Re: [PATCH v3 02/15] dax: increase granularity of dax_clear_blocks() operations

[Jan Kara]

On Mon 02-11-15 21:31:11, Dan Williams wrote:
> On Mon, Nov 2, 2015 at 8:48 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > 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@xxxxxxxxxxxxx> 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?
> 
> With a switch to non-temporal writes they wouldn't be synchronous,
> although it's doubtful that the subsequent writes after zeroing would
> also hit the store buffer.
> 
> If we had a method to flush by physical-cache-way rather than a
> virtual address then it would indeed be better to save up for one
> final flush, but when we need to resort to looping through all the
> virtual addresses that might have touched it gets expensive.

Similarly as Dave I'm somewhat confused by your use of "virtual addresses"
and I wasn't able to figure out what exactly are you speaking about. In
Ross' patches, fsync will iterate over all 4 KB ranges (they would be pages
if we had page cache) of the file that got dirtied and call wb_cache_pmem()
for each corresponding "physical block" - where "physical block" actually
ends up being an physical address in pmem. Is this iteration what you find
too costly?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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