Memcached 1.4.11 Released

Release notes from official site :

Overview

This release fixes race conditions and crashes introduced in 1.4.10, they should be rare, but users are strongly encouraged to upgrade.
Adds the ability to rebalance and reassign slab memory.

Fixes

  • Don’t compute incorrect argc for timedrun
  • Fix ‘age’ stat for stats items
  • Binary deletes were not ticking stats counters
  • Fix a race condition from 1.4.10 on item_remove
  • Close some idiotic race conditions
  • Initial slab automover
  • Slab reassignment
  • Clean do_item_get logic a bit. fix race.
  • Clean up the do_item_alloc logic
  • Shorten lock for item allocation more
  • Fix to build with cyrus sasl 2.1.25

New Features

Slab Reassign

Long running instances of memcached may run into an issue where all available memory has been assigned to a specific slab class (say items of roughly size 100 bytes). Later the application starts storing more of its data into a different slab class (items around 200 bytes). Memcached could not use the 100 byte chunks to satisfy the 200 byte requests, and thus you would be able to store very few 200 byte items.

1.4.11 introduces the ability to reassign slab pages. This is a beta feature and the commands may change for the next few releases, so please keep this in mind. When the commands are finalized they will be noted in the release notes.

Enable slab reassign on startup:

Once all memory has been assigned and used by items, you may use a command to reassign memory.

That will return an error code indicating success, or a need to retry later. Success does not mean that the slab was moved, but that a background thread will attempt to move the memory as quickly as it can.

Slab Automove

While slab reassign is a manual feature, there is also the start of an automatic memory reassignment algorithm.

The above enables it on startup, and it may also be enabled or disabled at runtime:

The algorithm is slow and conservative. 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.

There are lots of cases where this will not be sufficient, and we invite the community to help improve upon the algorithm. Included in the source directory is scripts/mc_slab_mover. See perldoc for more information:

It implements the same algorithm as built into memcached, and you may modify it to better suit your needs and improve on the script or port it to other languages. Please provide patches!

Slab Reassign Implementation

Slab page reassignment requires some tradeoffs:
All items larger than 500k (even if they’re under 730k) take 1MB of space
When memory is reassigned, all items that were in the 1MB page are evicted
When slab reassign is enabled, an extra background thread is used
The first item will be improved in later releases, and is avoided if you start memcached without the -o slab_reassign option.

New Stats

slab_reassign_running indicates if the slab thread is attempting to move a page. It may need to wait for some memory to free up, so it could take several seconds.

slabs_moved is simply a count of how many pages have been successfully moved.

You can download it from : http://memcached.googlecode.com/files/memcached-1.4.11.tar.gz

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