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
More info about table structure and delailed queries you can find on sysbench documentation page.
Tested box:
      Red Hat Enterprise Linux AS release 3 (Taroon Update 2)
      Kernel 2.4.21-15.ELsmp
      4 x Intel(R) XEON(TM) MP CPU 2.00GHz with HyperThreading
      4GB of RAM
    The results (queries/sec, more is better):
    select-primary
    ThreadsPreadMmapMmap/pread ratio
    18894.009393.731.06
    421774.8222650.341.04
    1626161.7127233.201.04
    6424330.4424847.331.02
    25623868.3324271.741.02

    image0011.gif

    select-range
    ThreadsPreadMmapMmap/pread ratio
    11555.052516.281.62
    43055.644749.041.55
    163193.564365.281.37
    643190.754301.651.35
    2563183.874225.031.33

    image004.gif

    OLTP ro
    ThreadsPreadMmapMmap/pread ratio
    1249.58272.381.09
    4608.79737.731.21
    16694.52799.641.15
    64684.06781.291.14
    256668.79767.621.15

    image005.gif

    update key
    ThreadsPreadMmapMmap/pread ratio
    15266.306586.481.25
    48443.8511446.371.36
    168080.2910974.551.36
    647654.2810317.601.35
    2566474.268577.431.32

    image003.gif

    update nokey
    ThreadsPreadMmapMmap/pread ratio
    17665.519114.751.19
    410963.0614964.021.36
    1610472.8114028.801.34
    6410013.0713161.231.31
    2568734.1110991.451.26

    image006.gif

    OLTP rw
    ThreadsPreadMmapMmap/pread ratio
    1183.48234.361.28
    4180.00243.101.35
    16177.23238.381.35
    64176.17237.311.35
    256175.86234.181.33

    image002.gif

    And finally the interesting results for select batch
    ThreadsPreadMmapMmap/pread ratio
    113.7933.532.43
    429.3699.153.38
    1620.48136.896.68
    6420.34137.986.78
    25620.69137.976.67

    image007.gif

    So in conclusion memory mapping gives benefit in different cases. The value of performance gain depends of count replaced pread/pwrite calls. For select-primary we replace only one pread call and gain is ~6%, for select-batch we replace 20.000 calls and the summary benefit is ~670%

    9 Comments
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    stanojr

    uuu, that is cool
    it will be hard to backport this feature to old versions of mysql ?

    Kevin Burton

    One thing that bothers me is that I’m not able to determine my OS cache efficiency.

    I use munin and RRD to graph as many stats about MySQL as possible. Number of slow queries, query cache efficiency, etc.

    The problem is I can’t figure out where Linux keeps stats on its file cache hit rate.

    Anyone have any idea? email me please and I’ll write a munin plugin for it 🙂

    Kevin

    Valeriy Kravchuk

    Vadim,

    Does –myisam_use_mmap=1 require any changes in ulimit -n settings? It is about MySQL bug #19800…

    Yuri

    Great, thank you! I’ve got about 10-15% performance on SELECTs with multiple simple joins
    using unindexes LIKE and REGEXP searches throught a table with 1m records.

    Michael

    As this article is several years old, would it be possible to post the 5.0 series patch if it will still apply to the current 5.0.54 (or other) release?

    Thanks,
    Michael

    Andrew

    I know this post is ancient, but I thought I’d ask anyway. Do you have a guideline for determining the value of myisam_mmap_size? The default is 18446744073709551615 (16 exabytes) which is clearly ridiculous. I currently have it set randomly to half the size of installed system RAM on a new cluster which is not yet in production. On an existing cluster, mmap was enabled before we noticed the default size, and we’re 99% certain this value caused MySQL to crash several times. No more crashes since disabling mmap. Thank you very much for any insight.