netfilter conntrack性能调整

这里我们可以调整两个参数:
-允许的最大跟踪连接条目,在这篇文档中我们叫作CONNTRACK_MAX
-存储跟踪连接条目列表的哈西表的大小,在这篇文档中我们叫做HASHSIZE(下面是这个结构的描述)

CONNTRACK_MAX是在内核内存中netfilter可以同时处理的“任务”(连接跟踪条目)。

一个跟踪连接的条目是存储在一个链接起来的列表的一个节点上,每个列表都是一个哈西表的元素。因此每个哈西表的条目(也叫一个桶-bucket)包含了一个链接起来的跟踪连接条目。
要访问一个特定包的跟踪连接条目,内核必须:
-针对一个包中的已经定义的一些字符计算哈西值。这是一个不间断的计算。
这个哈西值就会被当作哈西表的索引来使用,而跟踪连接条目的列表就存储在这里。
-反复的查看链接列表中的跟踪连接条目以找到匹配的那一个。
这是一个耗资源的操作,依赖于列表的大小(也依赖于列表中被操作的跟踪连接条目的位置)。

哈西表包含了HASHSIZE大小的链接条目。当条目满的时候(总的跟踪连接条目数达到了CONNTRACK_MAX),理想状态下,每个列表(在最优化的条件下)将包含大约CONNTRACK_MAX/HASHSIZE的条目数。

不管你是否有连接,哈西表都将占用一个固定大小的非交换内核内存。但是最大连接跟踪条目会检测最多可以存储多少条目(globally into the
linked lists),也就是说他们最多可以占用多少的内核内存。

默认并发连接数=内存大小*64     (内存大小以M为单位)
默认HASH表大小=内存大小*8      (内存大小以M为单位)

计算公式:

CONNTRACK_MAX = 内存大小 (以bytes记) / 16384 / (x / 32)

HASHSIZE = CONNTRACK_MAX / 8 = 内存大小 (以bytes记) / 131072 / (x / 32)

这里x是指针的bit数,(例如,32或者64bit)

-默认HASHSIZE的值不会小于16
-对于带有超过1G内存的系统,HASHSIZE的默认值会被限制在8192(但是可以手工设置成更大的值)

查看当前的Conntrack_max和HASHSIZE

# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max
# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_buckets

在系统有足够的内存并且性能真的很重要的时候,你可以试着使平均值是一个跟踪连接条目配一个哈西桶。

这意味着HASHSIZE = CONNTRACK_MAX。

附英文文档http://www.wallfire.org/misc/netfilter_conntrack_perf.txt

Netfilter conntrack performance tweaking, v0.8
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Herv?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:

http://www.wallfire.org/misc/netfilter_conntrack_perf.txt

——————————————————————————

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

Since Linux kernel version 2.4.23 (thus Linux 2.6 as well), 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.
Since Linux kernel version 2.4.24 (thus Linux 2.6 as well), 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

Since Linux kernel version 2.4.23 (thus Linux 2.6 as well), 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 chosen 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 (thus Linux 2.6 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.

Since 2.6.14, it is possible to set hashsize dynamically at runtime,
after boot and module load.

Between 2.6.14 and 2.6.19 (included), use:
# echo $HASHSIZE > /sys/module/ip_conntrack/parameters/hashsize

Since 2.6.20, use:
# echo $HASHSIZE > /sys/module/nf_conntrack/parameters/hashsize

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 your firewalling-only box has 512MB of RAM (a decent amount
of memory considering today’s memory prices). You have to spare a bit of
memory for a few applications (syslog, etc.): 128MB should really be big
enough for a firewall in console mode, for example.
The rest can be dedicated to conntrack entries.
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).
Since Linux 2.4.21 (thus Linux 2.6 as well), 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.

- – - – - – - – - – - – - – - – - – - – - – - – - – - – - – - – - – - – - – - -
Last changes on Jan 10, 2008

Revision history:
0.8 Make the “Ideal case: firewalling-only machine” paragraph a bit more clearer.
0.7 Hashsize parameter can be set dynamically since Linux 2.6.14.  Thanks
to Christopher A. Craig for the suggestion.
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!

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