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                              WORKLOG TASK
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TASK...........: Partitioned Key Cache for MyISAM
CREATION DATE..: Sun, 14 Feb 2010, 00:10
SUPERVISOR.....: Monty
IMPLEMENTOR....: 
COPIES TO......: Igor, Monty, Sergei
CATEGORY.......: Server-BackLog
TASK ID........: 85 (http://askmonty.org/worklog/?tid=85)
VERSION........: Benchmarks-3.0
STATUS.........: Assigned
PRIORITY.......: 60
WORKED HOURS...: 0
ESTIMATE.......: 80 (hours remain)
ORIG. ESTIMATE.: 80

PROGRESS NOTES:



DESCRIPTION:

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.


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