MyISAM mmap feature (5.1)

As you know MyISAM does not cache data, only indexes. MyISAM assumes OS cache is good enough and uses pread/pwrite system calls for reading/writing datafiles. However OS is not always good in this task, my benchmarks show Linux/Solaris aren’t scalable on intensive pread calls (I believe the same for Windows, but I did not test it).
In 5.1 I implemented a new feature: memory mapping for the datafiles. That can be enabled by –myisam_use_mmap=1 startup option.
In this case instead of systems call MyISAM will use memcpy function. There is a memory addressing limit for 32bit platforms – 2Gb, so the datafiles over 2GB will be used the old way – pread/pwrite functions. Mmap is available on all POSIX-compatible platforms. It will work faster for SELECT/UPDATE/INSERT inside file queries, and no performance gain (maybe a bit slower) for INSERT at the end of file. In case with INSERT at the end of file we have to use a remap technic – resize memory mmaped area to new extended size. Currenlty we call remap once per 1000 inserts at the end of file and on an exclusive operation (DELETE/UPDATE/INSERT inside file), for work with non-mmaped area we use pread/pwrite functions.
To approve effectiveness of memory mapping several benchmarks:

For benchmarks I used the sysbench tool and tested different workloads:

1. SELECT by PRIMARY KEY (select-primary for further link)
2. SELECT by range of PRIMARY KEY (select-range)
3. OLTP ro queries, group of different SELECT queries (OLTP ro)
4. UPDATE primary key (update-key)
5. UPDATE non key column (updaye-nokey)
6. OLTP rw queries, SELECT/INSERT/DELETE/UPDATE queries (OLTP rw)
7. batch select queries – scan of 20.000 rows by primary key