[ 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@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
