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**Contents**

[[!toc levels=3]]

#  Tuning the kernel 

##  Process and file descriptor limits 

**Before reading:**

_These are mostly only demonstative values on how to tune your system for different needs. They are not some kind of an ultimate optional values. This article mostly aims to provide a quick overview on the ways to fine tune your system settings and being aware of the limitations._

###  maxusers 

The name is a bit misleading, because it doesn't set the number of users on the system, but used in the formula to calculate maximal number of allowed processes. 

You can find it in your kernel configuration file, something like this: 
    
    maxusers        32
    

This is the default value, so if we look at the formulae we get process limit values: 
    
    /usr/src/sys/param.h:
       #define  NPROC    (20 + 16 * MAXUSERS)
    
    /usr/src/sys/conf/param.c:
       #define  MAXFILES (3 * (NPROC + MAXUSERS) + 80)
    

So we got 532 for NPROC (maximal number of processes) and 1772 for MAXFILES (maximal number of open file descriptors). 

Some say that the maxusers should be set to the amount of RAM in megabytes.  
For reference, FreeBSD sets is automaticaly by this formula, but limits it's maximum to 384. 

Setting it to 64 is always a safe bet if you don't want too much experimenting. Just change it in your kernel configuration file: 
    
    maxusers        64
    

Compile the new kernel with build.sh or manualy, install the new kernel and reboot. 

You can check your limits with sysctl: 

With maxusers 32 
    
    $ sysctl proc.curproc.rlimit.maxproc
    proc.curproc.rlimit.maxproc.soft = 160
    proc.curproc.rlimit.maxproc.hard = 532
    
    
    $ sysctl proc.curproc.rlimit.descriptors
    proc.curproc.rlimit.descriptors.soft = 64
    proc.curproc.rlimit.descriptors.hard = 1772
    

With maxusers 64 

You can check your limits with sysctl: 
    
    $ sysctl proc.curproc.rlimit.maxproc
    proc.curproc.rlimit.maxproc.soft = 160
    proc.curproc.rlimit.maxproc.hard = 1044
    
    
    $ sysctl proc.curproc.rlimit.descriptors
    proc.curproc.rlimit.descriptors.soft = 64
    proc.curproc.rlimit.descriptors.hard = 3404
    

###  login.conf 

So you can change the hard limits now. Let's see the soft limits. 

or with ulimit: 
    
    $ ulimit -a
    core file size          (blocks, -c) unlimited
    data seg size           (kbytes, -d) 131072
    file size               (blocks, -f) unlimited
    max locked memory       (kbytes, -l) 80920
    max memory size         (kbytes, -m) 242760
    open files                      (-n) 64
    pipe size            (512 bytes, -p) 1
    stack size              (kbytes, -s) 2048
    cpu time               (seconds, -t) unlimited
    max user processes              (-u) 160
    virtual memory          (kbytes, -v) 133120
    

  
You can set it with the file /etc/login.conf: 
    
    default:\
        :path=/usr/bin /bin /usr/sbin /sbin /usr/X11R6/bin /usr/pkg/bin /usr/pkg/sbin /usr/local/bin:\
        :umask=022:\
        :datasize-max=3072M:\
        :datasize-cur=1024M:\
        :maxproc-max=1044:\
        :maxproc-cur=512:\
        :openfiles-cur=256:\
        :stacksize-cur=8M:
    

Next time you start the sytem, all users belonging to the _default_ login group will have the following limits: 
    
    $ ulimit -a
    coredump(blocks)     unlimited
    data(KiB)            1048576
    file(blocks)         unlimited
    lockedmem(KiB)       124528
    memory(KiB)          373584
    nofiles(descriptors) 256
    processes            512
    stack(KiB)           8192
    time(cpu-seconds)    unlimited
    

You may set different limits for different user, thus different services: 
    
    database:\
       :ignorenologin:\
       :datasize=infinity:\
       :maxproc=infinity:\
       :openfiles-cur=1024:\
       :stacksize-cur=48M:
    

You should run this command after editing your login.conf: 
    
    $ cap_mkdb /etc/login.conf
    

You can assign the newly created login class to the desired user by doing something like this: 
    
    $ usermod -L database pgsql
    

Let's check our limits again with sysctl: 
    
    $ sysctl proc.curproc.rlimit.maxproc
    proc.curproc.rlimit.maxproc.soft = 512
    proc.curproc.rlimit.maxproc.hard = 1044
    
    $ sysctl proc.curproc.rlimit.descriptors
    proc.curproc.rlimit.descriptors.soft = 256
    proc.curproc.rlimit.descriptors.hard = 3404
    

Much reasonable for a modern system. 

##  System V interprocess communication 

Shared memory and semaphores are part of the System V IPC. Using and fine tuning shared memory and semaphores can give you increased performance on your NetBSD server. 

You can check it's settings with sysctl: 
    
    $ sysctl kern.ipc
    kern.ipc.sysvmsg = 1
    kern.ipc.sysvsem = 1
    kern.ipc.sysvshm = 1
    kern.ipc.shmmax = 8388608
    kern.ipc.shmmni = 128
    kern.ipc.shmseg = 128
    kern.ipc.shmmaxpgs = 2048
    kern.ipc.shm_use_phys = 0
    kern.ipc.msgmni = 40
    kern.ipc.msgseg = 2048
    kern.ipc.semmni = 10
    kern.ipc.semmns = 60
    kern.ipc.semmnu = 30
    

As you can see, the default maximum size of shared memory segment (shmmax) is 8 megabytes by default, but for a postgresql server you will most likely need about 128 megabytes. 

Note, that you cannot set shmmax directly with syctl, but you need to set the value in pages size with _kern.ipc.shmmaxpgs_. 

The default PAGE_SIZE is 4096, so if you want to set it to 128M, you have to do: 
    
    grimnismal# sysctl -w kern.ipc.shmmaxpgs=32768
    kern.ipc.shmmaxpgs: 4096 -> 32768
    

So the formula is: 128*1024*1024/4096 = 32768 

**You can make any sysctl change permanent by setting it in /etc/sysctl.conf**

You can also get detailed information on System V interprocess communication (IPC) facilities on the system with the following command: 
    
    
    $ ipcs
    IPC status from <running system> as of Mon Dec  3 18:52:00 2007
    
    Message Queues:
    T        ID     KEY        MODE       OWNER    GROUP
    
    Shared Memory:
    T        ID     KEY        MODE       OWNER    GROUP
    m     65536    5432001 --rw-------    pgsql    pgsql
    
    Semaphores:
    T        ID     KEY        MODE       OWNER    GROUP
    s     65536    5432001 --rw-------    pgsql    pgsql
    s     65537    5432002 --rw-------    pgsql    pgsql
    s     65538    5432003 --rw-------    pgsql    pgsql
    

  
You can also force shared memory to stay in physical memory. This means that they will be never paged out to swap.  
You may set this behaviour with the _kern.ipc.shm_use_phys_ sysctl. 

##  TCP Performance 

TCP uses what is called the “congestion window” to determine how many packets can be sent at one time. The larger the congestion window size, the higher the throughput. The maximum congestion window is related to the amount of buffer space that the kernel allocates for each socket. 

So on high bandwidth line the bottleneck could be the buffer sizes. 

Here's the formula for a network link's throughput: 
    
    Throughput = buffer size / latency
    

So if we reorganise it a bit, we get the formula of the ideal buffer size: 
    
    buffer size = 2 * delay * bandwidth
    

The delay is the network latency, which is most commonly known as "ping". 

I think I don't have to introduce this tool: 
    
    $ ping yahoo.com
    PING yahoo.com (66.94.234.13): 56 data bytes
    64 bytes from 66.94.234.13: icmp_seq=0 ttl=50 time=195.596 ms
    64 bytes from 66.94.234.13: icmp_seq=1 ttl=50 time=188.883 ms
    64 bytes from 66.94.234.13: icmp_seq=2 ttl=51 time=192.023 ms
    ^C
    ----yahoo.com PING Statistics----
    3 packets transmitted, 3 packets received, 0.0% packet loss
    round-trip min/avg/max/stddev = 188.883/192.167/195.596/3.359 ms
    

However ping(1) will give you the round-trip of the network link -- which is the twice of delay -- so the final formula is the following: 
    
    buffer size = RTT * bandwidth
    

Fortunately, there is an automatic control for those buffers in NetBSD. It can be checked and and enabled with sysctl: 
    
    net.inet.tcp.recvbuf_auto = 0
    net.inet.tcp.recvbuf_inc = 16384
    net.inet.tcp.recvbuf_max = 262144
    net.inet.tcp.sendbuf_auto = 0
    net.inet.tcp.sendbuf_inc = 8192
    net.inet.tcp.sendbuf_max = 262144
    

The automatic setting for sendbuf and recvbuf is disabled in the default installation.  
The initial value for maximal send buffer and receive buffer is both 256k, which is very tiny. 

A reasonable value is 16 megabytes, so you may set it to that value after you turned it on with sysctl: 
    
    net.inet.tcp.recvbuf_auto=1
    net.inet.tcp.sendbuf_auto=1
    net.inet.tcp.sendbuf_max=16777216 
    net.inet.tcp.recvbuf_max=16777216
    

##  Disk I/O 

You may enable **experimental** buffer queue strategy for better responsiveness under high disk I/O load.  
This options is _likely_ to stable but not yet the default. 

Enable them with the following lines in your kernel configuration file: 
    
      options         BUFQ_READPRIO
      options         BUFQ_PRIOCSCAN
    

##  Using optimized FLAGS with GCC 

NOTE: Trying to utilise heavy optimalisations can make your system hard to debug, cause unpredictable behaviour or kill your pet. Especially use of -mtune is highly discouraged, because it does not improve performance considerably or at all compared to -march=i686, and gcc4 can't handle it correctly at least on athlon CPUs. 

You can put something like this into your mk.conf, when you compile your packages and your system. 
    
    CPUFLAGS+=-march=i686
    COPTS+=-O2
    

FIXME: This is only for building world 
    
    CFLAGS+="-O2 -march=i686"
    

FIXME: For packages 

For more detailed information about the possible CFLAG values, please read the [GNU C Compiler documentation](http://netbsd.gw.com/cgi-bin/man-cgi?gcc++NetBSD-current). 

#  References 

  * "17.4. Managing Kernel Resources". PostgreSQL 8.3beta3 Documentation. PostgreSQL Global Development Group. December 2007. Retrieved December 2, 2007. [[1]](http://www.postgresql.org/docs/8.3/static/kernel-resources.html)

  * "Performance Tuning a NetBSD Server". Eric Radman. Retrieved December 3, 2007. [[2]](http://eradman.com/article/bsdtuning1)

  * "TCP Tuning Guide". Lawrence Berkeley National Laboratory. Nov 15, 2007. Retrieved December 4, 2007. [[3]](http://www-didc.lbl.gov/TCP-tuning/)

#  See also 

  * [[Configuration examples]]



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