On 09/02/2015 05:23 PM, Dave Hansen wrote:
> I'd be curious what the cost is in practice.  Do you have any actual
> numbers of the cost of doing it this way?
> Even if the instruction is a "noop", I'd really expect the overhead to
> really add up for a tens-of-gigabytes mapping, no matter how much the
> CPU optimizes it.

What tens-of-gigabytes mapping? I have yet to encounter an application
that does that. Our tests show that usually the mmaps are small.

I can send you a micro benchmark results of an mmap vs direct-io random
write. Our code will jump over holes in the file BTW, but I'll ask to also
run it with falloc that will make all blocks allocated.

Give me a few days to collect this.

I guess one optimization we should do is jump over holes and zero-extents.
This will save the case of a mostly sparse very big file.


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