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Re: revivesa status 2008/07/09
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...
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