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Date: Mon, 16 Nov 2015 09:28:59 -0800
Message-ID: <CAPcyv4jZjnkz2YYtGWmkA23KAUMT092kjRtFkJ3QrzgPfTucfA@mail.gmail.com>
Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling
From: Dan Williams <dan.j.williams@intel.com>
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Sender: owner-linux-mm@kvack.org
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To: Jan Kara <jack@suse.cz>
Cc: Andreas Dilger <adilger@dilger.ca>, Ross Zwisler <ross.zwisler@linux.intel.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, "J. Bruce Fields" <bfields@fieldses.org>, Theodore Ts'o <tytso@mit.edu>, Alexander Viro <viro@zeniv.linux.org.uk>, Dave Chinner <david@fromorbit.com>, Ingo Molnar <mingo@redhat.com>, Jan Kara <jack@suse.com>, Jeff Layton <jlayton@poochiereds.net>, Matthew Wilcox <willy@linux.intel.com>, Thomas Gleixner <tglx@linutronix.de>, linux-ext4 <linux-ext4@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, X86 ML <x86@kernel.org>, XFS Developers <xfs@oss.sgi.com>, Andrew Morton <akpm@linux-foundation.org>, Matthew Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@linux.intel.com>

On Mon, Nov 16, 2015 at 6:05 AM, Jan Kara <jack@suse.cz> wrote:
> On Mon 16-11-15 14:37:14, Jan Kara wrote:
[..]
> But a question: Won't it be better to do sfence + pcommit only in response
> to REQ_FLUSH request and don't do it after each write? I'm not sure how
> expensive these instructions are but in theory it could be a performance
> win, couldn't it? For filesystems this is enough wrt persistency
> guarantees...

We would need to gather the performance data...  The expectation is
that the cache flushing is more expensive than the sfence + pcommit.

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