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E.4. Comments about RTS Threads

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

They use old versions of many 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 currently 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:

  • Change functions MySQL uses from the thread library to POSIX. This shouldn't take that long.

  • Compile all libraries with the -DHAVE_rts_threads.

  • Compile thr_alarm.

  • If there are some small differences in the implementation, they may be fixed by changing my_pthread.h and my_pthread.c.

  • Run thr_alarm. If it runs without any “warning,” “error,” or aborted messages, you are on the right track. Here is a successful run on Solaris:

    Main thread: 1
    Thread 0 (5) started
    Thread: 5  Waiting
    process_alarm
    Thread 1 (6) started
    Thread: 6  Waiting
    process_alarm
    process_alarm
    thread_alarm
    Thread: 6  Slept for 1 (1) sec
    Thread: 6  Waiting
    process_alarm
    process_alarm
    thread_alarm
    Thread: 6  Slept for 2 (2) sec
    Thread: 6  Simulation of no alarm needed
    Thread: 6  Slept for 0 (3) sec
    Thread: 6  Waiting
    process_alarm
    process_alarm
    thread_alarm
    Thread: 6  Slept for 4 (4) sec
    Thread: 6  Waiting
    process_alarm
    thread_alarm
    Thread: 5  Slept for 10 (10) sec
    Thread: 5  Waiting
    process_alarm
    process_alarm
    thread_alarm
    Thread: 6  Slept for 5 (5) sec
    Thread: 6  Waiting
    process_alarm
    process_alarm
    
    ...
    thread_alarm
    Thread: 5  Slept for 0 (1) sec
    end
    

 
 
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