linux-ext4 - Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ

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Date:	Mon, 16 Nov 2015 09:28:59 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	X86 ML <x86@...nel.org>, XFS Developers <xfs@....sgi.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling

On Mon, Nov 16, 2015 at 6:05 AM, Jan Kara <jack@...e.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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