Re: [lkp] [xfs] fbcc025613: -5.6% fsmark.files_per_sec

[Christoph Hellwig <hch@xxxxxx>]

On Fri, Feb 19, 2016 at 05:49:32PM +1100, Dave Chinner wrote:
> That doesn't really seem right. The writeback should be done as a
> single ioend, with a single completion, with a single setsize
> transaction, adn then all the pages are marked clean sequentially.
> The above behaviour implies we are ending up doing something like:
> 
> fsync proc            io completion
> wait on page 0
>                       end page 0 writeback
>                       wake up page 0
> wait on page 1
>                       end page 1 writeback
>                       wake up page 1
> wait on page 2
>                       end page 2 writeback
>                       wake up page 2
> 
> Though in slightly larger batches than a single page (10 wakeups a
> file, so batches of around 100 pages per wakeup?). i.e. the fsync
> IO wait appears to be racing with IO completion marking pages as
> done. I simply cannot see how the above change would cause that, as
> it was simply a change in the IO submission code that doesn't affect
> overall size or shape of the IOs being submitted.

Could this be the lack of blk plugs, which will cause us to complete
too early?