On Wed, Aug 14, 2013 at 7:10 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Wed, Aug 14, 2013 at 09:11:01PM -0400, Theodore Ts'o wrote: >> On Wed, Aug 14, 2013 at 04:38:12PM -0700, Andy Lutomirski wrote: >> > > It would be better to write zeros to it, so we aren't measuring the >> > > cost of the unwritten->written conversion. >> > >> > At the risk of beating a dead horse, how hard would it be to defer >> > this part until writeback? >> >> Part of the work has to be done at write time because we need to >> update allocation statistics (i.e., so that we don't have ENOSPC >> problems). The unwritten->written conversion does happen at writeback >> (as does the actual block allocation if we are doing delayed >> allocation). >> >> The point is that if the goal is to measure page fault scalability, we >> shouldn't have this other stuff happening as the same time as the page >> fault workload. > > Sure, but the real problem is not the block mapping or allocation > path - even if the test is changed to take that out of the picture, > we still have timestamp updates being done on every single page > fault. ext4, XFS and btrfs all do transactional timestamp updates > and have nanosecond granularity, so every page fault is resulting in > a transaction to update the timestamp of the file being modified. I have (unmergeable) patches to fix this: http://comments.gmane.org/gmane.linux.kernel.mm/92476 I'll dust them off. Getting something like that merged will allow me to run an unmodified kernel.org kernel on my production system :) It should be a latency improvement (file times are deferred), a throughput improvement (one update per writepages call instead of one per page), and a correctness improvement (the current semantics violate SuS, IIRC, and are backwards from the point of view of anything trying to detect changes to files). --Andy > > That's why on XFS the log is showing up in the profiles. > > So, even if we narrow the test down to just overwriting existing > blocks, we've still got a filesystem transaction per page fault > being done. IOWs, it's still just a filesystem overhead test.... > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx -- Andy Lutomirski AMA Capital Management, LLC -- 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
