Re: Efficient IPC mechanism on Linux

[Jamie Lokier]

Arjan van de Ven wrote:
> > The overhead implied by a memcpy() is the same, in the oder of magnitude,
> > ***whatever*** kernel version you can develop.
> 
> yes a copy of a page is about 3000 to 4000 cycles on an x86 box in the
> uncached case. A pagetable operation (like the cpu setting the accessed
> or dirty bit) is in that same order I suspect (maybe half this, but not
> a lot less). Changing pagetable content is even more because all the
> tlb's and internal cpu state will need to be flushed... which is also a
> microcode operation for the cpu. And it's deadly in an SMP environment.

I have just done a measurement on a 366MHz PII Celeron.  The test is
quite crude, but these numbers are upper bounds on the timings:

	mean 813 cycles to:

		page fault
		install zero page
		remove zero page

		(= read every page from MAP_ANON region and unmap, repeatedly)

	mean 11561 cycles to:

		page fault
		copy zero page
		install copy
		remove copy

		(= write every page from MAP_ANON region and unmap, repeatedly)

I think that makes a clear case for avoiding page copies.

-- Jamie
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