linux-ext4 - Re: Subtle races between DAX mmap fault and write path

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Date:	Fri, 5 Aug 2016 21:27:39 +1000
From:	Dave Chinner <david@...morbit.com>
To:	"Kani, Toshimitsu" <toshi.kani@....com>
Cc:	"boaz@...xistor.com" <boaz@...xistor.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"jack@...e.cz" <jack@...e.cz>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: Subtle races between DAX mmap fault and write path

[ cut to just the important points ]
On Thu, Aug 04, 2016 at 06:40:42PM +0000, Kani, Toshimitsu wrote:
> On Tue, 2016-08-02 at 10:21 +1000, Dave Chinner wrote:
> > If I drop the fsync from the
> > buffered IO path, bandwidth remains the same but runtime drops to
> > 0.55-0.57s, so again the buffered IO write path is faster than DAX
> > while doing more work.
> 
> I do not think the test results are relevant on this point because both
> buffered and dax write() paths use uncached copy to avoid clflush.  The
> buffered path uses cached copy to the page cache and then use uncached copy to
> PMEM via writeback.  Therefore, the buffered IO path also benefits from using
> uncached copy to avoid clflush.

Except that I tested without the writeback path for buffered IO, so
there was a direct comparison for single cached copy vs single
uncached copy.

The undenial fact is that a write() with a single cached copy with
all the overhead of dirty page tracking is /faster/ than a much
shorter, simpler IO path that uses an uncached copy. That's what the
numbers say....

> Cached copy (req movq) is slightly faster than uncached copy,

Not according to Boaz - he claims that uncached is 20% faster than
cached. How about you two get together, do some benchmarking and get
your story straight, eh?

> and should be
> used for writing to the page cache.  For writing to PMEM, however, additional
> clflush can be expensive, and allocating cachelines for PMEM leads to evict
> application's cachelines.

I keep hearing people tell me why cached copies are slower, but
no-one is providing numbers to back up their statements. The only
numbers we have are the ones I've published showing cached copies w/
full dirty tracking is faster than uncached copy w/o dirty tracking.

Show me the numbers that back up your statements, then I'll listen
to you.

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