Re: [PATCH] dax, pmem: add support for msync

[Dave Hansen]

On 09/02/2015 03:27 AM, Boaz Harrosh wrote:
>> > Yet you're ignoring the fact that flushing the entire range of the
>> > relevant VMAs may not be very efficient. It may be a very
>> > large mapping with only a few pages that need flushing from the
>> > cache, but you still iterate the mappings flushing GB ranges from
>> > the cache at a time.
>> > 
> So actually you are wrong about this. We have a working system and as part
> of our testing rig we do performance measurements, constantly. Our random
> mmap 4k writes test preforms very well and is in par with the random-direct-write
> implementation even though on every unmap, we do a VMA->start/end cl_flushing.
> 
> The cl_flush operation is a no-op if the cacheline is not dirty and is a
> memory bus storm with all the CLs that are dirty. So the only cost
> is the iteration of vma->start-to-vma->end i+=64

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/