RE: [PATCH RFC] smt nice introduces significant lock contention

["Chen, Kenneth W" <kenneth.w.chen@intel.com>]

Chris Mason wrote on Thursday, June 01, 2006 3:56 PM
> Hello everyone,
> 
> Recent benchmarks showed some performance regressions between 2.6.16 and 
> 2.6.5.  We tracked down one of the regressions to lock contention in schedule 
> heavy workloads (~70,000 context switches per second)
> 
> kernel/sched.c:dependent_sleeper() was responsible for most of the lock 
> contention, hammering on the run queue locks.  The patch below is more of 
> a discussion point than a suggested fix (although it does reduce lock 
> contention significantly).  The dependent_sleeper code looks very expensive 
> to me, especially for using a spinlock to bounce control between two different 
> siblings in the same cpu.


Yeah, this also sort of echo a recent discovery on one of our benchmarks
that schedule() is red hot in the kernel.  I was just scratching my head
not sure what's going on.  This dependent_sleeper could be the culprit
considering it is called almost at every context switch.  I don't think
we are hitting on lock contention, but by the large amount of code it
executes.  It really needs to be cut down ....

- Ken