Netfilter conntrack performance tweaking, v0.6

           Netfilter conntrack performance tweaking, v0.6
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

             Hervé Eychenne 

This document explains some of the things you need to know for netfilter
conntrack (and thus NAT) performance tuning.

Latest version of this document can be found at:

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There are two parameters we can play with:
- the maximum number of allowed conntrack entries, which will be called
  CONNTRACK_MAX in this document
- the size of the hash table storing the lists of conntrack entries, which
  will be called HASHSIZE (see below for a description of the structure)

CONNTRACK_MAX is the maximum number of "sessions" (connection tracking entries)
that can be handled simultaneously by netfilter in kernel memory.

A conntrack entry is stored in a node of a linked list, and there are several
lists, each list being an element in a hash table.  So each hash table entry
(also called a bucket) contains a linked list of conntrack entries.
To access a conntrack entry corresponding to a packet, the kernel has to:
- compute a hash value according to some defined characteristics of the packet.
  This is a constant time operation.
  This hash value will then be used as an index in the hash table, where a
  list of conntrack entries is stored.
- iterate over the linked list of conntrack entries to find the good one.
  This is a more costly operation, depending on the size of the list (and on
  the position of the wanted conntrack entry in the list).

The hash table contains HASHSIZE linked lists.  When the limit is reached
(the total number of conntrack entries being stored has reached CONNTRACK_MAX),
each list will contain ideally (in the optimal case) about
CONNTRACK_MAX/HASHSIZE entries.

The hash table occupies a fixed amount of non-swappable kernel memory,
whether you have any connections or not.  But the maximum number of conntrack
entries determines how many conntrack entries can be stored (globally into the
linked lists), i.e. how much kernel memory they will be able to occupy at most.


This document will now give you hints about how to choose optimal values for
HASHSIZE and CONNTRACK_MAX, in order to get the best out of the netfilter
conntracking/NAT system.

Default values of CONNTRACK_MAX and HASHSIZE
============================================

By default, both CONNTRACK_MAX and HASHSIZE get average values for
"reasonable" use, computed automatically according to the amount of
available RAM.

Default value of CONNTRACK_MAX
------------------------------

On i386 architecture, CONNTRACK_MAX = RAMSIZE (in bytes) / 16384 =
RAMSIZE (in MegaBytes) * 64.
So for example, a 32 bits PC with 512MB of RAM can handle 512*1024^2/16384 =
512*64 = 32768 simultaneous netfilter connections by default.

But the real formula is:
CONNTRACK_MAX = RAMSIZE (in bytes) / 16384 / (x / 32)
where x is the number of bits in a pointer (for example, 32 or 64 bits)

Please note that:
- default CONNTRACK_MAX value will not be inferior to 128
- for systems with more than 1GB of RAM, default CONNTRACK_MAX value is
  limited to 65536 (but can of course be set to more manually).

Default value of HASHSIZE
-------------------------

By default, CONNTRACK_MAX = HASHSIZE * 8.  This means that there is an average
of 8 conntrack entries per linked list (in the optimal case, and when
CONNTRACK_MAX is reached), each linked list being a hash table entry
(a bucket).

On i386 architecture, HASHSIZE = CONNTRACK_MAX / 8 =
RAMSIZE (in bytes) / 131072 = RAMSIZE (in MegaBytes) * 8.
So for example, a 32 bits PC with 512MB of RAM can store 512*1024^2/128/1024 =
512*8 = 4096 buckets (linked lists)

But the real formula is:
HASHSIZE = CONNTRACK_MAX / 8 = RAMSIZE (in bytes) / 131072 / (x / 32)
where x is the number of bits in a pointer (for example, 32 or 64 bits)

Please note that:
- default HASHSIZE value will not be inferior to 16
- for systems with more than 1GB of RAM, default HASHSIZE value is limited
  to 8192 (but can of course be set to more manually).

Reading CONNTRACK_MAX and HASHSIZE
==================================

Current CONNTRACK_MAX value can be read at runtime, via the /proc filesystem.

Before Linux kernel version 2.4.23, use:
# cat /proc/sys/net/ipv4/ip_conntrack_max

As of Linux kernel version 2.4.23, use:
# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max
  (old /proc/sys/net/ipv4/ip_conntrack_max is then deprecated!)


Current HASHSIZE is always available (for every kernel version) in syslog
messages, as the number of buckets (which is HASHSIZE) is printed there at
ip_conntrack initialization.
As of Linux kernel version 2.4.24, current HASHSIZE value can be read at
runtime with:
# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_buckets


Modifying CONNTRACK_MAX and HASHSIZE
====================================

Default CONNTRACK_MAX and HASHSIZE values are reasonable for a typical host,
but you may increase them on high-loaded firewalling-only systems.
So CONNTRACK_MAX and HASHSIZE values can be changed manually if needed.

While accessing a bucket is a constant time operation (hence the interest
of having a hash of lists), keep in mind that the kernel has to iterate over
a linked list to find a conntrack entry.  So the average size of a linked
list (CONNTRACK_MAX/HASHSIZE in the optimal case when the limit is reached)
must not be too big.  This ratio is set to 8 by default (when values are
computed automatically).
On systems with enough memory and where performance really matters, you can
consider trying to get an average of one conntrack entry per hash bucket,
which means HASHSIZE = CONNTRACK_MAX.

Setting CONNTRACK_MAX
---------------------

Conntrack entries are stored in linked lists, so the maximum number of
conntrack entries (CONNTRACK_MAX) can be easily configured dynamically.

Before Linux kernel version 2.4.23, use:
# echo $CONNTRACK_MAX > /proc/sys/net/ipv4/ip_conntrack_max

As of Linux kernel version 2.4.23, use:
# echo $CONNTRACK_MAX > /proc/sys/net/ipv4/netfilter/ip_conntrack_max

where $CONNTRACK_MAX is an integer.

Setting HASHSIZE
----------------

For mathematical reasons, hash tables have static sizes.  So HASHSIZE must be
determined before the hash table is created and begins to be filled.

Before Linux kernel version 2.4.21, a prime number should be choosed for hash
size, ensuring that the hash table will be efficiently populated. Odd
non-prime numbers or even numbers are strongly discouraged, as the hash
distribution will be sub-optimal.

Since Linux kernel version 2.4.21 (and for 2.6 kernel as well), conntrack
uses jenkins2b hash algorithm which is happy with all sizes, but power
of 2 works best.


If netfilter conntrack is statically compiled in the kernel, the hash table
size can be set at compile time, or (since kernel 2.6) as a boot option with
ip_conntrack.hashsize=$HASHSIZE

If netfilter conntrack is compiled as a module, the hash table size can be
set at module insertion, with the following command:
# modprobe ip_conntrack hashsize=$HASHSIZE

where $HASHSIZE is an integer.


Ideal case: firewalling-only machine
------------------------------------

In the ideal case, you have a machine _just_ doing packet filtering and NAT
(i.e. almost no userspace running, at least none that would have a growing
memory consumption like proxies, ...).

The size of kernel memory used by netfilter connection tracking is:
size_of_mem_used_by_conntrack (in bytes) =
        CONNTRACK_MAX * sizeof(struct ip_conntrack) +
        HASHSIZE * sizeof(struct list_head)
where:
- sizeof(struct ip_conntrack) can vary quite much, depending on architecture,
  kernel version and compile-time configuration. To know its size, see the
  kernel log message at ip_conntrack initialization time.
  sizeof(struct ip_conntrack) is around 300 bytes on i386 for 2.6.5, but
  heavy development around 2.6.10 make it vary between 352 and 192 bytes!
- sizeof(struct list_head) = 2 * size_of_a_pointer
  On i386, size_of_a_pointer is 4 bytes.

So, on i386, kernel 2.6.5, size_of_mem_used_by_conntrack is around
CONNTRACK_MAX * 300 + HASHSIZE * 8 (bytes).

If we take HASHSIZE = CONNTRACK_MAX (if we have most of the memory dedicated
to firewalling, see "Modifying CONNTRACK_MAX and HASHSIZE" section above),
size_of_mem_used_by_conntrack would be around CONNTRACK_MAX * 308 bytes
on i386 systems, kernel 2.6.5.

Now suppose you put 512MB of RAM (a decent amount of memory considering today's
memory prices) into the firewalling-only box, and use all but 128MB for
conntrack, which should really be big enough for a firewall in console mode,
for example.
Then you could set both CONNTRACK_MAX and HASHSIZE approximately to:
(512 - 128) * 1024^2 / 308 =~ 1307315 (instead of 32768 for CONNTRACK_MAX,
and 4096 for HASHSIZE by default).
As of Linux 2.4.21 (and Linux 2.6), hash algorithm is happy with
"power of 2" sizes (it used to be a prime number before).

So here we can set CONNTRACK_MAX and HASHSIZE to 1048576 (2^20), for example.

This way, you can store about 32 times more conntrack entries than the
default, and get better performance for conntrack entry access.


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Last changes on Jan 20, 2005

Revision history:
0.6 Hashsize parameter can be set at boot time with Linux 2.6.  Thanks to
    Tobias Diedrich for pointing this out.
0.5 Added further notice about the varying length of the conntrack structure.
0.4 Since Linux 2.4.21, hash algorithm is happy with all sizes, not only
    prime ones.  However, power of 2 is best.
0.3 Various small precisions.
0.2 Information about Linux kernel versions and corresponding /proc entries.
    (/proc/sys/net/ipv4/netfilter/ip_conntrack_{max,buckets}).
0.1 Initial writing, largely based on my discussions with Harald Welte
    (netfilter maintainer) on the netfilter-devel mailing-list.  Many thanks
    to him!