| A large EnterpriseDB installation can quickly exhaust
various operating system resource limits. (On some systems, the
factory defaults are so low that you don't even need a really
"large" installation.) If you have encountered this kind of
problem, keep reading.
Shared memory and semaphores are collectively referred to as
"System V
IPC" (together with message queues, which are not
relevant for EnterpriseDB). Almost all modern
operating systems provide these features, but not all of them have
them turned on or sufficiently sized by default, especially systems
with BSD heritage. (For the QNX and
BeOS ports, EnterpriseDB
provides its own replacement implementation of these facilities.)
The complete lack of these facilities is usually manifested by an
Illegal system call error upon server start. In
that case there's nothing left to do but to reconfigure your
kernel. EnterpriseDB won't work without them.
When EnterpriseDB exceeds one of the various hard
IPC limits, the server will refuse to start and
should leave an instructive error message describing the problem
encountered and what to do about it. (See also Section 30.3.1.) The relevant kernel
parameters are named consistently across different systems; Table 30-2 gives an overview. The methods to set
them, however, vary. Suggestions for some platforms are given below.
Be warned that it is often necessary to reboot your machine, and
possibly even recompile the kernel, to change these settings.
Table 30-2. System V IPC parameters Name | Description | Reasonable values |
---|
SHMMAX | Maximum size of shared memory segment (bytes) | 250 KB + 8.2 KB * shared_buffers +
14.2 kB * max_connections up to infinity | SHMMIN | Minimum size of shared memory segment (bytes) | 1 | SHMALL | Total amount of shared memory available (bytes or pages) | if bytes, same as SHMMAX; if pages, ceil(SHMMAX/PAGE_SIZE) | SHMSEG | Maximum number of shared memory segments per process | only 1 segment is needed, but the default is much higher | SHMMNI | Maximum number of shared memory segments system-wide | like SHMSEG plus room for other applications | SEMMNI | Maximum number of semaphore identifiers (i.e., sets) | at least ceil(max_connections / 16) | SEMMNS | Maximum number of semaphores system-wide | ceil(max_connections / 16) * 17 plus room for other applications | SEMMSL | Maximum number of semaphores per set | at least 17 | SEMMAP | Number of entries in semaphore map | see text | SEMVMX | Maximum value of semaphore | at least 1000 (The default is often 32767, don't change unless asked to.) |
The most important
shared memory parameter is SHMMAX, the maximum size, in
bytes, of a shared memory segment. If you get an error message from
shmget like Invalid argument, it is
possible that this limit has been exceeded. The size of the required
shared memory segment varies both with the number of requested
buffers (-B option) and the number of allowed connections
(-N option), although the former is the most significant.
(You can, as a temporary solution, lower these settings to eliminate
the failure.) As a rough approximation, you can estimate the
required segment size by multiplying the number of buffers and the
block size (8 KB by default) plus ample overhead (at least half a
megabyte). Any error message you might get will contain the size of
the failed allocation request.
Less likely to cause problems is the minimum size for shared
memory segments (SHMMIN), which should be at most
approximately 256 KB for EnterpriseDB (it is
usually just 1). The maximum number of segments system-wide
(SHMMNI) or per-process (SHMSEG) should
not cause a problem unless your system has them set to zero. Some
systems also have a limit on the total amount of shared memory in
the system; see the platform-specific instructions below.
EnterpriseDB uses one semaphore per allowed connection
(-N option), in sets of 16. Each such set will also
contain a 17th semaphore which contains a "magic
number", to detect collision with semaphore sets used by
other applications. The maximum number of semaphores in the system
is set by SEMMNS, which consequently must be at least
as high as max_connections plus one extra for each 16
allowed connections (see the formula in Table 30-2). The parameter SEMMNI
determines the limit on the number of semaphore sets that can
exist on the system at one time. Hence this parameter must be at
least ceil(max_connections / 16). Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded No space
left on device, from the function semget .
In some cases it might also be necessary to increase
SEMMAP to be at least on the order of
SEMMNS. This parameter defines the size of the semaphore
resource map, in which each contiguous block of available semaphores
needs an entry. When a semaphore set is freed it is either added to
an existing entry that is adjacent to the freed block or it is
registered under a new map entry. If the map is full, the freed
semaphores get lost (until reboot). Fragmentation of the semaphore
space could, over time lead to fewer available semaphores than there
should be.
The SEMMSL parameter, which determines how many
semaphores can be in a set, must be at least 17 for
EnterpriseDB.
Various other settings related to "semaphore undo", such as
SEMMNU and SEMUME, are not of concern
for EnterpriseDB.
- BSD/OS
For those running 4.0 and earlier releases, use bpatch
to find the sysptsize value in the current
kernel. This is computed dynamically at boot time.
$ bpatch -r sysptsize
0x9 = 9
Next, add SYSPTSIZE as a hard-coded value in the
kernel configuration file. Increase the value you found using
bpatch. Add 1 for every additional 4 MB of
shared memory you desire.
options "SYSPTSIZE=16"
sysptsize cannot be changed by sysctl.
- FreeBSD
NetBSD OpenBSD The options SYSVSHM and SYSVSEM need
to be enabled when the kernel is compiled. (They are by
default.) The maximum size of shared memory is determined by
the option SHMMAXPGS (in pages). The following
shows an example of how to set the various parameters:
options SYSVSHM
options SHMMAXPGS=4096
options SHMSEG=256
options SYSVSEM
options SEMMNI=256
options SEMMNS=512
options SEMMNU=256
options SEMMAP=256
(On NetBSD and OpenBSD the key word is actually
option singular.)
You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
Use the sysctl setting
kern.ipc.shm_use_phys.
- HP-UX
The default settings tend to suffice for normal installations.
On HP-UX 10, the factory default for
SEMMNS is 128, which might be too low for larger
database sites.
IPC parameters can be set in the System
Administration Manager (SAM) under
->. Hit
Create A New Kernel when you're done.
- Linux
The default shared memory limit (both
SHMMAX and SHMALL) is 32
MB in 2.2 kernels, but it can be changed in the
proc file system (without reboot). For
example, to allow 128 MB:
$ echo 134217728 >/proc/sys/kernel/shmall
$ echo 134217728 >/proc/sys/kernel/shmmax
You could put these commands into a script run at boot-time.
Alternatively, you can use sysctl, if
available, to control these parameters. Look for a file
called /etc/sysctl.conf and add lines
like the following to it:
kernel.shmall = 134217728
kernel.shmmax = 134217728
This file is usually processed at boot time, but
sysctl can also be called
explicitly later.
Other parameters are sufficiently sized for any application. If
you want to see for yourself look in
/usr/src/linux/include/asm-xxx/shmparam.h
and /usr/src/linux/include/linux/sem.h.
- SCO OpenServer
In the default configuration, only 512 KB of shared memory per
segment is allowed, which is about enough for -B 24 -N
12. To increase the setting, first change to the directory
/etc/conf/cf.d. To display the current value of
SHMMAX, run
./configure -y SHMMAX
To set a new value for SHMMAX, run
./configure SHMMAX=value
where value is the new value you want to use
(in bytes). After setting SHMMAX, rebuild the kernel:
./link_unix
and reboot.
- AIX
At least as of version 5.1, it should not be necessary to do
any special configuration for such parameters as
SHMMAX, as it appears this is configured to
allow all memory to be used as shared memory. That is the
sort of configuration commonly used for other databases such
as DB/2. It may, however, be necessary to modify the global
ulimit information in
/etc/security/limits, as the default hard
limits for filesizes (fsize) and numbers of
files (nofiles) may be too low.
- Solaris
At least in version 2.6, the default maximum size of a shared
memory segments is too low for EnterpriseDB. The
relevant settings can be changed in /etc/system,
for example:
set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256
set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32
You need to reboot for the changes to take effect.
See also http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html for information on shared memory under Solaris.
- UnixWare
On UnixWare 7, the maximum size for shared
memory segments is 512 KB in the default configuration. This
is enough for about -B 24 -N 12. To display the
current value of SHMMAX, run
/etc/conf/bin/idtune -g SHMMAX
which displays the current, default, minimum, and maximum
values. To set a new value for SHMMAX,
run
/etc/conf/bin/idtune SHMMAX value
where value is the new value you want to use
(in bytes). After setting SHMMAX, rebuild the
kernel:
/etc/conf/bin/idbuild -B
and reboot.
Unix-like operating systems enforce various kinds of resource limits
that might interfere with the operation of your
EnterpriseDB server. Of particular
importance are limits on the number of processes per user, the
number of open files per process, and the amount of memory available
to each process. Each of these have a "hard" and a
"soft" limit. The soft limit is what actually counts
but it can be changed by the user up to the hard limit. The hard
limit can only be changed by the root user. The system call
setrlimit is responsible for setting these
parameters. The shell's built-in command ulimit
(Bourne shells) or limit (csh) is
used to control the resource limits from the command line. On
BSD-derived systems the file /etc/login.conf
controls the various resource limits set during login. See the
operating system documentation for details. The relevant
parameters are maxproc,
openfiles, and datasize. For
example:
default:\
...
:datasize-cur=256M:\
:maxproc-cur=256:\
:openfiles-cur=256:\
...
(-cur is the soft limit. Append
-max to set the hard limit.)
Kernels can also have system-wide limits on some resources.
The EnterpriseDB server uses one process
per connection so you should provide for at least as many processes
as allowed connections, in addition to what you need for the rest
of your system. This is usually not a problem but if you run
several servers on one machine things might get tight.
The factory default limit on open files is often set to
"socially friendly" values that allow many users to
coexist on a machine without using an inappropriate fraction of
the system resources. If you run many servers on a machine this
is perhaps what you want, but on dedicated servers you may want to
raise this limit.
On the other side of the coin, some systems allow individual
processes to open large numbers of files; if more than a few
processes do so then the system-wide limit can easily be exceeded.
If you find this happening, and you do not want to alter the
system-wide limit, you can set EnterpriseDB's max_files_per_process configuration parameter to
limit the consumption of open files.
In Linux 2.4 and later, the default virtual memory behavior is not
optimal for EnterpriseDB. Because of the
way that the kernel implements memory overcommit, the kernel may
terminate the EnterpriseDB server (the
edb-postmaster process) if the memory demands of
another process cause the system to run out of virtual memory.
If this happens, you will see a kernel message that looks like
this (consult your system documentation and configuration on where
to look for such a message):
Out of Memory: Killed process 12345 (edb-postmaster).
This indicates that the edb-postmaster process
has been terminated due to memory pressure.
Although existing database connections will continue to function
normally, no new connections will be accepted. To recover,
EnterpriseDB will need to be restarted.
One way to avoid this problem is to run
EnterpriseDB
on a machine where you can be sure that other processes will not
run the machine out of memory.
On Linux 2.6 and later, a better solution is to modify the kernel's
behavior so that it will not "overcommit" memory. This is
done by selecting strict overcommit mode via sysctl:
sysctl -w vm.overcommit_memory=2
or placing an equivalent entry in /etc/sysctl.conf.
You may also wish to modify the related setting
vm.overcommit_ratio. For details see the kernel documentation
file Documentation/vm/overcommit-accounting.
Some vendors' Linux 2.4 kernels are reported to have early versions
of the 2.6 overcommit sysctl. However, setting
vm.overcommit_memory to 2
on a kernel that does not have the relevant code will make
things worse not better. It is recommended that you inspect
the actual kernel source code (see the function
vm_enough_memory in the file mm/mmap.c)
to verify what is supported in your copy before you try this in a 2.4
installation. The presence of the overcommit-accounting
documentation file should not be taken as evidence that the
feature is there. If in any doubt, consult a kernel expert or your
kernel vendor.
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