From mboxrd@z Thu Jan  1 00:00:00 1970
From: Boaz Harrosh <boaz@plexistor.com>
Subject: Re: [PATCH] dax, pmem: add support for msync
Date: Wed, 02 Sep 2015 18:18:06 +0300
Message-ID: <55E7132E.104@plexistor.com>
References: <1441047584-14664-1-git-send-email-ross.zwisler@linux.intel.com> <20150831233803.GO3902@dastard> <20150901070608.GA5482@lst.de> <20150901222120.GQ3902@dastard> <20150902031945.GA8916@linux.intel.com> <20150902051711.GS3902@dastard> <55E6CF15.4070105@plexistor.com> <55E70653.4090302@linux.intel.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>,
 Christoph Hellwig <hch@lst.de>, linux-kernel@vger.kernel.org,
 Alexander Viro <viro@zeniv.linux.org.uk>,
 Andrew Morton <akpm@osdl.org>, "H. Peter Anvin" <hpa@zytor.com>,
 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@lists.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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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

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