Re: Subtle races between DAX mmap fault and write path

On Fri, Aug 05, 2016 at 07:58:33PM +0000, Boylston, Brian wrote: > > Dave Chinner wrote on 2016-08-05: > > > > [ 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. > > Here are some numbers for a particular scenario, and the code is > below. > > Time (in seconds) to copy a 16KiB buffer 1M times to a 4MiB NVDIMM > buffer (1M total memcpy()s).  For the cached+clflush case, the > flushes are done every 4MiB (which seems slightly faster than > flushing every 16KiB): > >                   NUMA local    NUMA remote > Cached+clflush      13.5           37.1 > movnt                1.0            1.3  So let's put that in memory bandwidth terms. You wrote 16GB to the NVDIMM.  That means:                   NUMA local    NUMA remote Cached+clflush      1.2GB/s         0.43GB/s movnt              16.0GB/s         12.3GB/s That smells wrong.  The DAX code (using movnt) is not 1-2 orders of magnitude faster than a page cache copy, so I don't believe your benchmark reflects what I'm proposing. What I think you're getting wrong is that we are not doing a clflush after every 16k write when we use the page cache, nor will we do that if we use cached copies, dirty tracking and clflush on fsync().

IOWs, the correct equivalent "cached + clflush" loop to a volatile copy with dirty tracking + fsync would be: dstp = dst; while (--nloops) { memcpy(dstp, src, src_sz); // pwrite(); dstp += src_sz; }         pmem_persist(dst, dstsz); // fsync(); i.e. The cache flushes occur only at the user defined synchronisation point not on every syscall.

Yes, if you want to make your copy slow and safe, use O_SYNC to trigger clflush on every write() call - that's what we do for existing storage and the mechanisms are already there; we just need the dirty tracking to optimise it.

Put simple: we should only care about cache flush synchronisation at user defined data integrity synchronisation points. That's the IO model the kernel has always exposed to users, and pmem storage is no different.