Memcached comprehensive analysis-2. Understanding memcached memory storage

The second part of memcached comprehensive analysis is as follows.

Posting date: 2008/7/9
Link: http://gihyo.jp/dev/feature/01/memcached/0002

Here is the link to this series of articles:

• 1st times: http://www.phpchina.com/html/29/n-35329.html
• 2nd times: http://www.phpchina.com/html/30/n-35330.html
• 3rd Times: http://www.phpchina.com/html/31/n-35331.html
• 4th times: http://www.phpchina.com/html/32/n-35332.html
• 5th times: http://www.phpchina.com/html/32/n-35333.html

• Slab Allocation mechanism: sort out the memory for Reuse
  • Slab Allocation terminology
• Principle of caching records in Slab
• Disadvantages of Slab Allocator
• Use Growth Factor FOR Tuning
• View the internal status of memcached
• View slabs usage
• Memory storage Summary

I am the precognition OF THE mixi Research and Development Group. The last article introduced that memcached is a distributed high-speed cache server. This section describes the implementation of the internal structure of memcached and the memory management mode. In addition, the vulnerabilities caused by the internal structure of memcached will also be described.

Slab Allocation mechanism: sort out the memory for Reuse

By default, the latest memcached uses the Slab Allocator mechanism to allocate and manage memory. Before the emergence of this mechanism, the memory is allocated by simply malloc and free all records. However, this method will cause memory fragmentation and increase the burden on the memory manager of the operating system. In the worst case, the operating system will be slower than the memcached process itself. Slab Allocator was born to solve this problem.

Next let's take a look at the Slab Allocator principle. Below are the objectives of slab allocator in the memcached document:

The primary goal of the slabs subsystem in memcached was to eliminate memory fragmentation issues totally by using fixed-size memory chunks coming from a few predetermined size classes.

That is to say, the basic principle of Slab Allocator is to divide the allocated memory into blocks of a specific length according to the predefined size to completely solve the memory fragmentation problem.

The principle of Slab Allocation is quite simple. Divide the allocated memory into chunk blocks of various sizes and divide the chunk blocks into groups (figure 1 ).

Figure 1 Structure of Slab Allocation

In addition, slab allocator can reuse allocated memory. That is to say, the allocated memory is not released, but reused.

Slab Allocation terminology

Page

Memory space allocated to Slab. The default value is 1 MB. After being allocated to Slab, it is split into chunks based on the size of slab.

Chunk

Memory space used to cache records.

Slab Class

A chunk group of a specific size.

Principle of caching records in Slab

The following describes how memcached selects slab for the data sent by the client and caches it to chunk.

Memcached selects the slab that best fits the data size based on the size of the received data (figure 2 ). Memcached stores the list of idle chunks in slab. Select chunks based on the list and then cache the data.

Figure 2 method for selecting a group for storing records

In fact, Slab Allocator also has advantages and disadvantages. The following describes its shortcomings.

Disadvantages of Slab Allocator

Slab Allocator solves the original memory fragmentation problem, but the new mechanism also brings new problems to memcached.

This problem occurs because the allocated memory is of a specific length, so the allocated memory cannot be effectively used. For example, if 100 bytes of data are cached to 128 bytes of chunk, the remaining 28 bytes are wasted (Figure 3 ).

Figure 3 usage of chunk Space

There is no perfect solution for this problem, but the document records a more effective solution.

The most efficient way to reduce the waste is to use a list of size classes that closely matches (if that's at all possible) common sizes of objects that the clients of this particle installation of memcached are likely to store.

That is to say, if you know the public size of the data sent by the client in advance, or only cache the data of the same size, you only need to use a list of groups suitable for the data size to reduce waste.

However, it is a pity that no optimization can be performed now. We can only look forward to future versions. However, we can adjust the differences in slab class size. The following describes the growth factor option.

Use Growth Factor FOR Tuning

Memcached specifies the Growth Factor (through the-f option) at startup to control the differences between slab to some extent. The default value is 1.25. However, before this option appeared, this factor was fixed to 2, called the "powers of 2" policy.

Let's start memcached in verbose mode with the previous settings:

$ memcached -f 2 -vv

The following figure shows the verbose output after startup:

slab class 1: chunk size 128 perslab 8192 slab class 2: chunk size 256 perslab 4096 slab class 3: chunk size 512 perslab 2048 slab class 4: chunk size 1024 perslab 1024 slab class 5: chunk size 2048 perslab 512 slab class 6: chunk size 4096 perslab 256 slab class 7: chunk size 8192 perslab 128 slab class 8: chunk size 16384 perslab 64 slab class 9: chunk size 32768 perslab 32 slab class 10: chunk size 65536 perslab 16 slab class 11: chunk size 131072 perslab 8 slab class 12: chunk size 262144 perslab 4 slab class 13: chunk size 524288 perslab 2

It can be seen that, starting from the 128-byte group, the size of the group increases to 2 times that of the original group. The problem with this setting is that the slab is quite different and sometimes it is quite a waste of memory. Therefore, to minimize memory waste, the growth factor option was added two years ago.

Let's take a look at the output of the current default settings (f = 1.25) (limited space, only 10th groups are written here ):

slab class 1: chunk size 88 perslab 11915 slab class 2: chunk size 112 perslab 9362 slab class 3: chunk size 144 perslab 7281 slab class 4: chunk size 184 perslab 5698 slab class 5: chunk size 232 perslab 4519 slab class 6: chunk size 296 perslab 3542 slab class 7: chunk size 376 perslab 2788 slab class 8: chunk size 472 perslab 2221 slab class 9: chunk size 592 perslab 1771 slab class 10: chunk size 744 perslab 1409

It can be seen that the gap between groups is much smaller than the factor of 2, and it is more suitable for caching hundreds of bytes of records. From the above output, some calculation errors may be found. These errors are intentionally set to keep the number of bytes aligned.

When you introduce memcached into a product or directly use the default value for deployment, it is best to recalculate the expected average length of data and adjust the growth factor for the most appropriate settings. Memory is a precious resource, which is a waste of resources.

Next we will introduce how to use the memcached stats command to view various information such as slabs utilization.

View the internal status of memcached

Memcached has a command named stats, which can be used to obtain various information. There are many ways to execute commands. telnet is the simplest:

$ Telnet host name port number

After connecting to memcached, enter stats and press enter to obtain information including resource utilization. In addition, enter "stats slabs" or "stats items" to obtain information about cache records. Enter quit to end the program.

For more information, see protocol.txt in the memcachedsoftware package.

$ telnet localhost 11211 Trying ::1... Connected to localhost. Escape character is '^]'. stats STAT pid 481 STAT uptime 16574 STAT time 1213687612 STAT version 1.2.5 STAT pointer_size 32 STAT rusage_user 0.102297 STAT rusage_system 0.214317 STAT curr_items 0 STAT total_items 0 STAT bytes 0 STAT curr_connections 6 STAT total_connections 8 STAT connection_structures 7 STAT cmd_get 0 STAT cmd_set 0 STAT get_hits 0 STAT get_misses 0 STAT evictions 0 STAT bytes_read 20 STAT bytes_written 465 STAT limit_maxbytes 67108864 STAT threads 4 END quit

In addition, if the libmemcached client library for C/C ++ is installed, the memstat command is installed. It is easy to use. You can use fewer steps to obtain the same information as telnet and obtain information from multiple servers at a time.

$ memstat --servers=server1,server2,server3,...

Libmemcached can be obtained from the following address:

• Http://tangent.org/552/libmemcached.html

View slabs usage

Using memcached to create a Perl script written by Brad named memcached-tool, you can easily get the usage of slab (it organizes the returned values of memcached into easy-to-read formats ). You can obtain the script from the following address:

• Http://code.sixapart.com/svn/memcached/trunk/server/scripts/memcached-tool

The usage is extremely simple:

$ Memcached-tool Host Name: port option

You do not need to specify an option to view slabs usage. Use the following command:

$ Memcached-tool Host Name: Port

The obtained information is as follows:

 # Item_Size Max_age 1MB_pages Count Full? 1 104 B 1394292 s 1215 12249628 yes 2 136 B 1456795 s 52 400919 yes 3 176 B 1339587 s 33 196567 yes 4 224 B 1360926 s 109 510221 yes 5 280 B 1570071 s 49 183452 yes 6 352 B 1592051 s 77 229197 yes 7 440 B 1517732 s 66 157183 yes 8 552 B 1460821 s 62 117697 yes 9 696 B 1521917 s 143 215308 yes 10 872 B 1695035 s 205 246162 yes 11 1.1 kB 1681650 s 233 221968 yes 12 1.3 kB 1603363 s 241 183621 yes 13 1.7 kB 1634218 s 94 57197 yes 14 2.1 kB 1695038 s 75 36488 yes 15 2.6 kB 1747075 s 65 25203 yes 16 3.3 kB 1760661 s 78 24167 yes

The meaning of each column is:

Column description # slab class no. Item_SizeChunk size Max_ageLRU oldest record survival time 1MB_pages allocated to Slab page count CountSlab Full? Whether the Slab contains idle chunk

The information obtained from this script is very convenient for tuning and is strongly recommended.

Memory storage Summary

This section briefly describes the cache mechanism and Optimization Method of memcached. I hope you can understand the memory management principles and advantages and disadvantages of memcached.

Next time, we will continue to explain the principles of LRU and Expire, as well as the latest development direction of memcached, the Extensible System (pluggable architecher )).

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