Re: [benchmark] 1% performance overhead of paravirt_ops on nativekernels

From: Peter Zijlstra
Date: Wed Jun 10 2009 - 02:29:33 EST

On Tue, 2009-06-09 at 09:26 -0700, Linus Torvalds wrote:
> On Tue, 9 Jun 2009, Nick Piggin wrote:
> >
> > The idea seems nice but isn't the problem that kmap gives back a
> > basically 1st class kernel virtual memory? (ie. it can then be used
> > by any other CPU at any point without it having to use kmap?).
> No, everybody has to use kmap()/kunmap().
> The "problem" is that you could in theory run out of kmap frames, since if
> everybody does a kmap() in an interruptible context and you have lots and
> lots of threads doing different pages, you'd run out. But that has nothing
> to do with kmap_atomic(), which is basically limited to just the number of
> CPU's and a (very small) level of nesting.

One of the things I did for -rt back when I rewrote mm/highmem.c for it
was to reserve multiple slots per kmap() user so that if you did 1 you
could always do another.

With everything in task context like rt does 2 seemed enough, but you
cuold ways extend that scheme and reserve enough for the worst case
nesting depth and be done with it.

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