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Date: Wed, 02 Sep 2015 18:18:06 +0300
From: Boaz Harrosh <boaz@plexistor.com>
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To: Dave Hansen <dave.hansen@linux.intel.com>,
        Boaz Harrosh <boaz@plexistor.com>, Dave Chinner <david@fromorbit.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
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        Hugh Dickins <hughd@google.com>, Ingo Molnar <mingo@redhat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-nvdimm@ml01.01.org, Matthew Wilcox <willy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org
Subject: Re: [PATCH] dax, pmem: add support for msync
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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.

Thanks
Boaz