[LTP] [PATCH v2] io: fix really slow dio_sparse on certain systems

[Cyril Hrubis]

Hi!
> This is definetly a possible solution. I sent this patch by waiting for
> some feebacks in order to have other opinions. What puzzles me is that
> it's only happening in POWER10 on a random node during kernel tests.
> Other architectures seem to work fine.
> 
> kernel 6.6+ seems to be the affected one.
> 
> >
> > However the test should finish as fast as the writer finishes writing
> > the file. So slow readers shouldn't matter unless there is some serious
> > contention on the disk I/O. That's probably the reason you are aligning
> > the writer as well.
> 
> Exactly, I would expect that.
> 
> >
> > What is the difference in runtime between test before and after this
> > patch on the slow hardware?
> 
> DS009 from 4 hours to 30 seconds. I also profiled the list of syscalls
> with perf, obtaining a 63+ % of io_read() time consumption. Still, this
> patch moves the execution from ~10 secs to ~3 secs on my laptop. There's
> a big difference between 4h and 10 secs runtime, no matter the hard disk
> which is running below.

Honestly that does sound like a kernel bug. You shouldn't get several
orders of magnitude slower just because your buffer is 4 times smaller.

-- 
Cyril Hrubis
chrubis@suse.cz