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

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Date:	Wed, 2 Sep 2015 07:23:15 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	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 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.
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