Messages in this thread | | | Date | Wed, 10 Sep 2003 10:52:55 +0100 | From | Jamie Lokier <> | Subject | Re: Efficient IPC mechanism on Linux |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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