linux-kernel - Re: [PATCH v3 02/15] dax: increase granularity of dax_clear

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Message-ID: <20151103044802.GP10656@dastard>
Date:	Tue, 3 Nov 2015 15:48:02 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Jens Axboe <axboe@...com>, Jan Kara <jack@...e.cz>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>, Jan Kara <jack@...e.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 02/15] dax: increase granularity of dax_clear_blocks()
 operations

On Mon, Nov 02, 2015 at 07:27:26PM -0800, Dan Williams wrote:
> On Mon, Nov 2, 2015 at 4:51 PM, Dave Chinner <david@...morbit.com> wrote:
> > On Sun, Nov 01, 2015 at 11:29:53PM -0500, Dan Williams wrote:
> > The zeroing (and the data, for that matter) doesn't need to be
> > committed to persistent store until the allocation is written and
> > committed to the journal - that will happen with a REQ_FLUSH|REQ_FUA
> > write, so it makes sense to deploy the big hammer and delay the
> > blocking CPU cache flushes until the last possible moment in cases
> > like this.
> 
> In pmem terms that would be a non-temporal memset plus a delayed
> wmb_pmem at REQ_FLUSH time.  Better to write around the cache than
> loop over the dirty-data issuing flushes after the fact.  We'll bump
> the priority of the non-temporal memset implementation.

Why is it better to do two synchronous physical writes to memory
within a couple of microseconds of CPU time rather than writing them
through the cache and, in most cases, only doing one physical write
to memory in a separate context that expects to wait for a flush
to complete?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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