From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 3 Nov 2015 15:48:02 +1100 From: Dave Chinner Subject: Re: [PATCH v3 02/15] dax: increase granularity of dax_clear_blocks() operations Message-ID: <20151103044802.GP10656@dastard> References: <20151102042941.6610.27784.stgit@dwillia2-desk3.amr.corp.intel.com> <20151102042952.6610.7185.stgit@dwillia2-desk3.amr.corp.intel.com> <20151103005113.GN10656@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Dan Williams Cc: Jens Axboe , Jan Kara , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" , Jeff Moyer , Jan Kara , Ross Zwisler , Christoph Hellwig List-ID: On Mon, Nov 02, 2015 at 07:27:26PM -0800, Dan Williams wrote: > On Mon, Nov 2, 2015 at 4:51 PM, Dave Chinner 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@fromorbit.com