Re: revivesa status 2008/07/09

[Mindaugas Rasiukevicius <rmind%NetBSD.org@localhost>]

Jason Thorpe <thorpej%shagadelic.org@localhost> wrote:
> 
> On Jul 12, 2008, at 10:07 AM, Gary Thorpe wrote:
> 
> > The advantages of SA are supposed to be on *I/O* bound workloads  
> > because it can reduce overhead due to kernel context switches by  
> > doing userland switches where appropriate (according to previous  
> > benchmarks, research etc.), but I could be wron on this.
> 
> No.  I/O bound switching in SA requires a kernel context switch  
> (because the thread blocks in the kernel).

Not only context switch, but it also needs a new LWP (kernel thread) when
blocking. In a case of new pthread (userland) or cold "LWP-cache" (actually,
it is pool) it means: creation of new LWP + context switch. Also, at some
point LWP should be destroyed or put back to the "LWP-cache", what means
more overhead, or well.. wasting of memory.

Imagine a case of 1000 (new) pthreads which block - that would mean:
1000 * (LWP creation + SA context switch) operations. Plus, LWPs for VPs...

-- 
Best regards,
Mindaugas
www.NetBSD.org