http://dev.mysql.com/doc/mysql/en/source-notes-linux.html

Note that Linux kernel and the LinuxThread library can by default only have 1,024
threads. If you plan to have more than 1,000 concurrent connections, you will need
to make some changes to LinuxThreads: 

Increase PTHREAD_THREADS_MAX in sysdeps/unix/sysv/linux/bits/local_lim.h to 4096 and
decrease STACK_SIZE in linuxthreads/internals.h to 256KB. The paths are relative to
the root of glibc. (Note that MySQL will not be stable with around 600-1000
connections if STACK_SIZE is the default of 2MB.) 

Recompile LinuxThreads to produce a new libpthread.a library, and relink MySQL
against it. 



http://software.newsforge.com/software/04/12/27/1238216.shtml?tid=15
2&tid=72&tid=29