memcached comprehensive analysis of –2. Understanding the memory storage of memcached

Slab allocation mechanism: Defragment memory for reuse

The most recent memcached by default uses a mechanism called slab allocator to allocate and manage memory. Prior to the advent of this mechanism, the allocation of memory was performed simply by malloc and free for all records. However, this approach can lead to memory fragmentation, aggravating the burden on the operating system memory manager, and in the worst case, cause the operating system to be slower than the memcached process itself. Slab Allocator was born to solve the problem.

Let's look at the principle of slab allocator. The following is the goal of slab allocator in the memcached documentation:

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

In other words, the basic principle of the Slab allocator is to divide the allocated memory into blocks of a specific length in a predetermined size to completely resolve the memory fragmentation problem.

The principle of Slab allocation is quite simple. Divide the allocated memory into blocks of various sizes (chunk) and divide the same size blocks into groups (the Chunk collection) (Figure 1).

Fig. 1 structure diagram of Slab allocation

Also, slab allocator has the purpose of reusing allocated memory. In other words, the allocated memory is not freed, but reused.

Main terms of Slab allocation

Page

The memory space allocated to slab, which is 1MB by default. After assigning to slab, the slab is divided into chunk according to the size of the.

Chunk

The memory space used to cache records.

Slab Class

A group of chunk of a specific size.

The principle of caching records in slab

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

Memcached based on the size of the data received, select the slab that best fits the data size (Figure 2). Memcached holds a list of idle chunk within slab, selects chunk based on the list, and caches the data in it.

Figure 2 How to select a group to store records

In fact, Slab allocator also has advantages and disadvantages. Here's a look at its drawbacks.

Disadvantages of Slab Allocator

Slab allocator solved the original memory fragmentation problem, but the new mechanism also brought new problems to memcached.

The problem is that the allocated memory cannot be effectively exploited because it allocates memory of a specific length. For example, by caching 100 bytes of data into a 128-byte chunk, the remaining 28 bytes are wasted (Figure 3).

Figure 3 Use of chunk space

There is no perfect solution for this problem, but the more effective solution is documented in the documentation.

The most efficient-on-the-waste-to-use-a list of size classes that closely matches (if that's at all Possibl e) Common sizes of objects, the clients of this particular installation of memcached is likely to store.

That is, if you know in advance the common size of the data sent by the client, or if you only cache data of the same size, you can reduce waste if you use a list of groups that fit the data size.

Unfortunately, it is not possible to perform any tuning at this time, only to look forward to future versions. However, we can adjust the difference in the size of the slab class. Next, the growth factor option is described.

Tuning with growth factor

memcached specifies the growth factor factor at startup (with the-f option) to control the difference between slab to some extent. The default value is 1.25. However, before this option occurs, this factor was once fixed to 2, called the "Powers of 2" policy.

Let's try using the previous settings to start memcached in verbose mode:

$ memcached-f 2-VV

The following is the verbose output after startup:

Slab class   1:chunk size    perslab  8192
Slab class   2:chunk size    perslab  4096
Slab class   3:chunk size    perslab  2048
Slab class   4:chunk size   1024x768 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

As you can see, starting with a 128-byte group, the size of the group increases to twice times the original. The problem with this setting is that the difference between slab is large, and in some cases it is quite a waste of memory. Therefore, to minimize memory waste, two years ago this option was appended with growth factor.

Take a look at the current default settings (f=1.25) when the output (space limit, this is only written to the 10th group):

Slab class   1:chunk size     Perslab 11915
Slab class   2:chunk size    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

As can be seen, the gap between groups is much smaller than the factor of 2 o'clock, which is more suitable for caching hundreds of-byte records. From the above output, you may find some calculation errors, which are deliberately set to keep the number of bytes aligned.

When introducing memcached into a product or deploying it directly using default values, it is best to recalculate the expected average length of the data and adjust the growth factor to get the most appropriate settings. Memory is a precious resource, and a waste of it is too bad.

Here's how to use memcached's stats command to see a wide variety of information such as slabs utilization.

View the internal state of the memcached

Memcached has a command called stats that can be used to obtain a wide variety of information. There are many ways to execute commands, with Telnet the simplest:

$ telnet Host name Port number

After connecting to memcached, enter stats and press ENTER to get a variety of information including resource utilization. Also, enter "stats slabs" or "stats items" to get information about the cache record. To end the program, enter quit.

The details of these commands can refer to the Protocol.txt documentation within the memcached 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

Additionally, if you install libmemcached, the client library for the C + + language, the MemStat command will be installed. Using the method is simple, you can get the same information as Telnet with fewer steps, and you can get information from multiple servers at once.

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

Libmemcached can be obtained from the following address:

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

View the usage status of slabs

Using memcached, a Perl script named Memcached-tool, written by Brad, makes it easy to get slab usage (IT organizes memcached return values into easy-to-read formats). You can get the script from the following address:

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

The method of use is also extremely simple:

$ memcached-tool Host Name: Port option

You do not need to specify options when viewing slabs usage, so you can use the following command:

$ memcached-tool Host Name: Port

The information obtained is as follows:

#  item_size   max_age  1mb_pages Count full   ?
1     104 B  1394292 s    1215 12249628    Yes
2     136 B  1456795 s  400919     Yes
3     176 B  1339587 s  196567     Yes
4     224 B  1360926 s     109  510221     Yes
5     280 B  1570071 s  183452     Yes
6     352 B  1592051 s  229197     Yes
7  1517732 s  157183     Yes
8     552 B  1460821 s  117697     Yes
9     696 B  1521917 s     143  215308     Yes
Ten     872 B  1695035 s     205  246162     Yes
     1.1 KB 1681650 s     233  221968     Yes
     1.3 KB 1603363 s     241  183621     Yes
     1.7 KB 1634218 s      94   57197     Yes
     2.1 KB 1695038 s   36488     Yes
     2.6 KB 1747075 s   25203     Yes
     3.3 KB 1760661 s   24167     Yes

The meanings of each column are:

Column	Meaning
#	Slab class Number
Item_size	Chunk size
Max_age	Lifetime of oldest record in LRU
1mb_pages	Number of pages assigned to slab
Count	Number of records in slab
Full?	Whether the slab contains idle chunk

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

Summary of memory storage

This paper simply explains the caching mechanism and tuning method of memcached. I hope readers can understand the principle of memcached memory management and its advantages and disadvantages.

Next time, we will continue to explain the principles of LRU and expire, as well as the latest development direction of memcached-expandable system (pluggable Architecher).

Memcached Comprehensive Anatomy of –2. Understanding memcached Memory Storage