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Re: New (by Igor): Partitioned Key Cache for MyISAM (85)


Hi all

On 14/02/2010, at 8:10 AM, worklog-noreply@xxxxxxxxxxxx wrote:

A partitioned key cache is a collection of structures for regular MyiSAM key caches called key cache partitions. Any page from a file can be placed into a buffer of only one partition. The number of the partition is calculated from the file number and the position of the page in the file, and it's always the same for the page. The function that maps pages into partitions takes care of even
distribution of pages among partitions.

Partition key cache mitigate one of the major problem of simple key cache: thread contention for key cache lock (mutex). Every call of a key cache interface function must acquire this lock. So threads compete for this lock even in the case when they have acquired shared locks for the file and pages they want read from are in the key cache buffers. When working with a partitioned key cache any key cache interface function that needs only one page has to acquire the key cache lock only for the partition the page is ascribed to. This makes the chances for threads not compete for the same key cache lock better.

The idea and the original of the partitioned key cache was provided by one of
our external contributers.

Great idea, but something it needs accessible stats to make it usable.

Right now we can already have multiple key caches, but there's no way to see the stats of them, so that makes it very difficult to use in a production environment - I prefer to not have to guess how well a cache is doing ;-)

 - FIRST figure out a way to make stats for multiple key cache visible
 - implement that for the current multi keycaches
 - then add the partitioned key cache feature


Arjen Lentz, Exec.Director @ Open Query (http://openquery.com)
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