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G Comments on porting to other systems

A working Posix thread library is needed for the server. On Solaris 2.5 we use SUN PThreads (the native thread support in 2.4 and earlier versions are not good enough) and on Linux we use LinuxThreads by Xavier Leroy, Xavier.Leroy@inria.fr.

The hard part of porting to a new Unix variant without good native thread support is probably to port MIT-pthreads. See `mit-pthreads/README' and Programming POSIX Threads.

The MySQL distribution includes a patched version of Provenzano's Pthreads from MIT (see MIT Pthreads web page). This can be used for some operating systems that do not have POSIX threads.

It is also possible to use another user level thread package named FSU Pthreads (see FSU Pthreads home page). This implementation is being used for the SCO port.

See the `thr_lock.c' and `thr_alarm.c' programs in the `mysys' directory for some tests/examples of these problems.

Both the server and the client need a working C++ compiler (we use gcc and have tried SparcWorks). Another compiler that is known to work is the Irix cc.

To compile only the client use ./configure --without-server.

There is currently no support for only compiling the server. Nor is it likly to be added unless someone has a good reason for it.

If you want/need to change any `Makefile' or the configure script you must get Automake and Autoconf. We have used the automake-1.2 and autoconf-2.12 distributions.

All steps needed to remake everything from the most basic files. 

/bin/rm */.deps/*.P
/bin/rm -f config.cache
aclocal
autoheader
aclocal
automake
autoconf
./configure --with-debug --prefix='your installation directory'

# The makefiles generated above need GNU make 3.75 or newer.
# (called gmake below)
gmake clean all install init-db

If you run into problems with a new port, you may have to do some debugging of MySQL! See section G.1 Debugging a MySQL server.

Note: Before you start debugging mysqld, first get the test programs mysys/thr_alarm and mysys/thr_lock to work. This will ensure that your thread installation has even a remote chance to work!

G.1 Debugging a MySQL server

If you are using some functionality that is very new in MySQL, you can try to run mysqld with the --skip-new (which will disable all new, potentially unsafe functionality) or with --safe-mode which disables a lot of optimization that may cause problems. See section 18.1 What to do if MySQL keeps crashing.

If mysqld doesn't want to start, you should check that you don't have any my.cnf file that interferes with your setup! You can check your my.cnf arguments with mysqld --print-defaults and avoid using them by starting with mysqld --no-defaults ....

If you have some very specific problem, you can always try to debug MySQL. To do this you must configure MySQL with the option --with-debug. You can check whether or not MySQL was compiled with debugging by doing: mysqld --help. If the --debug flag is listed with the options then you have debugging enabled. mysqladmin ver also lists the mysqld version as mysql ... -debug in this case.

If you are using gcc or egcs, the recommended configure line is: 

CC=gcc CFLAGS="-O6" CXX=gcc CXXFLAGS="-O6 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-debug

This will avoid problems with the libstdc++ library and with C++ exceptions (many compilers has problems with C++ exceptions in threaded code).

If mysqld stops crashing when you compile it with --with-debug, you have probably found a compiler bug or a timing bug within MySQL. In this case you can try to add -g to the CFLAGS and CXXFLAGS variables above and not use --with-debug. If mysqld now dies, you can at least attach to it with gdb or use gdb on the core file to find out what happened.

If you can cause the mysqld server to crash quickly, you can try to create a trace file of this:

Start the mysqld server with a trace log in `/tmp/mysql.trace'. The log file will get very BIG.

mysqld --debug --log

or you can start it with

mysqld --debug=d,info,error,query,general,where:O,/tmp/mysql.trace

which only prints information with the most interesting tags.

When you configure MySQL for debugging you automatically enable a lot of extra safety check functions that monitor the health of mysqld. If they find something ``unexpected,'' an entry will be written to stderr, which safe_mysqld directs to the error log! This also means that if you are having some unexpected problems with MySQL and are using a source distribution, the first thing you should do is to configure MySQL for debugging! (The second thing, of course, is to send mail to mysql@lists.mysql.com and ask for help. Please use the mysqlbug script for all bug reports or questions regarding the MySQL version you are using!

On most system you can also start mysqld from gdb to get more information if mysqld crashes.

With some 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.

If you are using gdb 4.17.x 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

Here follows an example how to debug mysqld: 

shell> gdb /usr/local/libexec/mysqld
gdb> run
...
back   # Do this when mysqld crashes
info locals
up
info locals
up
...
(until you get some information about local variables)

quit

Include the above output in a mail generated with mysqlbug and mail this to mysql@lists.mysql.com.

If mysqld hangs you can try to use some system tools like strace or /usr/proc/bin/pstack to examine where mysqld has hanged.

If mysqld starts to eat up CPU or memory or if it ``hangs'', you can use mysqladmin processlist status to find out if someone is executing some query that takes a long time. It may be a good idea to run mysqladmin -i10 processlist status in some window if you are experiencing performance problems or problems when new clients can't connect.

If mysqld dies or hangs, you should start mysqld with --log. When mysqld dies again, you can check in the log file for the query that killed mysqld. Note that before starting mysqld with --log you should check all your tables with myisamchk. See section 13 Maintaining a MySQL installation.

If you are using a log file, mysqld --log, you should check the 'hostname' log files, that you can find in the database directory, for any queries that could cause a problem. Try the command EXPLAIN on all SELECT statements that takes a long time to ensure that mysqld are using indexes properly. See section 7.22 EXPLAIN syntax (Get information about a SELECT). You should also test complicated queries that didn't complete within the mysql command line tool.

If you find the text mysqld restarted in the error log file (normally named `hostname.err') you have probably found a query that causes mysqld to fail. If this happens you should check all your tables with myisamchk (see section 13 Maintaining a MySQL installation), and test the queries in the MySQL log files if someone doesn't work. If you find such a query, try first upgrading to the newest MySQL version. If this doesn't help and you can't find anything in the mysql mail archive, you should report the bug to mysql@lists.mysql.com. Links to mail archives are available at the online MySQL documentation page.

If you get corrupted tables or if mysqld always fails after some update commands, you can test if this bug is reproducible by doing the following:

The command mysqladmin debug will dump some information about locks in use, used memory and query usage to the mysql log file. This may help solve some problems. This command also provides some useful information even if you haven't compiled MySQL for debugging!

If the problem is that some tables are getting slower and slower you should try to optimize the table with OPTIMIZE TABLE or myisamchk. See section 13 Maintaining a MySQL installation. You should also check the slow queries with EXPLAIN.

You should also read the OS-specific section in this manual for problems that may be unique to your environment. See section 4.11 System-specific issues.

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. See section 20.5.2 The DBI interface.

G.2 Debugging a MySQL client

To be able to debug a MySQL client with the integrated debug package, you should configure MySQL with --with-debug. See section 4.7.3 Typical configure options.

Before running a client, you should set the MYSQL_DEBUG environment variable: 

shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
shell> export MYSQL_DEBUG

This causes clients to generate a trace file in `/tmp/client.trace'.

If you have problems with your own client code, you should attempt to connect to the server and run your query using a client that is known to work. Do this by running mysql in debugging mode (assuming you have compiled MySQL with debugging on): 

shell> mysql --debug=d:t:O,/tmp/client.trace

This will provide useful information in case you mail a bug report. See section 2.3 How to report bugs or problems.

If your client crashes at some 'legal' looking code, you should check that your `mysql.h' include file matches your mysql library file. A very common mistake is to use an old `mysql.h' file from an old MySQL installation with new MySQL library.

G.3 Comments about RTS threads

I have tried to use the RTS thread packages with MySQL but stumbled on the following problems:

They use old version of a lot of POSIX calls and it is very tedious to make wrappers for all functions. I am inclined to think that it would be easier to change the thread libraries to the newest POSIX specification.

Some wrappers are already written. See `mysys/my_pthread.c' for more info.

At least the following should be changed:

pthread_get_specific should use one argument. sigwait should take two arguments. A lot of functions (at least pthread_cond_wait, pthread_cond_timedwait) should return the error code on error. Now they return -1 and set errno.

Another problem is that user-level threads use the ALRM signal and this aborts a lot of functions (read, write, open...). MySQL should do a retry on interrupt on all of these but it is not that easy to verify it.

The biggest unsolved problem is the following:

To get thread-level alarms I changed `mysys/thr_alarm.c' to wait between alarms with pthread_cond_timedwait(), but this aborts with error EINTR. I tried to debug the thread library as to why this happens, but couldn't find any easy solution.

If someone wants to try MySQL with RTS threads I suggest the following:

G.4 Differences between different thread packages

MySQL is very dependent on the thread package used. So when choosing a good platform for MySQL, the thread package is very important.

There are at least three types of thread packages:

In some systems kernel threads are managed by integrating user level threads in the system libraries. In such cases, the thread switching can only be done by the thread library and the kernel isn't really ``thread aware''.

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