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Subject: Re: [PATCH] dax, pmem: add support for msync
To: Boaz Harrosh <boaz@plexistor.com>, Dave Chinner <david@fromorbit.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Christoph Hellwig <hch@lst.de>, linux-kernel@vger.kernel.org,
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        linux-nvdimm@ml01.01.org, Matthew Wilcox <willy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org
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From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <55E70653.4090302@linux.intel.com>
Date: Wed, 2 Sep 2015 07:23:15 -0700
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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.