[ 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:

quoted

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-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org