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
>> 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:
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).
> 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....
> Dave Chinner
AMA Capital Management, LLC