RE: huge number of context switches under 2.2.x with SMP & threa

[Davide Libenzi]

On 17-Jul-2001 bert hubert wrote:
> 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'.

If the lock is contended the thread start spinning with sched_yield() for a
given number of times.
This could result in an high ctx switch rate with quite long runqueue also.

- Davide