Re: Slab cache reap and CPU availability

[Andrew Morton]

Dimitri Sivanich <sivanich@xxxxxxx> wrote:
>
> Hi all,
> 
> I have a fairly general question about the slab cache reap code.
> 
> In running realtime noise tests on the 2.6 kernels (spinning to detect periods
> of CPU unavailability to RT threads) on an IA/64 Altix system, I have found the
> cache_reap code to be the source of a number of larger holdoffs (periods of
> CPU unavailability).  These can last into the 100's of usec on 1300 MHz CPUs.
> Since this code runs periodically every few seconds as a timer softirq on all
> CPUs, holdoffs can occur frequently.
> 
> Has anyone looked into less interruptive alternatives to running cache_reap
> this way (for the 2.6 kernel), or maybe looked into potential optimizations
> to the routine itself?
> 

Do you have stack backtraces?  I thought the problem was via the RCU
softirq callbacks, not via the timer interrupt.  Dipankar spent some time
looking at the RCU-related problem but solutions are not comfortable.

What workload is triggering this?
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