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

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Date:	Wed, 02 Sep 2015 18:18:06 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>,
	Boaz Harrosh <boaz@...xistor.com>,
	Dave Chinner <david@...morbit.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...l.org>,
	"H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-nvdimm@...ts.01.org, Matthew Wilcox <willy@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [PATCH] dax, pmem: add support for msync

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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