Charsets mutex meets InnoDB

When InnoDB compares data (like, when looking up in indexes), it actually asks help from MySQL – character sets may provide different rules for evaluation. MySQL then looks up character set information. This is where the fun begins – if character set used for comparison is default server character set, or latin1 – there are shortcuts. If not (say, some smart developer decided to use Unicode without forcing DBA to set default server charset, as it isn’t needed) – a very nice internal routine is called, which at the very beginning does exactly this:

 /*
    To make things thread safe we are not
    allowing other threads to interfere
    while we may changing the cs_info_table
  */
  pthread_mutex_lock(&THR_LOCK_charset);

Apparently, ha_innodb.cc has such comment too:

/* Use the charset number to pick the right charset struct for
the comparison. Since the MySQL function get_charset may be
slow before Bar removes the mutex operation there, we first
look at 2 common charsets directly. */

if (charset_number == default_charset_info->number) {
    charset = default_charset_info;
} else if (charset_number == my_charset_latin1.number) {
    charset = &my_charset_latin1;
} else {
    charset = get_charset(charset_number, MYF(MY_WME));
    [...]

I’ll avoid going into discussions why such global lock at every row operation is harmful, but in case anyone is hitting lots of mutex contention there – just set default server character set to what your databases are in (or use binary or latin1, or add a clause for utf8 up here, or remove mutex, nobody is changing your character sets anyway ;-)

Update: Some of the discussion is at bug#42649