huge number of context switches under 2.2.x with SMP & threaded apps

[bert hubert <ahu@ds9a.nl>]

A threads related question - I have a nameserver with 8 active threads,
which in turn leads to 6 (in this case) MySQL connections. When
stresstesting this nameserver, we see a *huge* number of context switches.
50.000 has been observered. When raising this to ~50 active threads and ~50
MySQL connections we've seen 100.000 context switches/second. Performance
suffers.

This is a RedHat 6.2 system with a 2.2.16 kernel, 2*PIII, 900MHz.

I saw some mention of this problem on the MySQL site with regards to
processes holding a pthread_mutex_lock() for short amounts of time. They
advise to use 2.4 but right now that is not within the scope of my options.

My question: is there a 2.2 kernel in which this is resolved? And secondly,
is there a way to prevent this problem purely from userspace? In other
words, what causes this problem.

The MySQL site also mentions that 2.4 could do better in some ways,
especially regarding 'overspin'.

Thanks for your time.


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