Only if the Virtual Environments needs to send and receive very large
datagrams, the barrier
s for both othersockbuf
and
dgramrcvbuf
parameters should be raised.
Hitting dgramrcvbuf
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 dgramrcvbuf
the barrier
should be set to the limit
.
Dgramrcvbuf
limits can't be set arbitrarily high.
The total amount of dgramrcvbuf
consumable by all Virtual Environments
in the system plus the kmemsize
and other socket buffers
is limited by the hardware resources of the system.
This total limit is discussed in “low memory”.
oomguarpages
The guaranteed amount of memory for the case the memory is “over-booked” (out-of-memory kill guarantee).
Oomguarpages
parameter is related to vmguarpages
.
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[1].
Oomguarpages
parameter accounts the total amount of
memory and swap space used by the processes of a particular
Virtual Environment.
The barrier
of the oomguarpages
parameter is the out-of-memory
guarantee.
If the current usage of memory and swap space
(the value of oomguarpages
) plus the amount of used kernel memory
(kmemsize
) and socket buffers is below the barrier
,
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 oomguarpages
excess, applications in the
Virtual Environment with the biggest excess will be killed first.
The failcnt
counter of oomguarpages
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 privvmpages
limit to a value not greater than the
oomguarpages
guarantee significantly reduce the likelihood of
the application being killed,
and setting it to a half of the oomguarpages
guarantee completely
prevents it.
Such configurations are not popular because they significantly reduce
the utilization of the hardware.
The meaning of the limit for the
oomguarpages
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.
Privvmpages
parameter
allows controlling the amount of memory allocated by applications.
The barrier
and the limit
of privvmpages
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 vmguarpages
guarantee.
Privvmpages
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 oomguarpages
parameter.
Since the memory accounted into privvmpages
may not be actually used,
the sum of current privvmpages
values for all Virtual Environments
may exceed the RAM and swap size of the computer.
There should be a safety gap between the barrier
and the limit
for privvmpages
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 barrier
if
privvmpages
is reached.
Total privvmpages
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 VEs.
Both these configuration requirements are discussed in UBC systemwide configuration#Allocated memory.
There's also an article describing how user pages accounting works.
Units
Oomguarpages
and privvmpages
values are measured in memory pages.
For other secondary parameters, the values are in bytes.
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
- ↑ The possible reasons of out-of-memory situations are the excess of total
vmguarpages
guarantees the available physical resources or high memory consumption by system processes. Also, the kernel might allow some Virtual Environments to allocate memory above theirvmguarpages
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.