E.1.3. Debugging mysqld under gdb
On most systems you can also start mysqld
from gdb to get more information if
mysqld crashes.
With some older gdb versions on Linux you
must use run --one-thread if you want to be
able to debug mysqld threads. In this case,
you can only have one thread active at a time. We recommend you
to upgrade to gdb 5.1 ASAP as thread debugging works much better
with this version!
NTPL threads (the new thread library on Linux) may cause
problems while running mysqld under
gdb. Some symptoms are:
In this case, you should set the following environment variable
in the shell before starting gdb:
LD_ASSUME_KERNEL=2.4.1
export LD_ASSUME_KERNEL
When running mysqld under
gdb, you should disable the stack trace with
--skip-stack-trace to be able to catch
segfaults within gdb.
In MySQL 4.0.14 and above you should use the
--gdb option to mysqld. This installs an
interrupt handler for SIGINT (needed to stop
mysqld with ^C to set
breakpoints) and disable stack tracing and core file handling.
It's very hard to debug MySQL under gdb if
you do a lot of new connections the whole time as
gdb doesn't free the memory for old threads.
You can avoid this problem by starting mysqld
with --thread_cache_size='max_connections+1'.
In most cases just using --thread_cache_size=5'
helps a lot!
If you want to get a core dump on Linux if
mysqld dies with a SIGSEGV signal, you can
start mysqld with the
--core-file option. This core file can be used
to make a backtrace that may help you find out why
mysqld died:
shell> gdb mysqld core
gdb> backtrace full
gdb> exit
See Section A.4.2, “What to Do If MySQL Keeps Crashing”.
If you are using gdb 4.17.x or above on
Linux, you should install a .gdb file, with
the following information, in your current directory:
set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint
If you have problems debugging threads with
gdb, you should download gdb 5.x and try this
instead. The new gdb version has very
improved thread handling!
Here is an example how to debug mysqld:
shell> gdb /usr/local/libexec/mysqld
gdb> run
...
backtrace full # Do this when mysqld crashes
Include the above output in a bug report, which you can file
using the instructions in Section 1.8, “How to Report Bugs or Problems”.
If mysqld hangs you can try to use some
system tools like strace or
/usr/proc/bin/pstack to examine where
mysqld has hung.
strace /tmp/log libexec/mysqld
If you are using the Perl DBI interface, you
can turn on debugging information by using the
trace method or by setting the
DBI_TRACE environment variable.
