The following was done on Ubuntu 14.04.1.
sudo apt-get install memcachedand it seemed to start automatically,
$ ps -ax | grep memcached
11760 ? Sl 0:00 /usr/bin/memcached -m 64 -p 11211 -u memcache -l 127.0.0.1memcached can be controlled via,
sudo service memcached start|stop|status
and the configuration can be found in /etc/memcached.conf.
Commands can be sent to memcached from the command line using netcat,
$ echo "version" | nc 127.0.0.1 11211
VERSION 1.4.14 (Ubuntu)- Storage is broken up into 1 megabyte pages.
- A page is assigned a slab class, which denotes that that page stores items a particular size.
- These size ranges are determined by a configurable growth factor.
$ memcached -vv
slab class 1: chunk size 96 perslab 10922
slab class 2: chunk size 120 perslab 8738
slab class 3: chunk size 152 perslab 6898
slab class 4: chunk size 192 perslab 5461
slab class 5: chunk size 240 perslab 4369
slab class 6: chunk size 304 perslab 3449
slab class 7: chunk size 384 perslab 2730
slab class 8: chunk size 480 perslab 2184
slab class 9: chunk size 600 perslab 1747
...
Thus, if I wanted to store a 140 byte item, memcached would have to make sure that at least one page was assigned to slab class 3.
The allocation of pages to slab classes happens dynamically as memcached receieves set commands for items of a given size.
e.g. If I had allocated 9MB to memcached and sent set commands for:
- 9000 items of 105 bytes
- 4300 items of 500 bytes the allocation of pages would look like the following:
+---+---+---+
| 2 | 2 | 9 |
+---+---+---+
| 9 | 9 | |
+---+---+---+
| | | |
+---+---+---+
The default behaviour is that slabs aren't reassigned. If the cache fills up with pages predominantly allocated to a few slab classes and then the size of items being stored changes (thus requiring different slab classes), you could end up with only a few pages actually being used for the items you now want to cache. This can be overcome by switching on the automove feature, which is described in the next section.
To allow reassignment of slabs (a feature introduced in v1.4.11
I added the following line to /etc/memcached.conf and restarted the
memcached server (slab reassignment can only be enabled at start time).
-o slab_reassign
Automove can be enabled by specifying an option on startup (slab_automove)
or the following command,
echo "slabs automove 1" | nc localhost 11211A demonstration of automove can be seen by running the automove_example.py
file in this repo (output shown below),
$ python automove_example.py
Disabled autmove
{'32': {'total_pages': '6', 'cmd_set': '120000', 'evicted': '119940'}, '40': {'total_pages': '59', 'cmd_set': '300', 'evicted': '236'}}
Fill cache with 500KB items
{'32': {'total_pages': '6', 'cmd_set': '120000', 'evicted': '119940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
Start adding 100KB items
{'32': {'total_pages': '6', 'cmd_set': '122000', 'evicted': '121940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '124000', 'evicted': '123940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '126000', 'evicted': '125940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '128000', 'evicted': '127940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '130000', 'evicted': '129940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '132000', 'evicted': '131940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '134000', 'evicted': '133940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '136000', 'evicted': '135940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '138000', 'evicted': '137940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '140000', 'evicted': '139940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
Enabled automove
{'32': {'total_pages': '6', 'cmd_set': '142000', 'evicted': '141940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '144000', 'evicted': '143940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '146000', 'evicted': '145940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '148000', 'evicted': '147940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '150000', 'evicted': '149940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '152000', 'evicted': '151940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '6', 'cmd_set': '154000', 'evicted': '153940'}, '40': {'total_pages': '59', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '7', 'cmd_set': '156000', 'evicted': '155930'}, '40': {'total_pages': '58', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '7', 'cmd_set': '158000', 'evicted': '157930'}, '40': {'total_pages': '58', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '160000', 'evicted': '159920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
Disabled autmove
{'32': {'total_pages': '8', 'cmd_set': '162000', 'evicted': '161920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '164000', 'evicted': '163920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '166000', 'evicted': '165920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '168000', 'evicted': '167920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '170000', 'evicted': '169920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '172000', 'evicted': '171920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '174000', 'evicted': '173920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '176000', 'evicted': '175920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '178000', 'evicted': '177920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}
{'32': {'total_pages': '8', 'cmd_set': '180000', 'evicted': '179920'}, '40': {'total_pages': '57', 'cmd_set': '450', 'evicted': '386'}}As can be seen above, enabling automove results in pages being reallocated from slab 40 to slab 32. The algorithm for automove is
"If a slab class is seen as having the highest eviction count 3 times 10 seconds apart, it will take a page from a slab class which has had zero evictions in the last 30 seconds and move the memory."
(see v1.4.11 release notes).
When multiple instances of memcached are used the client decides which node a a key resides via an algorithm like,
node_index = hash(key) % number_of_nodes
Adding nodes is difficult as adding a node would result in many items being relocated thus resulting in many misses for a period after adding the new node.